Conserving Canvas

This glossary is based on the Handbook of Terms Used in the Lining of Paintings, selected and edited by Westby Percival-Prescott and Gillian Lewis for the Conference on Comparative Lining Techniques, Greenwich, London, 1974, and reprinted in Lining Paintings: Papers from the Greenwich Conference on Comparative Lining Techniques (Villers 2003b). The main contributors to the Handbook were David Bomford, Alan Cummings, Gerry Hedley, Gillian Lewis, Joyce Plesters, and Westby Percival-Prescott.

In this revised and expanded version, the editors, Cynthia Schwarz, Jim Coddington, and Ian McClure, have reviewed the terms used in the 1974 version, removing those no longer current and adding terms not current in 1974, taking notice of terms used in the papers included in this publication.

abrasion: Damaged area of paint, resulting from the scraping, rubbing down, or grinding away of the upper paint layers. A combination of an iron that is too hot plus careless application of pressure during hand lining will easily produce this form of damage, and frequently the excess heat, unevenly maintained in local areas, will succeed in softening, scorching, and burning the paint surface so that it adheres to any protective layers of paper and is subsequently removed with these facing layers after the lining is completed. On a smaller scale, an overheated spatula used for fixing down flaking paint will have a similar effect. Often seen as a regular pattern mirroring the canvas. See also weave emphasis.
absorption: (1) The concentration or retention of a substance within the porosity of another material (compare adsorption). (2) Optical light and radiant heat are absorbed by matter in varying degrees, leading to increase in temperature.
acrylic resins: Synthetic resins of a general formula where R1 and R2 are alkyl groups, or H. R1 is commonly CH₃, which gives methyl methacrylate, ethyl methacrylate, and butyl methacrylate where Ri = CH₃, C₂H5, C₃H7 etc. Some have a distinct tendency to cross-link, while for others there is no recorded tendency to do so. Solubility also varies widely. Examples of acrylic resins in conservation are polymethyl methacrylate, methylacrylate/ethyl methacrylate copolymer, and various other resins and emulsions.
acrylic sheet: Polymethyl methacrylate sheet. Available as a clear, transparent plastic in a range of thicknesses. It finds use as a backing board or a rigid support for marouflage, where its transparency is sometimes desirable. Also used for glazing, particularly if a nonreflective coating is applied.
adherend: A body secured to another body by an adhesive.
adhesion: Sticking of one surface to another, the result of forces of attraction between molecules, among other factors.
adhesive: A material that binds other materials together by forces of molecular attraction, chemical bonds, or interlocking action at the interfaces.
adhesive failure: See bond strength.
adhesive strength: See bond strength.
adsorption: The concentration or retention of a substance on the surface of a material. See also absorption.
aging test (accelerated): To test the possible deterioration of materials with time they may be exposed to more extreme conditions of heat, light, air, and others than normally encountered, in order to age them artificially.
air conductor: For lining processes under vacuum pressure. Extraction of air present between the canvases. The working support and the upper membrane is usually facilitated by (1) strips of open-weave fabric, felt, webbing, corrugated paper, and other materials placed around the edges of the object and leading from it to the extraction points; by (2) similar strips actually fitted permanently in the edges; or by (3) channels built into the hot tab around the edge of the lining area See also hessian and vacuum hot table.
aluminum sheet: Aluminum alloy sheet used as a rigid support or in composite supports for marouflage, also used as a surface material for hot tables. It is available in a variety of types of differing alloy composition and surface properties and in a range of thicknesses.
animal glues (bone and hide): Glues that are prepared from collagen, the principal constituent of skin, bone, and sinew, by treatment with acids or hot water to yield a soluble product. There are two main types of animal glues: those obtained from the hides and those made from the bones (most commonly of sheep and cattle). Hide glues are stronger than bone glues. Animal glues give viscous aqueous solutions that set first by gelation on cooling and then by loss of water. They swell and lose strength in humid conditions. Common additives are humectants and plasticizers to offset embrittlement on aging, and fungicides to deter mold growth. See also fish glue.
backing, backboards, backing boards: Protection applied to the reverse of a painting and/or picture frame to prevent mechanical and atmospheric damage to picture fabrics.
balsam: A general term for the resinous exudate from trees of the order Coniferae. The balsams are soft semiliquids containing essential oil, terpenes, and resinous bodies. Distillation yields turpentine and the residue, resin or colophony. Common balsams are Venice turpentine, Strasbourg turpentine, and Canada balsam. Venice turpentine particularly finds use as a plasticizer and tackifier in both wax-resin and traditional aqueous adhesives such as Italian pasta. See also oleoresin.
batting: A layer between the backing board and the back of the canvas in back boards, also referred to as backing boards. Can be made from a variety of fibers.
beeswax: A secretion from honeybees from which the honeycomb is built. Natural beeswax is a yellowish-brown solid with a granular fracture, brittle when cold, plastic when warmed, and melting between 65°C and 68°C. It contains varying proportions of hydrocarbons, esters, free acids and other compounds, depending on where it was produced. Color and texture vary with origin. Bleaching is usually affected by (a) simple purifying and de-colorizing with charcoal or Fuller’s earth (or similar bleaching earth) or boiling with water; (b) via oxidation by exposure to air or by treatment with oxidizing agents, such as ozone, chlorine, permanganate, potassium dichromate. Use of strong oxidizing agents (e.g., dichromates or chlorine) produces a more brittle and crystalline product. Beeswaxes are widely used in hotmelt wax-resin lining and facing adhesives because of their stability, inertness, and relative impermeability to moisture.
benzine: See mineral spirits.
Beva 371: Adhesive developed by Gustav Berger in the 1970s as a replacement for wax resin. A mixture of synthetic resins and waxes. The recipe has been modified over time, resulting in different thicknesses, properties, melting points, and performance. Available as both a gel and a film.
biodegradation: The breakdown of organic matter by microorganisms, such as bacteria and fungi.
blanket (heating): A rubber sheet with embedded electric heating elements. Used (usually in conjunction with a metal plate) as a versatile extension of the hot-table method. Successfully used to reline outsize paintings, section by section.
blind stretcher: See panel back stretcher.
blister: A small, raised area of paint indicating cleavage of paint and/or ground layers either from each other or from the support.
Bloom strength: Bloom strength testing measures the strength of a gel or gelatin at a specified temperature. The test determines the weight in grams needed by a specified plunger (normally with a diameter of 1/2 inch) to depress the surface of the gel by 4 mm without breaking it. Also called Bloom number.
bobbinet: A fabric invented by John Heathcote in 1806. The warp threads are wound around the weft threads, producing a characteristic hexagonal mesh pattern, which is very strong and dimensionally stable. Widely used in fashion to support and shape garments, it has been used in conservation to support damaged large-scale fabrics, such as panoramas. Traditionally made from silk threads, currently a wide variety of threads—natural, synthetic, and metal—are used for numerous purposes.
bond strength: The force required to break an adhesive assembly. Three types of failure may occur, either separately or in combination: (1) Failure at the interface between the adhesive and adherend, called adhesive failure. The ability to resist adhesive failure is termed the adhesive strength. (2) Failure within the adhesive layer. This is called cohesive failure (of the adhesive), and occurs, for example, when wax-resin-impregnated canvases are separated. The ability of an adhesive to resist such failure is the cohesive strength (of the adhesive). (3) Failure of one or other of the adherents. This occurs when the strength of one of the adherends is less than either the adhesive strength or cohesive strength of the adhesive. Bond strength varies according to how the load is applied and is expressed accordingly: tensile strength, peel strength, cleavage strength, impact strength, sheer strength, and so forth.
bone glue: See animal glues (bone and hide).
buckling: The appearance of waves or bulges in a canvas that has slackened on its stretcher. See also cockling and corner draws.
bulge: Irregular distortion, wrinkling, and swelling of stretched fabric support caused by uneven dimensional change or accidental pressure against the canvas.
burlap: A coarse loose woven canvas. Often made from jute fibers, its texture is favored as a painting support by some artists.
butt joint: A joint where the ends of the material are joined without any overlap.
cami-lining: A technique for adding support to an unlined canvas. It is done by attaching fabric, usually polyester for its stability, to the reverse of the stretcher using staples along the reverse of the outside members and feeding the fabric under the stretcher cross members to create a tensioned structure in which the lining is in contact or near contact with the original at only one point in the middle of the canvas. Also called stretcher bar lining.
canvas: A generic term for the fabric support for a painting and for the finished painting itself. Natural fibers were traditionally used, but use of synthetic fibers also occurs, either on their own or in combination with natural fibers.
canvas pliers: Pliers or pincers with wide corrugated jaws used in stretching canvas over a stretcher or strainer. A projection below one jaw acts as a fulcrum for levering against the back of the stretcher. Also called stretching pliers.
casein: A strong proteinaceous compound obtained from skimmed milk that forms an insoluble adhesive on mixing with an alkali (usually calcium hydroxide [lime water] to give calcium caseinate) or with formalin. Prepared casein glues are available that have only to be mixed with water. A very strong adhesive, casein has been used for centuries, particularly for wood, and was occasionally employed in the past for transferring paintings and as a pigment binder.
cellulose: A complex polysaccharide carbohydrate consisting of parallel unbranched chains of glucose units. It is the structural and principal tissue forming the walls or skeletons of plants. Cotton fibers and delignified wood are the most important raw materials for preparation of cellulose derivatives, such as cellulose acetate, cellulose nitrate, and methyl and ethyl cellulose. The chief component of fabric fibers of vegetable origin, cellulose is hygroscopic, subject to oxidation, is decomposed by acid action, and acts as a culture for bacteria and fungi. Sodium carboxymethyl cellulose (SCMC) is an important derivative, a stable nonyellowing water-soluble compound used as a general-purpose adhesive in paper and textile conservation. It is also used as a thickener for other water-soluble adhesives.
ceresin: A white waxy substance but not a true wax. Like paraffin wax, ceresin is a mixture of high-melting-point hydrocarbons. It differs from paraffin wax in being plastic and noncrystalline in character. It is soluble in alcohol, benzene, and other solvents and has a melting point of 65°C–80°C.
