By Manfred Koller

	The 1754 portrait of Jan van Dijk, painted by Jan ten Compe. Between 1746 and 1766, the City of Amsterdam made numerous payments to van Dijk for the cleaning and restoration of paintings owned by the municipality. Photo: Courtesy the Amsterdams Historisch Museum.

The cleaning of works of art and historic monuments generally results in substantial physical changes to the surface of the objects. From the subjective standpoint of the viewer, there are visual changes as well. Because visual sensations and reactions depend on the viewer's consciousness, experience, and knowledge of art and history, discussions regarding cleaning have to respect differences in cultural background and visual education.

While cleaning can be a technical necessity for proper conservation, it usually is done for aesthetic or other reasons. Cleaning as a technical necessity has been nearly overlooked in the past; cleaning for aesthetic reasons has been under discussion since early times. In many instances, as a consequence of cleaning, other issues—such as how to present the areas of loss or whether or not to apply new protective and unifying coatings on the surface—have to be solved. In the following review of cleaning and conservation, my focus is primarily on painting (representing an indoor environment) and on monuments (representing an outdoor one).

The Evolving Idea of Cleaning

The literal significance of clean is free of dirt, stains, or anything that dulls luster or transparency. But since ancient times the condition of cleanliness has also been understood as a symbol of purity and integrity. Cleaning served as a purgative for religious purposes—for example, as a component of the rules for the dressing and the food of priests or the immaculate presentation of venerated statues. Even in profane life, the idea of cleanliness was important. The buildings and places of Greek and Roman towns followed certain guidelines for public order. In medieval central Italy, new communities set up rules to maintain the visual harmony of their buildings in order to assure positive public representation of townships. This concept continued until the 20th century with public control of civic buildings. Often cleaning has been connected with the celebration of anniversaries, done to show physical and visual rebirth—in Latin called renovatio or restitutio. Between 1625 and 1775, churches in Rome were whitewashed every 25 years to celebrate "holy years."

With the development of science and of philosophical concepts of rationalism in the Age of Enlightenment, a basic reformation in economics, society, and the arts occurred. When Isaac Newton identified the spectral composition of light around 1670, the nature of pure color became evident for the first time. It cannot be accidental that shortly thereafter, following debates in the academy of arts in Paris, the priority of color was officially emphasized. Similarly, in Venice, the dark tonality of paintings (from the so-called tenebrosi painters) changed to a bright and clear palette. The theory and the practice of art have always influenced those of restoration, as has the general development of society and mind. (For example, after the American wars for independence and the French Revolution, the ideals of liberation and freedom became dominant; in the following period of romanticism, cleaning was given even a moral value as liberation from all earlier alterations—a recovery of the "original" nature of material and the believed authentic creation of the artist.)

The first definitions of cleaning and patina related to works of art were noted by Tuscan artist Filippo Baldinucci in the latter part of the 17th century. In his Italian vocabulary, to clean (pulire) means not only to take away dirt and stains but also to polish—mainly marble and metal. He called patina (patena) "some universal darkness which time made to appear on pictures and that sometimes favors them." This remark reflects the view already held by the antiquarians and connoisseurs of the time. But patina and other darkness were appreciated in other respects. It was forbidden to clean certain venerated religious statues or paintings such as icons, and only tinted varnishes were periodically applied; over time, these became nearly black, a condition that was desirable for its mystic appearance (an example is The Black Madonna of Czestochowa, Poland).

Another antiquarian idea against cleaning during the 18th century was the appreciation of decay and patina as testimony of genuine origin and true age. Pieces in this kind of condition brought a good price in the art trade. William Hogarth offered a critique of this esteem for age in his famous and ingenious 1761 engraving Time Smoking a Picture. The inscription below the engraving connects it to the paradox of a contemporary intellectual controversy: "As Statues moulder into Worth." From the late 18th century and into the 19th century, the application of colored varnishes and dark surfaces became fashionable as a way to evoke sentimental feelings of mystery and ideal harmony. This corresponded to the contemporary philosophy of aesthetic idealism.

	St. Peter's Basilica in the Vatican after being cleaned in honor of the Jubilee Year 2000. In the 17th and 18th centuries, Roman churches were washed every 25 years in honor of "holy years." Photo: Marta de la Torre.

Technical problems in cleaning pictures were discussed in many painters' books before and after 1800, following the growing professional specialization of "restorer." This specialization was guided by leading members of the art