cheesecloth: Cheesecloth, also referred to as butter muslin, is a thin, loosely woven cloth with fine mesh (traditionally used for wrapping butter), useful for straining impurities from adhesive mixtures (e.g., lumps or twigs out of natural resins), prior to their application for lining. Traditionally used in transfers as a support layer for the detached paint layer before reattachment to a new support. See also muslin.
chemisorption: Distinct from absorption and adsorption, chemisorption creates a chemical bond formed in the material. As a result, the liquid or gas taken up cannot be expelled by moderate heat.
cleavage: Separation between paint layers, paint and ground layers, or ground and support. Cleavage occurs where adhesion between layers has deteriorated and may often be found where a heavy glue layer has been placed between support and ground. Common treatment takes the form of local or total infusion with an appropriate adhesive; sometimes this involves lining or relining and full impregnation with the lining mixture.
clippings: Small pieces cut from full animal skins as waste and used in the preparation of animal glues, particularly parchment glue.
cockling: A ripple or wrinkle distortion occurring in lining and original canvases, usually during hand lining, when the canvas reacts severely to localized heat, expanding or contracting unevenly. The adhesive may then lock in this distortion. See also buckling.
cohesion: Force holding a solid or liquid together owing to attraction between the molecules.
cohesive failure: See bond strength.
cold flow: Commonly confused with creep but is in fact plastic deformation without the action of external forces.
colletta: Italian term, strictly meaning rabbit-skin glue but often used to mean that glue plus several other additives, together forming a thin, animal-glue composition. Used as a consolidant to the reverse of canvas paintings prior to lining with pasta, as a facing adhesive, for fixing flaking paint, and so forth.
colloid: A state of subdivision of matter that consists either of single, large molecules (proteins, organic polymers, etc.) or of aggregates of smaller molecules (colloidal gold, sulfur, etc.). There are eight recognized classes of colloids: solid sols (solid in solid), such as alloys; suspensions (solid in liquid), such as paint; smokes (solid in gas); gels (liquid in solid), such as glue gel and fruit jelly; emulsions (liquid in liquid), such as milk; fogs (liquid in gas), such as clouds or visible steam: solid foams (gas in solid), such as sponge rubber or pumice; foams (gas in liquid), such as soap lather. The colloidal particles are called the disperse phase and the surrounding medium, the continuous or external phase.
colophony: The residue that remains after turpentine has been distilled off from a species of the Pinus family. It is soft resin with a melting point of 100°C–130°C and is soluble in a variety of organic solvents. In the past, was sometimes used in wax-resin lining adhesives to improve the flow properties of the melt and as a plasticizer. Also called rosin.
compression: Decrease in dimension of a body or material by application of external inward-directed forces.
compression hold: A type of surface deformation associated with impregnation lining systems (particularly hot-melt adhesive) using vacuum hold in conditions where the paint structure can be deformed by this pressure and set in that deformed configuration on chilling. The degree of compression hold that will be distributed throughout the lining complex is directly related to the amount of vacuum pressure and degree of impregnation employed. The use of an unimpregnated interleaf between painting and support can reduce compression hold distortion. See also nap bond lining.
complex weaves: A repeating weave pattern that is other than plain, basket, or twill weave, such as herringbone, or weaves with repeating patterns such as the mantelillo canvases used in seventeenth-century Spain.
consistency: The viscosity or fluidity of a liquid or paste.
consolidant: An adhesive used in consolidation. A few examples used in the conservation of paintings are fish and animal glues, synthetic resins, and resin emulsions. See also consolidation.
consolidation: The use of an adhesive (consolidant) to re-adhere detached layers in a painting structure or to add cohesion to the layer.
constant tension stretcher: A stretcher that is constructed to be able to move in concert with expansion and contraction of the painting it supports.
contact lining: Contact lining is a low-pressure lining process where the adhesive adhering the lining canvas to the original is applied to the lining canvas only, and occasionally a thin layer is applied to the original canvas without impregnation. The bond is achieved without any penetration of the adhesive into the structure of the original canvas.
contraction: Decrease in dimension of an object or material resulting from internal structural changes rather than external compressive forces.
convection crackle: In a paint surface, a type of age crackle that predominates in areas affected by barriers (e.g., stretcher bars) or bottlenecks (e.g., behind keys) impeding the flow of air at the reverse of the painting. The buildup and consequent absorption by the canvas of moisture at these points induces differential local dimensional change and stress. The crackle will often be less or absent in the areas of the barriers themselves, where a more stable microclimate is maintained. See also stretcher bar marks and convection patterns.
convection patterns: Dust deposits visible on a wall on which a painting has been hanging. The restricted conditions for circulation, which seldom allow an unrestrained passage to warm, moisture-bearing air currents, cause dust patterns to accumulate. These can show detailed images of the stretcher, frame, canvas, keys, labels, and other components of the picture, even though it is not in contact with the wall. These patterns may take twenty years or so to form but can clearly indicate wide variations within the microclimate, direction of air flow, and the like behind the picture.
cooling iron: A large, heavy, unheated iron used in manual thermoplastic-adhesive lining processes to rapidly cool a surface where heat setting adhesives have been introduced, to quickly set the adhesive. The iron is bulky to assist in pressing together the cooling canvases and adhesive and also to draw away heat as quickly as possible.
copolymer: A polymer that results from the joining of two or more different types of monomer molecules.
corner draws: A series of undulations in the canvas radiating from a corner of the stretched canvas. These can be caused by the canvas at the corners becoming detached from excessive tensioning, from distortions in the stretcher construction (for example, protruding tenons), or by the canvas becoming slack on the stretcher or strainer. See also buckling and cockling.
cotton: The seed hair of the cotton plant. Long, fine fibers make fine yarns, while short, coarse fibers make coarser yarns, the four main types ranging from coarse to fine are Indian, American, Egyptian, and Sea Island, but there is considerable variation within each type. The breaking strength of cotton increases with increased RH, and is good when wet, but the fibers are nearly pure cellulose and thus are readily affected by acids and oxidizing agents, have low resistance to mold attack, and react rapidly to RH change. Once degraded their wet strength will be lower than the dry strength. However, in hot, dry climates they exhibit good resistance to breakdown. Cotton came into wider use as a paint support in the middle of the twentieth century when it provided large quantities of cheap ready-made artists’ canvas, but it developed a reputation for impermanence. Consequently, its use as a lining fabric is not common.
cotton duck: A heavy, plain-woven cotton fabric.
crackle: Breaks in the paint layers, or paint and ground layers, forming a network over the surface of a painting, occurring in two main forms: (1) Drying cracks (vehicular) caused by failure of the film to withstand its own contraction during drying or by the artist’s incorrect use of paint. (2) Age cracks (mechanical) caused by strain from movement of the support. This second type of crackle is one of the first stages in mechanical breakdown of the complex structure of a painting, but it requires treatment only when it begins to form buckling or flaking.
creep: Plastic deformation (see elasticity) of materials under stress at room temperature. In lined pictures, creep refers to the gradual extension of a lining canvas impregnated with adhesive after the painting has been replaced on its stretcher. Over a period of time, this results in sagging of the painting under its own new total weight. Contrast with cold flow.
cross linking: The joining of long-chain polymer molecules by lateral chemical bonds. Through cross linking, a collection of effectively one-dimensional molecules becomes a two- or three-dimensional network. This has the effect of making the polymer insoluble in normal solvents. Materials susceptible to cross linking should be avoided in conservation, where reversibility is desirable.
cupping: Islands of aged paint, bounded by cracks, with upward curving edges forming saucer shapes; these often draw a less stiff canvas support with them. They are induced by slight shrinkage of the canvas support or by chemical contraction of the upper stratum of the paint and/or differential contraction of the paint and varnish films, which prevents the cracked paint from lying completely flat. Frequently, excess pressure and heat are used in an attempt to reduce cupping during the lining process, which can result in the tops of the cracks being crushed and the paint surface abraded.
curing: The irreversible hardening of a synthetic resin by action of heat, a chemical catalyst, or other means.
cusping: Tacking marks, visible in the original canvas as a regular scalloping of the threads at the perimeter of the painting. The peaks of the scallops indicate the original tack points (and therefore the overall dimensions) on a painting that has been lined subsequently. They are clearly visible on radiographs. Also called stress garlands or scalloping.
dammar: A pale yellow, brittle natural resin, completely soluble in aromatic hydrocarbons and turpentine to give light-colored solutions. It melts at 100°C–115°C and is used as a varnish, in facing mixtures and in wax-resin lining materials.
denaturation: The process of modifying the molecular structure of a protein. Denaturation involves the breaking of many of the weak linkages, or bonds (e.g., hydrogen bonds), within a protein that are responsible for the highly ordered structure of the protein in its natural state. Denatured proteins have a looser, more random structure. Denaturation can be brought about in various ways, for example by heating or by treatment with alkali, acid, urea, or detergents.
delamination: The failure of adhesion between layers of a painting, often manifested by dimensional change or loss of paint. Delamination is a mode of failure where a material fractures into layers. Surface coatings such as paints and films can delaminate from the coated substrate.
dextrin: A generic name for the degradation products of starch produced by heating in the presence or absence of hydrolytic agents, used commercially as a thickening agent. There are a number of types of dextrin, usually comprising a mixture of soluble starch, true dextrin, and sugar (maltose and dextrose). They yield syrupy aqueous solutions with moderate adhesive properties.
dibutyl phthalate: A plasticizer that has been used in wax resin and synthetic-resin adhesives. It is an organic solvent with a high boiling point (340°C) that is immiscible with water and has insect-repellent properties. In time, its high vapor pressure causes it to gradually leave the film.
distensibility: The capability of being lengthened or extended in any direction.
double boiler: A device based on the principle of the water bath for heating materials to the boiling temperature of water without danger of burning. An inner saucepan holds the material and fits into a larger pan containing boiling water. Also called bain-marie.
double lining: In large pictures, a single lining may not be strong enough to support the weight of the original canvas and paint. In such cases a second lining may be carried out on the back of the first, and both lining canvases are fastened to the stretcher. Double lining may also be used to support seams or tears of any length. Sometimes, different adhesives are used for the two linings—first an aqueous (glue-paste) adhesive and then a wax-resin adhesive. See also interleaf.
dry mounting: An idea developed from a method for mounting photographs. The original canvas and lining canvas are bonded together with a thin layer of thermoplastic material (such as polyvinyl acetate) under the action of heat and pressure; a vacuum hot table may be used. Suitable where only strengthening of the original canvas is required—cleavage and cupping of the paint layer would be treated separately.
ductility: The ability of a material to undergo plastic deformation by tension.
durability: The capacity of a material to remain unchanged under normal conditions.
Dutch method: A term sometimes used for the wax-lining process. After the beginning of the eighteenth century in The Netherlands, the traditional glue-paste lining adhesives were gradually abandoned in favor of wax or wax-resin lining adhesives. In France in the mid- eighteenth century, the Comte de Caylushad succeeded in impregnating a painting with wax from the back in front of a fire. Charles Wilson Peale (1741–1827), an American artist, used wax-resin mixtures to impregnate his own paintings in the 1780s. See also wax-resin adhesives.
dynamic mechanical analysis (DMA): A technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers. The temperature of the sample or the frequency of the stress are often varied, leading to variations in the complex modulus. This approach can be used to locate the glass transition temperature of the material.
elasticity: The property that enables a stretched or compressed body or material to return to its original shape and size when the forces acting on it are removed. The three stages a stretched material can experience are (1) elastic deformation, in which the material returns to its original size when the stress is removed; (2) plastic deformation, in which the material is irreversibly stretched and does not return to its original size; and (3) breaking point. (1) and (2) can happen simultaneously in some materials, notably canvases.
elemi: A term covering a number of oleoresins. The best known is Manila elemi, a soft, yellow resin that is soluble in a variety of solvents other than mineral spirit. It is a common constituent of wax-resin lining adhesives, its purpose being to provide tack.
Eltoline tissue: A fine, long-fibered paper tissue used as an alternative to the more expensive mulberry paper tissue as an interleaf or facing material. Eltoline is the trade name of a range of high-quality papers available in several grades.
embrittlement: Loss of flexibility by a material. The increasing inability of, for example, a resin film to be bent without cracking. Plasticizers are introduced to reduce embrittlement.
emulsion: An emulsion consists of drops of one liquid dispersed in another liquid, in which it is immiscible. Generally, a third component, the emulsifying agent, is incorporated to stabilize the emulsion and prevent coalescence of the drops. Both water-in-oil and oil-in-water emulsions can be prepared. See also colloid.
emulsion (or dispersion) glues: The well-known “emulsion-glues” are in fact not true emulsions but dispersions of solid globules of adhesive polymer in an aqueous matrix. Commercial emulsions may contain various additives unsuitable for use on paintings and have an unsatisfactory pH. Emulsion glues are commonly used as low-hold, nap-bond lining adhesives.
enzyme: A catalytic substance produced by living cells that has a specific action in causing the decomposition or synthesis of compounds into new ones. Occasionally enzymes are used for the removal of old lining adhesives.
epoxy resins: Epoxy adhesives comprise a liquid or a fusible solid containing epoxide groups and a curing agent containing functional groups with which the epoxide groups combine to form a cross-linked polymer. The curing is an irreversible reaction that results in a thermoset resin with only slight shrinkage. A wide range of properties can be obtained by the use of different resin-hardener adhesive systems. Epoxy resins such as Araldite are used in preparing honeycombed auxiliary supports. Because of their irreversible nature they are not used directly as lining adhesives, but occasionally they can be employed for joining tears prior to lining. Trade name: Araldite (Huntsman Corporation).
ethylene vinyl acetate copolymers: See vinyl resins.
examination techniques: Basic information concerning the characteristics and deformation present in any painting intended for lining is obtainable from examination by the following means: (1) raking light (reveals buckling, bulging, cupping, cleavage, flaking, shrinkage, heat and pressure distortion, etc.); (2) transmitted light (reveals structure of original canvas ground and paint distribution, losses and tears, joins and patches); (3) reflected light (reveals planar distortions, surface texture). Photographs made in raking light are a valuable means of recording changes before and after lining. Radiography is used to reveal canvas weave type (especially for lined paintings where the original canvas may not be readily visible), losses in each layer, original stretching marks, joins, and distribution of paint ground layers.
expansion: Increase in dimension of an object or material due to internal structural changes rather than as a result of an applied mechanical stress.
Fabri-Sil: A Teflon-impregnated glass cloth coated on one side with a silicone-based pressure sensitive adhesive. It is supplied with a protective release layer on the adhesive side. The threads are anisotropic. The fabric has 13 threads per centimeter in the warp direction and 8 threads per centimeter in the weft.
facing: A process whereby an adaptable material (very often a thin tissue such as Eltoline tissue, mulberry paper, sulfite paper, fine silk, very thin cartridge paper) is glued to the face of picture to protect the paint layer during lining or other mechanical manipulation of the support. A chosen facing adhesive is applied either directly through the facing fabric onto the paint surface or separately to the tissue, which is then carefully applied to the paint to avoid wrinkling. In major support treatment, such as transfer, composite facings may be necessary, using different types of material and adhesive in successive layers. See also facing patterns.
facing adhesives: These adhesives (often mixtures) prepared for use in facing paintings or on other materials must be readily reversible. They include hot melts, resin solutions or emulsions, and water-soluble glues.
facing patterns: Markings left on the surface of a painting by the facing material during or after the lining. May be caused by excessive heat softening the final paint layers, or by the facing mix having an affinity for a soft paint surface (e.g., a natural wax-resin facing adhesive for a final resinous glaze), or by excessive pressure imprinting the edges, joins, or wrinkles in the facing material into the paint texture.
feathering: In strip-lining and patching canvas pictures, the piece of canvas used for the repair should not have a hard edge, which could show through as a ridge on the front of the picture. Thus, some threads along each side are removed, leaving only perpendicular strands at the edges. This creates a soft frayed or feathered transition, similar to a slight chamfer. The edge is sometimes not straight to further diminish the risk of it showing through.
fiber: See textile fiber.
Fieux lining: A lining system developed by Robert Fieux. It used a silicone-based adhesive preapplied to a synthetic fabric as the lining fabric. Lining was achieved by attaching the painting to the lining fabric using electrostatic hold to create a light pressure. See also Fabri-Sil.
filler: (1) An inert material added to an adhesive (or paint) to modify it, usually to improve its strength, flow, or other properties. Whiting or precipitated chalk, gypsum, and titanium dioxide are commonly used as fillers, for instance as a component in wax-resin lining mixtures. (2) Filler, or gap-filler, also refers to an adhesive material (i.e., putty) used to fill losses in a paint or ground film.
fish glue: Proteinaceous glue prepared from the heads, bones, and skins of fish, marketed in liquid form or in readily soluble cakes or sheets. Fish glue has weaker setting powers than mammalian glue, lower molecular weight, and different amino acid composition. Commercial glues generally contain preservatives and essential oils. Trade name: Seccotine. See also sturgeon glue.
fixative: A term usually applied to a dilute solution of a resin or adhesive that is sprayed onto chalk drawings or pastels. In the context of lining, a fixative is an adhesive solution or dispersion, or a hot melt, applied to the front of the painting in the treatment of cleavage flaking, blistering, and the like. Low viscosity and good wetting properties are usually required to allow good penetration beneath the flaking paint.
flaking: The breaking away or detachment of one or all paint and ground layers from the support in either small particles or larger areas. Flaking is an extreme stage of blistering, buckling, cleavage, and crackling. See also blister, buckling, cleavage, and crackle.
flocking: A method of spraying adhesives onto canvas that produces strands of adhesive in a fiber-like texture. The back of the canvas can conform to more thickly applied flocked adhesive layers.
flax: Plant from whose fibers linen is made. These fibers are fairly long, having a compound structure, and the twists in the yarn is usually fewer per unit length than that of cotton yarns because of the greater length. With time, continuous adjustment to changes of moisture content in the air can cause these compound fibers of the woven linen to fall apart and become so weak that the twist of the yarns and the crossing of the yarns in the fabric can no longer hold them in place. Some aged flax fibers have been known to develop a twist like a cotton fiber. Canada is currently the world’s largest flax producer, and the very fine linens are made in Belgium, Ireland, and Scotland.
flexibility: An inexact concept referring to the degree to which a material may be bent or stretched.
flour paste: An adhesive differing from starch paste in working properties, in that flour contains gluten, a proteinaceous material, as well as starch. Different flour pastes also differ in working properties. Wheat, rye, rice, and linseed flours are those commonly chosen. Flour pastes are widely used in the Italian pasta lining adhesives, which are mixtures of animal glue, flour paste, Venice turpentine, and various other materials. The purpose of the flour paste is to act as a filler in the mixture, having lower contracting forces than animal glue and giving a higher solid-to-liquid ratio; it also provides “slip” and aids spreading. Can be susceptible to mold and insect attack.
formalin: A 40% aqueous solution of formaldehyde used as a disinfectant. Used very dilute, it hardens, embrittles, and renders insoluble gelatin and animal glues in general, egg tempera, and casein.
fungicide: A substance that destroys fungi and mold and prevents their growth. Commonly added to aqueous lining adhesives containing natural carbohydrate and protein materials that support mold, including compo, pasta, fish glues, and cellulose derivatives such as sodium carboxymethyl cellulose (SCMC). Names for fungicides used by restorers are topane, thymol, Formalin (formaldehyde), and sodium fluoride.
fungistat: A substance capable of preventing the growth of fungi, molds, and the like.
gacha: A traditional Spanish glue paste adhesive comprising animal glue, flour, and additives, similar to traditional Italian pasta recipes.
garlanding: See cusping.
gel: A jelly-like colloidal substance composed of a liquid and a solid. The solid phase forms a network of macromolecules and the liquid phase is distributed through it, for example, gelatin and water. See also colloid.
gelatin: The purest of the adhesives obtained from animal hide and bone. See also animal glues (bone and hide).
gesso: The word is Italian for gypsum. Gesso is a pale, creamy white priming composed of burnt gypsum (plaster of Paris) mixed with glue. Two kinds of gesso grounds were used by early Italian painters: (1) gesso grosso, a mixture of plaster and glue size, which was applied direct to the painting support (usually panel), and (2) gesso sottile, a finer crystalline gypsum (slaked plaster of Paris) mixed with glue size, which could be used as a final surface over gesso grosso but also as a priming for canvas. Gesso has come to have a wider meaning today, which now includes grounds made from chalk (whiting) or another inert white pigment, bound with acrylic emulsions, glue size, parchment size, calfskin glue, rabbit-skin glue, or isinglass. Modified gesso is also common, in which the addition of white lead bound with flour paste and drying oil is used to produce “gesso” grounds more suited to oil techniques.
glass fiber woven fabric: Blown or drawn glass that has been made into fibers when molten and subsequently woven. The fibers are usually lubricated to assist weaving (if a nondrying oil is used as the lubricant, it can reduce the bond strength of some lining adhesives). The resulting fabric may be obtained in a variety of weaves, weights, and textures, with the yarn locked or unlocked. As an inorganic fabric, it is inert and offers resistance to agents of organic decay, molds, and so forth. It provides a degree of permanence plus an even, smooth surface texture that cannot be matched by natural-fiber woven fabrics. Probably first used as a lining support in London in 1952, it initially found fairly wide use in conservation, especially to produce transparent linings in combination with hot-melt adhesives, but is rarely used today because the fabric is difficult and hazardous to handle.
glass transition temperature: The glass-to-liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle “glassy” state into a viscous or rubbery state as the temperature is increased. The glass-transition temperature (T_g) of a material characterizes the range of temperatures over which this glass transition occurs. It is always lower than the melting temperature (T_m) of the crystalline state of the material, if it has one.
glassine paper: A transparent, glazed wrapping paper.
glaze: A layer of transparent or semitransparent paint through which the light passing to the surface beneath is reflected back such that the color of the glaze modifies that of the lighter underlying color. Essentially, a glaze uses transparent pigment—one with a low refractive index, such as lake, ultramarine, copper resinate, Prussian blue. Because of the higher vehicle-pigment ratio and the often resinous content, glazes can be affected by any excessive heat and pressure during lining; sometimes they will retain the imprint of facing paper fibers or joins in the facing material.
glue paste: An adhesive for lining made from flour and animal glue, with various additions to improve plasticity and fungicides to prevent mold. Recipes vary from conservator to conservator, often reflecting wider regional variations, such as Italian pasta and Spanish gacha.
glycerin/glycerol: A syrupy hygroscopic liquid used as a plasticizer and humectant for aqueous adhesives such as animal glue.
ground: A paint-like composition, usually containing inert fillers, earth colors, white or red lead, and driers. Ground is traditionally applied with a grounding knife in several layers over isolating layers of glue size. Used for filling the open weave of a canvas and for mechanical purposes, such as bonding subsequent paint layers to the support and supplying the necessary color base, uniform texture, and degree of absorbency. Also called preparation. See also priming.
gum elemi: See elemi.
hand-lining: A general term for adhering a lining canvas to an original canvas using devices controlled by hand, such as hand irons and rollers, with or without a vacuum system.
hardboard: A sheet material made by compressing wood pulp (often spruce) to which a thermosetting resin is added. The resin accounts for the dark color of hardboard, its relative resistance to water, and its strength and stability. Available in several grades. Trade name: Masonite.
heat-seal adhesives: Heat-activated adhesives generally consisting of two or more resins. One of these has a high softening point and high molecular weight and gives cohesive strength to the adhesive. Another may have a lower softening (or melting) point and lower molecular weight. At a given elevated temperature, the low-molecular-weight resin fuses and dissolves the high-molecular-weight resin, yielding a viscous solution. With application of pressure and after cooling the solution solidifies to form the adhesive bond. Bond strength may vary according to the temperature to which the assembly is raised. Individual resins with a sufficiently low softening point can be used alone as heat-seal adhesives. Other components such as waxes may be added to increase tack or flow during bond formation and to adjust the heat-seal temperature. The adhesives are generally applied in solution. They differ in this respect and in their viscosity from hot melt adhesives such as wax-resin. Examples of heat-seal adhesives used in lining are Beva 371 and PVA formulations. See also dry mounting.
heat sources: Depending on type of adhesive and method of lining required to effect impregnation and/or adhesion, the heat source may take the form of: (1) actual contact with the painting, or lining surfaces, passing by thermal conduction from irons, spatulas, heated rubber blankets, hot tables, and similar devices into the lining materials, or (2) radiant heat, derived from infrared lamps or heaters, hot air blowers, photographic light bulbs, or electric heating elements held at a distance from the lining area. Heat may be applied locally or over the whole area; usually the former will involve heat for a shorter time while the latter produces overall greater heat and often a much lengthier process. The heat may be applied from one or both sides alternately or simultaneously, depending on technique. With overall heating methods, such as hot tables, insulating layers (such as Melinex) may be employed to cut down heat loss to the surrounding air, thus enabling optimum temperatures for the lining process to be reached more quickly and in a controllable manner.
Heiber glue: A 1:1 mixture of sturgeon glue and wheat starch paste used by Winfried Heiber.
Heiber mend: A mend to a tear in canvas that reweaves the original structure of the canvas and attaches the original threads together using a strong glue. Pioneered by Winfried Heiber. Also called thread-by-thread tear mending.
hemp: Plant producing bast fibers from the stem that has been used for centuries to make rope, sailcloth, yarns, and textiles. Rarely found in painting canvases, and then probably chiefly in the nineteenth and twentieth centuries—although this is uncertain, as it is difficult to distinguish aged hemp and flax fibers from one another.
herringbone: See complex weaves.
hessian: Strong coarse cloth of hemp or jute. Chiefly finds a use in lining on the vacuum table, where strips of hessian webbing are used as breathers to draw out the air from the picture area and toward the vacuum ports.
honey: Produced by the eight species of honeybee, which are native to Asia, Europe and Africa, but are now globally distributed. Composed of the sugars dextrose and levulose, and other compounds, with variable amounts of water. It retains moisture and hence is used as a humectant and tackifier in aqueous lining adhesives—an important component of Russian sturgeon-glue lining adhesive.
honeycomb paper: A resin-impregnated paper structure that folds out into an array of hexagonal cells, like a honeycomb. It is available in a range of cell depths and cell densities. It is used in the preparation of composite supports for marouflage, glued between “veneers” of hardboard, Sundeala, or aluminum sheet, and provides rigidity with minimal increase in weight.
hot-melt adhesives: An adhesive that is solid at room temperature, on heating melts to a mobile liquid, and resolidifies on cooling to form the adhesive bond. The advantages of hot melts (such as the well-known wax-resin lining adhesives) are their low viscosity, and hence ready flow, and good penetration during bond formation. The 100% solids factor means there is no shrinkage problem due to loss of solvent. See also thermoplastic.
hot table: Widely used for lining pictures, a hot table consists of a large, polished (usually metal) tabletop with facilities for extremely uniform heating and cooling. Early versions of the tabletop were made of slate, plate glass, marble, and the like. See also vacuum hot table.
humectant: A substance that absorbs or retains water, such as glycerol or honey. Humectants are commonly added to aqueous adhesives of the carbohydrate or protein types to plasticize the glue film and to reduce brittleness.
humidity: The amount of water vapor present in the air. Can be stated as the absolute humidity, which is the mass of water present in a cubic meter of air, but usually quoted as the relative humidity (RH): the ratio of the mass of water vapor per unit volume of the air to the mass of water vapor per unit volume of saturated air at the same temperature, which is expressed as a percentage and measured with a hygrometer.
hygroscopic: Used to describe a substance that will readily absorb moisture from the air.
impasto: A thick, often opaque area of paint, applied with a brush or palette knife, which stands up above the surface to which it has been applied. (Can also be thin, low relief, with highly defined brushstrokes.) Successive heavy linings have reduced the relief of many impastos, thus drastically altering the effect of spontaneity in the handling of the paint. Also called pastosity. See also moating.
impregnation: The complete permeation of a porous material, such as lining canvas and paint layers, by an adhesive or consolidant, often under the action of heat and pressure.
infilling: The filling, with a compound, of holes and worn areas in the back of the original canvas prior to lining or relining. If this is not done, pressure applied during lining may cause the surface of the picture to form hollows in these thinner areas. Fabric of the same weight, cut to fit, is often used in addition to various filling compounds. Infilling also refers to the filling and texturing of losses on the front of the picture prior to retouching.
infrared heater: Groups of infrared (IR) bulbs or IR elements mounted within a metal frame, often suspended above a hot table. Useful for maintaining areas of a picture at an elevated temperature during lining and blister-laying processes. Precisely calibrated hot-air sources are more often used today.
insert: Pieces of canvas (sometimes fragments of other paintings with matching texture) are set into large losses in a painting during lining. See also infilling.
interleaf: A material introduced between the original and lining canvases intended either to give greater rigidity to the lining support (and hence prevent the reappearance of plane distortions and tears after lining) or to suppress weave interference between the fabrics. Mulberry or Eltoline tissue, paper, muslin, net, fine silk, and nonwoven polyester synthetic fabrics are used.
irons: Hot irons used in lining often have a large surface area, even heat spread, and a reliable thermostat. Precisely temperature-regulated irons designed for painting conservation are commercially available. See also cooling iron and tacking iron.
isinglass: Very pure fish gelatin made from the swim bladders of certain fish, especially sturgeon. See also fish glue and sturgeon glue.
isolating layer: (1) A sheet of nonstick material used in most stages of lining to prevent the painting from sticking to surfaces in contact with it. Commonly used materials are Melinex or (Mylar), silicone-coated Melinex/Mylar, and silicone paper. (2) In inpainting, the layer of varnish or other film that is applied to isolate the inpainting from the paint surface.
Japanese tissue: Fine, strong long-fibered paper made by hand in Japan from fibers of the paper mulberry tree and other trees and plants. In lining it is used as an interleaf or facing material. It is also widely used as a backing support for works on paper.
jute: A plant producing bast fibers, suitable for matting and sackcloth, that have poor durability; jute fabric is therefore less commonly found as a support for a painting. However, its coarse texture and slubs have been preferred by some modern and contemporary artists.
key: Thin, triangular piece of wood tapped into the corner of a stretcher. The key forces the stretcher members apart, thus tightening the canvas. Also called wedge.
kraft paper: Brown wrapping paper (also available as a gummed tape). In certain lining processes kraft paper is glued to the edges of the painting and to a surrounding loom or stretcher that is several inches larger than the actual painting. The paper follows a hysteresis cycle when moistened and allowed to dry. The overall contraction on drying places the painting in tension, allowing it to be worked on and treated. It should be noted that tensioning by this means is uncontrolled. Waxed kraft paper was often used in wax-resin lining in the past, but it is no longer widely available.
lacuna: Area of loss, a cavity, where one, some, or all layers of the painting have flaked away.
latex: A milky juice from the Hevea brasiliensis tree that is used to make natural rubber.
latex rubber sheet: The flexible membrane stretched over the vacuum hot table to seal the surface was typically made of rubber in the past. It has now been largely superseded by polyvinyl chloride (PVC), polyethylene, Melinex, or Mylar sheeting.
linen: Textile made from the bast fiber of the flax plant. As with cotton canvases, when wet the undegraded linen fabric can have an increased breaking strength of up to 30%. The reverse is true for degraded canvases that have undergone the damaging effects of chemicals, heat, and light; then their wet strength will be less than their dry strength. Linen has been the most common painting support for centuries, and is still the most commonly chosen lining fabric, although synthetic fabrics or mixtures of linen and synthetic fibers are increasingly used. See also canvas.
lining: The adhering of a fabric (traditionally a fine linen canvas) or solid support to the reverse side of a painting where the support has degraded to provide insufficient structural support. The purpose may be to counteract structural weakness in the original canvas itself and/or to secure cleavage between the paint, ground, and canvas layers. Past practice also used the adhesive chosen in the lining procedure to impregnate the canvas and the ground and paint layers from the reverse. Currently, consolidation and stabilization of the ground and paint layers would be a separate process, with the lining providing support. Several techniques are used employing a range of adhesives and supports. See also hand-lining, mist-lining, contact lining, and Fieux lining.
lithographic paper: A strong, thin, absorbent, adaptable cartridge paper that is used as a facing to “prestretch” a painting prior to lining.
locked weave: (1) In lining, the lining canvas on its loom can be impregnated with adhesive. When the adhesive is set and the lining canvas is cut from the loom it maintains its stretched condition because the adhesive has locked the weave of the material. (2) A particular weave type of glass fiber fabric. (3) Describes herringbone, twill, and similar types of woven fabrics.
loom: Stout wooden or metal frame, larger than the picture to be lined, over which the lining canvas is stretched. Some designs have tensioning systems. See also working stretcher.
loose-lining: A layer of fabric, which might be also sized and painted, that is in contact with but not adhered to the reverse of a canvas or lining. It provides extra support and also buffers against environmental fluctuations of relative humidity and temperature. Original loose linings have been found on eighteenth-century paintings and the practice was common in nineteenth-century England, when commercial artists’ colormen would supply stretched canvases with two layers of primed canvas; the layer on the reverse would have the primed face exposed. Paintings prepared in this manner have proved to be exceptionally well preserved. Adding a “tooth” to the loose lining by raising the fibers can improve the support it provides. See also cami-lining.
low tack: Used to describe adhesives of low cohesive strength that should be essentially stable, with resistance to creep, providing an adhesive hold that can be peeled apart.
lumps: Foreign particles in the support fabric, the lining adhesive, or on the release layers in between. With the application of heat and pressure, these produce lumps in the painted surface owing to the enlargement of the area covering the original fault.
malleability: The ability of a material to undergo plastic deformation by compression.
marouflage: The sticking of a canvas picture to a rigid support. Originally used to describe murals on canvas attached to a wall with white lead in oil as the adhesive, but now encompassing a wide range of supports and adhesives. See also hardboard.
mastic: A pale yellow, brittle, natural resin from the mastic tree (Pistacia lentiscus) that is soluble in a number of solvents, including the aromatic hydrocarbons, but not in mineral spirits. It melts at about 95°C and has been used in facing mixtures and wax-resin lining adhesives. More widely used as a natural resin varnish, but prone to yellowing.
Melinex: See Mylar.
melting point: The temperature (for a given pressure) at which the solid phase of a substance changes to the liquid phase. Usually quoted as measured at standard atmospheric pressure: 760 mm Hg.
membrane: The flexible sheet material that lies over or on either side of the painting, thereby enabling a vacuum to be created by means of a vacuum pump. The properties desirable in a membrane material are that it should be nonporous, thin and flexible enough to accommodate textures such as impasto, and have no texture of its own. Melinex/Mylar (polyethylene terephthalate), high-density polyethylene, and polyvinyl chloride sheeting are the most common membrane materials. Coatings with release materials such as silicon can reduce unwanted adhesion.
microclimate: A set of conditions that differ from the dominant or surrounding climate. Sometimes can be manifested in differing crack patterns, due to local differences in temperature and relative humidity (RH), along the stretcher bar area of a painting. Microclimates can be actively managed by controlling RH and temperature within an enclosure, to protect from differing conditions outside the enclosure. Microclimates can be passive, relying on insulation and hygroscopic materials within the enclosure, or active, where stable conditions are produced by specialist equipment.
microcrystalline wax: Wax derived from the heavy residual lubricating oil fraction of crude oil after the removal of paraffin wax. Its microcrystalline structure gives it a plasticity not possessed by the paraffin waxes, and it is further characterized by its high melting point, viscosity, flexibility at low temperatures, and high adhesion and cohesion. Available in a range of hardnesses. Used as a component of wax-resin lining mixtures and as an additive to some synthetic resin adhesives. Trade names: Cosmolloid, Multiwax, Victory White.
microorganism: Any minute organism visible only through a microscope, such as a bacterium.
mineral spirits: A solvent in painting conservation that is a mixture of petroleum hydrocarbons obtained from fractional distillation of petroleum. Boiling range is 150°C–210°C. Several grades are available. Mineral spirits was a common solvent for wax-resin facing adhesives and is regularly used for the removal of the residues of hot-melt adhesives after lining or impregnation. Also called white spirit.
miscibility: The property whereby certain liquids will mix together in all proportions to form a homogeneous mixture, for example, alcohol and water."
mist-lining: A process that employs adhesive sprayed exclusively onto the lining canvas. As practiced by Jos van Och and at SRAL in Maastricht, it is a complete and carefully calibrated procedure using precisely measured solvent to activate the adhesive within a vacuum envelope. The principle of the method was developed by Phenix and Hedley at the Courtauld Institute in the 1970s and by Mehra independently.
moating: Any raised portion or particle of paint (e.g., impasto) or any loose particle introduced between the paint surface and the pressure source of the lining process (particularly in any face-down lining technique), can be pressed down level with the rest of the paint surface, creating a small hollow all around it, like a moat around a castle—hence the term. This is distinct from flattening impasto, which is caused by a combination of pressure and heat softening the paint.
modulus of elasticity: Measures the tensile stiffness of a solid material, quantifying the relationship between tensile stress σ and axial strain ε.
moisture barrier: A layer with water vapor impermeability. Often applied as the last stage of treatment to the back of a glue-paste relining. Also applied to the reverse and edges of paintings to protect them from changes in atmospheric humidity. A layer of card or paper, or a rigid panel attached to the back of the stretcher can act in the same way. See also backing, backboards, backing boards.
monomer: A single molecule of a chemical compound that by repeated combination with others forms a polymer.
mulberry paper: A general heading for the strong, pure, long-fibered papers made by hand from the paper mulberry tree. They are available in a variety of weights and find use as interleaves or facing materials in lining. See also Japanese tissue.
muslin: A thin, loosely woven, plain-weave cotton fabric. Used for straining impurities from adhesives and varnishes and traditionally as an interleaf. See also cheesecloth.
Mylar: Trade name for polyethylene terephthalate sheet. It is available in very thin films due to its high strength and in a range of thicker grades. It is highly resistant to solvents, light, and heat. It is used in lining as a general-purpose protective and nonstick isolating material—particularly when supplied with a silicone coating—and as a membrane in vacuum lining. It has also been used as a lining support for some synthetic adhesives. It has a low propagation tear strength and cannot therefore be tacked or stapled. Melinex is a similar product.
nap bond lining: A lining system in which impregnating the original canvas with lining adhesive is avoided. The adhesive is applied to the lining support fabric or an interleaf, not to the original canvas, providing a uniform hold at the nap surface. See also contact lining.
nap: The raised smooth-cut, short fibers on the surface of some woven textiles. Also referred to as pile.
newton: The newton (N) is a unit of force. It is defined as 1 kg⋅m/s², the force that gives a mass of 1 kilogram an acceleration of 1 meter per second per second.
nylon: Originally a trade name for a brand of a polyamide fabric. Now a generic term. Soluble nylon is also used as a term for polyamide welding powder. See also polyamide fabric.
oleoresin: (1) A natural combination of resinous substances and essential oils occurring in or exuding from plants. It is usually a soft semiliquid in which the resin is in solution in the essential oil. Four subgroups are classified: the varnish group, derived chiefly from plants of the Anarcardiaceae family; the copaiba group, sweet-smelling resins similar to the balsams; the turpentine group, from Coniferae soft resins; and the elemi group, which are soft resins containing above 10% ethereal oil. Among the oleoresins most common in pictorial painting are Venice turpentine and copaiba, both used in the older practice of picture restoration and in the compounding of some surface films. (2) Occasionally used to mean a mixture of drying oil and resin (e.g., sandarac) which, according to literary sources were used from early times as constituents of glazes, but the use of oleoresin balsams in painting media is rarely mentioned before the mid-eighteenth century. See also balsam.
oxgall: The bile obtained from the gall bladders of oxen. Ropey, mucous transparent liquid of greenish brown color and complex composition. Used as a wetting agent, for example where it is necessary to increase the penetration of an aqueous adhesive.
padded backing: A layer, most commonly polyester wadding, usually attached to a backing board, which cushions the canvas layer of a painting. Often used to reduce the flexing of unlined or large paintings during transport.
panel back stretcher: A painting stretcher that has rigid panels inserted into the stretcher bars and completely covers the reverse of the painting. The panels can be set close to the reverse of the painting to provide some structural support. Widely used in the nineteenth century, panel back stretchers significantly reduce degradation of the canvas, even when non-acid-free materials such as wood are used.
paraffin wax: Mixtures of saturated aliphatic hydrocarbons obtained in the distillation of petroleum and available in grades melting in various ranges, such as 48°C–50°C, 50°C–52°C, up to 75°C. Hardness, density, and lack of crystallinity increase with increased melting point. They are white or bluish white translucent materials, soluble in nonpolar and weakly polar solvents, and have a tendency to become brittle. They have been used in wax-resin lining adhesives, often as a minor constituent to adjust the melting point of the mixture.
pasta: The term applied to a lining adhesive widely used in Italy. The principal components are flour paste (approximately 60% by volume), animal glue (about 20%), and Venice turpentine (20%). Other additives include disinfectants, linseed flour, and honey or molasses.
pastosity: See impasto.
patching: A canvas with a tear or hole that otherwise does not need lining may be patched on the reverse side using a feathered piece of fine canvas and normal lining adhesive. However, this is not good practice as it very often distorts the canvas in the region of the patch. See also thread-by-thread tear mending.
pavimenteuse: A type of ground/canvas structure common in Italy in the seventeenth century. It was made by spreading a thick paste ground with a knife into an open weave canvas. Over time, a regular crackle system of tiny cubes in the paint surface develops, which is easily accentuated by lining pressure or may be detached by careless treatment. Also called pavementing.
peel test: The resistance to peeling apart of original and lining canvases, typically employed to assess a variety of lining adhesives. A strip of canvas of given width is attached for a definite part of its length to a fixed vertical plate. The free end is bent down through 180 degrees (in the same direction as the fixed end) and weighted until the test strip peels away from the plate.
permeability: (1) The property of a material that allows the passage of another substance. (2) Yielding passage to fluids, penetrable.
petroleum spirit: See mineral spirits.
pH: Measure of the acidity or alkalinity of a solution. Defined as pH = log10(1/ [H+]) (where H+ is the hydrogen ion concentration) and expressed on a continuous scale from 0–14: pH= 7 represents neutrality; 6–1 is increasingly acid, and 8–14 is increasingly alkaline. Can be determined by electrometric measurement or by use of colored indicators.
pigment-vehicle ratio: In artists oil paint the proportion of pigment to vehicle varies widely, between dense, low absorbent pigments (e.g., white lead requires 8% W/W vehicle) to more absorbent earth colors (e.g., raw sienna requires 35% vehicle; lakes, 55%) to the highly absorbent pigments (e.g., lamp black, 85%–100% vehicle; asphaltum, 150% vehicle). Paints with a high proportion of pigment to vehicle (usually the lighter parts of the picture) are more stable to heat and pressure than those paints with a higher vehicle content (found in the darker areas), which often show more signs of deformation (wrinkling, imprinting, flattening, cupping). See also absorption.
plasticizer: A substance added to a resin or adhesive film to increase or retain flexibility. Usually a nonvolatile or only slightly volatile liquid, which is retained in the film when the volatile solvents have evaporated.
plasticity: A material exhibits plasticity when it undergoes permanent deformation under the influence of applied stress. Substances that can be shaped or molded by heat, pressure, or both, are said to be plastic. See also elasticity.
platen: See suction plate.
polyamide fabric: A textile woven from fibers made from a group of synthetic polymers available under trade names (e.g., nylon). These fabrics have with differing properties according to spinning processes and aftertreatment, but commonly they have low moisture absorption, are quick drying, soften at 180°C, and tend to stretch, which can be partially irreversible. The latter makes these fabrics generally unsuitable as lining or patching supports. Occasionally used as a facing material.
polyamide welding powder: A resin in powder form distributed by Lascaux. It melts at approximately 100° C. Used to mend tears.
polycyclohexanone resins: Synthetic resins composed of chains of cyclohexanone and methylcyclohexanone units (or similar units with slight modifications). Average chain length is several units. The polycyclohexanones have similar visual and handling properties to the natural resins dammar and mastic but superior stability. They have thus found use as alternative resins in varnishes. They are all soluble in mineral spirits, are very brittle, and soften at 80°C–90°C. Trade names: AW2, M52. MS2A, MS2B, MS3, Ketone-N (MS2A is no longer commercially available and has been replaced with MS3. The cost of MS3 makes this resin an unlikely component of adhesives; its use is confined to varnishing paintings).
polyester fabrics: Woven textiles from fibers made from another group of synthetic polymers variously manufactured. Generally, their properties include extremely low water absorption (even when immersed), no swelling and thus quick drying, and stable to heating up to 130°C. The fibers extend less than those of polyamide fibers (nylon), and when processed on the cotton spinning system, the extensibility is about the same as cotton and linen. The light resistance is better than for polyamide fibers. Polyester fabrics are used fairly widely in textile conservation and are increasingly used as a component of lining fabrics. A common type is polyester sailcloth. Trade names: Cerex, Dacron, Hollytex, Kodel, Tergal, Terital, Terylene.
polymer: The majority of polymers are organic long-chain molecules. Combination of two or more molecules of the same compound results in formation of a new compound that has the same atoms in the same ratios but greater molecular weight. Thus, polymers are formed by the repeated joining, end to end, of single molecules called monomers. For example, the monomer ethylene gives rise to the polymer polyethylene. Polymers can be both naturally occurring (e.g., cellulose) or manufactured (e.g., synthetic resins, nylon). See also copolymer and terpolymer.
polyolefins: Purely aliphatic hydrocarbon polymers such as polyethylene and polypropylene. Fabrics woven from their fibers are finding increasing use in structural treatments, including facing, temporary supports, and storage.
polypropylene fabric: See polyolefins.
polyvinyl acetal/butyral/formal: See vinyl resins.
polyvinyl acetate: See vinyl resins.
polyvinyl alcohol: See vinyl resins.
pressure sources for lining: (1) Various sources of positive pressure have been used in lining procedures, ranging from early heated tailors’ irons (mobile sources of local pressure), the modern vacuum hot table, and other pressure systems, such as vacuum envelopes and air vortices in low-pressure systems. Other systems include weighting with flexible water containers or sandbags over the entire surface of the lining composite. Flexible synthetic sheets can be used as adaptable pressure sources over paintings. Rollers (covered with foam, nylon, rubber, felt) can also provide auxiliary mobile pressure sources. The heated spatula, though light, can bring high-pressure loads to bear on local areas. Cooling irons of between 6 and 30 pounds are commonly used in both glue- and wax-resin lining methods to maintain contact between the canvases as the lining adhesive is cooling and setting. (2) Positive pressure exerted during lining will be complemented by an equal amount of inverse pressure, the source of which usually takes the form of a flat, hard surface, such as a terrazzo floor, marble slab, composite board table, or hot table. Soft materials are often used as buffers between the painting and the pressure sources. Sandboxes, sawdust beds, and, more recently, silicone rubber molds have all been used to accommodate the relief of the painting within an overall firm support. A tensioned lining canvas during the lining process can also act as an inverse pressure source, the degree of flexibility being determined by the tautness. Other efforts to overcome the flatness characteristic of many of these inverse sources has led to the development of balanced pressure forms. See also vacuum envelope.
prestretching: (1) Where the canvas to be lined has shrunk (indicated by cupping or buckling of the paint layer), or has been distorted by a previous lining adhesive, stretching and flattening of the original canvas to accommodate the full size of the paint layers is often carried out as a separate procedure before the lining process. Methods are various, usually involving the presence of moisture and/or heat, and can be achieved by edge attachment and local retensioning over a considerable period. It is important to remove inhibiting material (old lining canvases, adhesive, etc.). (2) Preparing a linen lining canvas by moistening after it has been stretched on a loom. The increased tension as it dries removes local variations in the weave. Adjustments to the stretching are usually needed after this process.
priming: Layer following the ground layer, providing a modified color base and/or textured surface on which to paint. Today, priming and primer, meaning a preparation coating for canvas, are synonymous with ground.
putty: A material prepared for filling losses in a paint-ground film prior to inpainting. Puttying can be carried out before or after lining and can be applied from the front or the back of the painting. Putties generally consist of an inert filler (whiting, kaolin, titanium dioxide, and/or other pigments) dispersed in an adhesive to give a liquid or paste that is brushed, dropped, or pressed into losses; after setting it is modeled to match the texture of surrounding areas. Inserting putty into losses before lining reduces the risk of the original canvas being pushed to the surface in these areas during the lining process. The adhesive may be a synthetic resin solution or emulsion, wax or wax resin, or a natural glue such as gelatin, and it may contain a variety of additives (e.g., stand oil). The properties important in a filler are ease of handling and modeling, minimal shrinkage on setting, ease of removal from surrounding paint, flexibility, and retention of color with age.
rabbit-skin glue: An animal glue traditionally prepared from the skins of rabbits. However, rabbit skin is now generally used to describe the properties of the glue, including rapid solubility and lower bond strength, and these glues are made from other sources.
raking light: When an intense point-source lamp (incorporating a heat filter) is positioned with the beam at an acute angle across the surface of the painting any relief or deformation is accentuated and can be easily observed or photographed. The physical state of the painting can thus be studied before, during, and after lining.
reflectance transformation imaging (RTI): A technique that measures texture and relief of a surface from all angles, imaged on a computer, where a number of images with light sources in different positions are combined. Changes to distortions on a painting’s surface can be visualized in precise detail.
refractive index (RI): The RI of a medium is the ratio of the velocity of light in free space to that in the medium. Mathematically expressed as the sine of the angle of incidence divided by the sine of the angle of refraction, when light is traveling from a vacuum (or, as an approximation, air) into the medium. The importance of RI for paint films and lining techniques lies in the fact that the relative transparency or opacity of a paint film depends in part on the difference between the RI of the pigment and that of the suspending medium. A pigment with a high RI (such as titanium dioxide or vermilion) in oil will give an opaque film. Similarly, a pigment of comparatively low RI (such as whiting) will appear opaque in an aqueous medium and transparent in a medium of comparatively higher RI (such as oil). The increasing RI of oil with age increases the transparency of particular oil-pigment mixtures, producing the well-known phenomenon of pentimenti. In lining techniques, it is important not to saturate a low RI ground with a relining adhesive of similar RI because it would reduce the opacity of the ground and lead to a general darkening in tone.
relative humidity (RH): See humidity.
relining: The lining of a canvas painting that has been lined before. Involves removal of the old lining canvas and adhesive and mounting on a new lining canvas with new adhesive.
resins, natural: Amorphous organic compounds of complex composition that are secreted or excreted by certain plants. Generally insoluble in water and soluble in organic solvents.
resins, synthetic: Substances with resinous properties produced by chemical synthesis. This was a term originally applied to a group of synthetic substances whose properties resembled those of the natural resins but is now applied more generally to the whole range of synthetic materials produced by polymerization. They can be classified in a variety of ways, most importantly as thermosetting or thermoplastic, by chemical constitution and structure, and by degree of polymerization and extent of cross linking. They are widely used in conservation in adhesives, consolidants, surface coatings, paint media, fibers/fabrics, nonwoven fabrics, films, and rigid supports.
retreatability: See reversibility.
reversibility: A theoretical basic criterion for conservation. A process or material applied to an art object should be completely reversible or removable by means that do not endanger that object. In practice, few treatments are totally reversible, so ensuring the object can be retreated safely is the more realistic goal.
reweaving: Filling a tear or a lacuna in canvas by recreating the weave in the area of damage by realigning and rejoining broken threads or weaving a similar section to fill a loss by joining and extending threads.
rheology: The study of the deformation and flow of materials.
rigidity: The resistance of a body to twisting and bending.
roller: Heavy iron heated rollers were in use at the end of the eighteenth century for reducing surface distortions during lining. Heavy floor layers rollers have been known to be used for lining ceiling paintings until recently. Today, small hand rollers—of some fairly soft material—may be used in lining (especially on a vacuum hot table) to assist in the expulsion of air trapped between the original and lining canvases. A soft foam rubber roller can be used in glue-paste lining, or a nylon flock roller may be used to apply adhesive in the required amount for a wax-resin lining.
rosin: See colophony.
RTI: See reflectance transformation imaging (RTI).
sailcloth: A general term for a strong, plain-weave cloth used for boat sails. Sailcloth can be made from cotton, linen, jute, hemp, nylon, aramid, or polyester, but polyester is most common in painting conservation. Sailcloth has also been used as a support for paintings.
sand bed: Historically used in lining pictures with impasto. The painting is treated face down on a tray of fine sand, which, while providing a firm support, is sufficiently fluid to adopt the conformation of the paint surface.
sawdust box: Similar in principle to a sand bed, used as a bedding for lining pictures of high impasto.
scalloping: See cusping.
secant modulus: Using a stress-strain graph, one of the ways to calculate the modulus of elasticity of a material.
selvage: The continuous border formed by the weft threads returning at the edge of the warp of a woven textile. Also known as selvedge.
shear test: Used for testing shear properties of lining adhesives. Two strips of canvas of a given width are overlapped for a certain proportion of their length and joined with the adhesive. The free end of one strip is fixed, and the free end of the other is increasingly weighted until failure of the adhesive occurs. The results are expressed as shear strength and are given in the units of MPa or pounds per square inch (psi). The most common measurement obtained from a shear test is the shear strength, which is the maximum load a material can withstand in a direction parallel to the face of the material, as opposed to perpendicular to its surface. The shear strength is calculated by dividing the force required to shear the specimen by the area of the sheared edge. Creep can be measured by a similar procedure, but it is measured under constant load.
shellac: A yellowish-reddish brittle natural resin obtained from the lac insect, mainly in India. It melts between 77°C and 82°C and is soluble in alcohols.
shrinkage: Reduction in size that occurs naturally in the presence of moisture with some canvases; it is thus a major cause of blistering and cleavage of a hardened paint layer. Before lining a painting with an aqueous adhesive (and before treating bulges and distortions in the canvas with moisture) tests are made to check the response of the original canvas to water, and allowance is made for the escape of the moisture during the lining process and during any subsequent drying time that may be necessary.
shrinker: A painting that is suspected to or does demonstrate shrinkage when moisture is applied to correct distortion.
silicone paper: Paper impregnated with a silicone resin that finds general use as a release material or nonstick isolating layer where paintings are being treated with adhesives.
silk: The natural product of certain moths that is processed to produce a lustrous, highly absorbent fiber that can be readily impregnated or wetted with water. Can be heated to 110°C but decomposes at 170°C. Apparently rarely used as a paint support in Europe, but commonly employed as such for many centuries in Asia.
size: In its broadest sense, size means any material used to seal a porous surface. The term is frequently applied to gelatin or the pure forms of glue. Raw canvas is normally sized before application of the ground or priming.
spatula: Literally, a large blade, usually of flexible steel. In conservation, may refer to a small electrically heated iron, often with a variety of shaped heads, used for applying heat to a localized area on a painting or other art object (e.g., in the laying of flaking paint).
starch glues: Chemically, starch is a polymer of glucose. It has two constituents: amylose and amylopectin. For the preparation of starch glues, the starch from rice, wheat, corn, or arrowroot is commonly employed. When mixed with water and heated the starch gelatinizes and forms a viscous paste, which is a solution of amylose thickened by jelly-like amylopectin. Because adhesive films encourage mold growth and become brittle with age, humectants and fungicides are common additives. Soluble starch can be prepared from starch by treating it with acid or using various other methods. It dissolves to give a clear solution in hot water and can be used as an alternative size to gelatin.
strain: The deformation of a material from stress. It is unitless and is shown as simply a ratio of the change in length to the original length.
strainer: A rigid wooden framework over which canvas paintings may be fastened. Unlike a stretcher it has fixed corners and so cannot be expanded to tighten the canvas.
stress: Stress is a physical quantity. The term is closely associated with internal force. It is the measure of average amount of force exerted per unit area over a material.
stress-strain curve: In materials science, a stress-strain curve can be used to interpret data such as the Young’s modulus and the tensile strength of a material.
stretcher: A wooden frame over which canvas paintings are stretched. The corners are jointed but not fixed, so by driving in wedges or other mechanical means the stretcher can be expanded and the canvas tightened. Various stretchers have been devised using springs, rollers, and other devices with the aim of maintaining a constant tension on the painting. See also constant tension stretcher.
stretcher bar lining: See cami-lining.
stretcher bar marks: The appearance on the paint surface of the form of the stretcher bars as cracks or ridges that follow the inner edges of the stretcher bars or both sides of stretcher cross bars. The area between these marks and the outer edge of the canvas generally has areas of relatively uncracked or uncupped paint in a picture where the surface exhibits crackle. Indicative of the protective properties of any form of free, independent backing material behind the canvas; here the bars limit canvas movement by partially isolating the canvas from changes in the external climate and by guarding against accidental mechanical pressure or damage.
stretching: Associated with several stages of the lining process. (1) The stretching of the lining canvas onto the working stretcher prior to lining. (2) The stretching of the original picture canvas before and during lining to eliminate deformations. (3) The mounting of the lined picture on the stretcher and subsequent tapping out with wedges. See also prestretching.
strip-lining: Where a picture as a whole does not need lining, but the edges of a canvas painting are not strong enough to be tacked to a stretcher, strip-lining is carried out. Strips of canvas, a few inches wide and feathered on the inside edges, are lined onto the four edges of the picture; these strips are used to attach the painting to its stretcher.
sturgeon glue: Collagen glue made from the swim bladders of the sturgeon; known as isinglass in its purest form. Low melting point, plus high initial tack and oily spreading consistency, have led to its unique place as an adhesive in Russian lining methods, where it is used in combination with honey.
suction plate: A small plate with a perforated surface that when connected to a vacuum source can be used on the verso of a painting for a variety of uses, from humidification and flattening to consolidation. Also called a platen.
suction table: Refers to tables developed in the late 1970s and 1980s for lining that continue to be in use today. Suction is applied through perforations in the top of the table. Some suction tables can simultaneously provide heat or humidity to the painting.
support: Rigid or flexible substance on which a picture is made or painted.
supports for lining: Size, weight, evenness of weave, tensile strength, and expected endurance are all factors governing the choice of a lining support. Linen canvas has served in the past, mainly suiting glue-paste adhesives, having a typical life span of 80–100 years; it has also been used with wax-resin and synthetic-resin adhesives. Woven glass fiber fabric, offering evenness of weave, greater rigidity, and absence of creep, became common in the 1970s and 1980s, but have limited use today. Aluminum sheet or honeycomb sandwich panels have also been used as supporting materials (usually incorporating an interleaf between the original canvas and the backing panel). Woven polyester fabrics are often used and offer a limitless range of properties.
surface texture, changes in: The original surface of a painting is affected by the weave textures of the fabric; the texture layers, such as brush marks, impasto, and crackle; and structure of the paint, ground, and/or varnish. Lining procedures may cause marked changes of various types in surface texture. See also pressure sources for lining, lumps, and weave emphasis.
surfactant: Material added to a liquid that alters its surface tension and hence its spreading and wetting properties. Also called surface-active agent (SAA). See also wetting agent.
tabby: A weave type or binding system based on a unit of two warp threads and two weft threads.
tack: Tackiness or stickiness of an adhesive or varnish layer.
tacking iron: A small, lightweight iron, usually electrically heated. Often used when attaching adhesive tissue to an object. Can be used for lining by hand.
tensile strength: Ability of a material to withstand stretching forces. See also elasticity.
terpolymer: A particular type of copolymer in which the polymer molecule is made up of three specific monomers.
textile fiber: Fiber (either natural or synthetic) that can be spun, woven, or otherwise interlaced with other fibers as yarn to make fabric, as distinct from paper fibers, brush, and mat fibers.
texture loss: The creation of a flatter surface texture as a result of the lining process, for example in paintings lined face down on a rigid surface. See also impasto.
thermoplastic: Capable of being softened and made to flow by heat (and pressure). The term is commonly applied to artificial resins and plastics that are resoftened by heating. See also hot-melt adhesives.
thread-by-thread tear mending: A method of repairing tears in a canvas support, by realigning and rejoining individual threads broken by the tear, or adding fibers of similar weight if portions of threads are missing, then reweaving the torn section together. Broken threads are usually butt-jointed with a sturgeon glue/starch paste adhesive. Often tension is introduced to close the tear if it has opened (see Trekker entry), although tears are often repaired before any planar distortions are addressed. The method was developed by Winfried Heiber in the early 2000s and has been widely adopted.
time-temperature superposition: A principle used to determine temperature-dependent mechanical properties of viscoelastic materials using a reference temperature.
transfer: Removal of the support from the reverse of the paint and ground layers and subsequent mounting of these on a new support. Historically carried out when the condition of the canvas or panel has deteriorated so much that it cannot be further consolidated. In some cases, the ground is removed where it is in bad condition and a new ground is applied to the reverse of the paint film. This method was practiced widely in France from the middle of the eighteenth century, when many panel paintings were transferred to canvas, but was in use much earlier in Italy.
Trekker/Trecker: Introduced into use by Winfried Heiber, a device usually clamped onto the bars of a working stretcher with attached tensionable lines used to draw tears together before and during mending.
turpentine: A term originally applied to the balsams or oleoresins but now commonly used for the volatile liquid obtained from their distillation. The distillate is a mixture of various closely related hydrocarbons known as terpenes. When freshly distilled and pure it is a clear volatile liquid (boiling point 150°C–180°C) with characteristic odor. Widely used as a thinner or solvent for varnishes and adhesives.
twill: A weave type or binding system based on a unit of three or more warp threads and three or more weft threads. Canvases woven in the various kinds of twill (zigzag or chevron, herringbone, diamond) have frequently been used as paint supports by artists; use of twills as lining supports is rare.
twist: To obtain a continuous strand of fibers suitable for weaving, it must be twisted. The twist is the number of turns per unit of length; the product of this twisting is yarn. See also yarn.
vacuum: A pressure that is less than the surrounding atmospheric pressure. Essentially it is a difference in pressure, or differential, that can be used to do work.
vacuum envelope: The system has since been applied to the lining of canvas paintings. An envelope of plastic sheeting encloses both the painting and lining support, which are thus held together under vacuum pressure during the lining process. An external heat source and vacuum extraction points may also be incorporated. The method aims to minimize changes in surface texture by allowing the lining canvas and lining surface to conform to the reverse texture of the painting, rather than pressing it through to the paint surface. In this way lining pressure can be balanced, although the degree of tensioning of the lining canvas will affect this. The system allows full observation of both sides of the lining sandwich throughout the process, and rapid heating and cooling. First tested in 1964 in New York for the treatment of paint cleavage on panel paintings.
vacuum hot table: A hot table equipped with a membrane and vacuum pump that utilizes atmospheric pressure to hold the painting and relining canvas in close contact during the lining process. Air is withdrawn by breathers laid around the painting and through ports let into the table surface. A hot table was developed by Stephen Rees Jones at the Courtauld Institute in 1948 and the use of a vacuum to introduce pressure was added soon after.
vacuum pump: A mechanical device for withdrawing air to create a partial vacuum. Pressure is measured by a gauge calibrated in inches or millimeters of mercury. Readings should, strictly speaking, vary from 760 mm Hg (no vacuum) to 0 mm Hg (total vacuum), but they are often quoted in the reverse way as the apparent external pressure on the system.
Venice turpentine: The balsam obtained from the European larch, once widely used as a plasticizer and tackifier in lining adhesives.
vinyl resins: Thermoplastic synthetic polymers of the general formula (–CH=CH2). Four types of vinyl resins have found use in conservation as adhesives are (1) Ethylene vinyl acetate copolymers: synthetic resins in which the polymer chains contain both ethylene and vinyl acetate monomer units. They are available in a variety of grades of differing molecular weight and vinyl acetate content; these factors govern the strength and solubility properties of the resin. They are generally soluble in less polar solvents than polyvinyl acetate—in aliphatic hydrocarbons or mixtures of aliphatic and aromatic hydrocarbons—and are also available as emulsions. Grades with a vinyl acetate content greater than 25% have been found to exhibit cross linking in aging tests. They are used in solvent-applied heat-seal adhesives. (2) Polyvinyl acetate (PVA) synthetic resin is available in a range of grades of increasing degree of polymerization. Properties include stable to light; no evidence of cross linking; some tendency to swell in water; soluble in aromatic hydrocarbons, ketones, esters, and lower alcohols, but insoluble in aliphatic hydrocarbons. Viscosity of solutions, mechanical strength, and softening point (60°C– 200°C) increase with increasing degree of polymerization. PVA has been widely used as an adhesive in conservation in solvent types, heat-seal adhesives, and the well-known emulsion glues. Unlike wax-resin and other adhesives, its low refractive index means it does not appreciably stain porous substrates. (3) Polyvinyl alcohol, a water-soluble synthetic resin. It has good light stability and retains solubility after drying. Weak solutions, but not films, tend to deteriorate and lose cohesion. The degree of hydrolysis and polymerization govern ease of solution and film strength, the latter being also dependent on humidity. Polyvinyl alcohol is a possible component of aqueous lining adhesives but is more often used for paper and textiles and is a common stabilizer for PVA emulsions. (4) Polyvinyl acetals (especially polyvinyl formal and polyvinyl butyral). Thermoplastic synthetic polymers obtained from polyvinyl alcohols by partial reaction with formaldehyde, acetaldehyde, and butyraldehyde respectively. They are available in various grades depending on degree of polymerization and polyvinyl alcohol content. They have good stability to light and form tough films varying from rubbery (the butyrals) to hard (the formals). The acetals and butyrals are soluble in aromatic hydrocarbons, alcohols, and acetone. The formals have limited solubility.
viscoelastic: Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Synthetic polymers, wood, and human tissue, as well as metals at high temperature, display significant viscoelastic effects.
viscosity: (1) The frictional forces within a fluid influencing its rate of flow. (2) The resistance to relative motion inside a liquid or gas. The coefficient of viscosity is a measure of the difference in flow between two adjacent layers of moving fluid.
warp: Parallel threads, stretched lengthwise on loom before a fabric is woven. The weft threads cross through the sheet of warp threads. The selvage runs parallel to the warp threads. See also weave.
warping, of canvas or stretcher: Crooked state produced by uneven expansion or contraction.
waxes: A term often applied to organic, solid, nongreasy, hydrophobic substances that melt at fairly low temperatures. The waxes, such as beeswax, are composed chiefly of esters of fatty acids with higher alcohols, together with free alcohols, acids, and hydrocarbons. The waxes of mineral origin consist of mainly higher paraffin hydrocarbons and include paraffin waxes, microcrystalline waxes, and ceresin. The perceived moisture impermeability and stability of waxes led to their widespread use in the treatment of paintings.
wax-resin adhesives: Hot-melt adhesives once widely used in lining, most commonly composed of beeswax with up to 40% natural or synthetic resin and other additives such as paraffin wax, gum elemi, and/or Venice turpentine.
wax-resin lining: A lining procedure using wax-resin adhesive. When heated, wax-resin flows readily and penetrates porous materials. It has some resistance to moisture. The technique was commonly used in Holland from the mid-nineteenth century and linings from this time survive in good condition. It was widely used up to the 1970s when adhesives were developed to address the serious drawbacks of the procedure, such as its overall penetration when heated into the canvas and paint structure, its capacity to darken porous substrates by saturation and to make some paint films more soluble to solvents.
weave: (1) To make a fabric by interlacing threads or yarn. (2) The binding system of crossing, interlaced warp and weft threads, characterized by overall design and type of weaving unit: tabby, twill, satin, or variations of these three. See also warp and weft.
weave emphasis: Accentuation of the weave texture of a fabric. Usually a result of lining under vacuum pressure, caused by the exertion of one-sided pressure against the flat, rigid surface of the lining table. The effect can occur in both lining and painting canvases and may combine to a greater or lesser extent. The paint suffers vertical distortion, adopting the accentuated texture of the underlying fabrics.
weave interference patterns: Wave patterns caused by the superimposition of the weave of the original canvas and the new weave of the lining canvas. These effects may be reduced or eliminated by placing a suitable interleaf or surfacing veil between the canvases, by using canvases of dissimilar weave, or reducing the pressure used in lining.
wedge: See key.
weft: The crossing threads that are woven into a sheet of warp threads to form a continuous fabric. The compactness of the fabric will be determined by the number of warp and weft threads per measured unit (thread count), by the fineness of the yarns, and the weave type. See also warp.
wet strength tissue: A thin, nonwoven plant fiber tissue that retains cohesive strength when wet. Often used in facing applications.
wetting agent: A particular type of surfactant that increases spreading of a liquid (usually water) in contact with a solid. Detergents are an example, and act by lowering the surface tension of the water.
whipstitch: A sewing stitch that joins two pieces of fabric by spiraling around both pieces’ edges.
white spirit: See mineral spirits.
working stretcher: A stretcher or strainer used during the conservation treatment phase for keeping the canvas under tension. Sometimes used during lining, where the lining canvas may also be attached to a working stretcher.
wrinkling: A pattern of small creases on the paint surface on a macro scale can be caused by the use of too much heat during lining. It may occur locally or totally.
xylene: Dimethylbenzene, an aromatic hydrocarbon with solvent properties.
yarn: A continuous strand of textile fibers made by spinning or twisting or otherwise binding. It is characterized by twist, direction of twist, and fineness. The fineness is measured in terms of mass per unit length (the unit is denier) and is the most common system today for measuring all kinds of yarns, filaments, and fibers.
yield point: The point on a stress-strain curve when a material loses elasticity and becomes plastic.
Young’s modulus: Quantifies the relationship between tensile stress σ (force per unit area) and axial strain ε (proportional deformation) in the linear elastic region of a material.

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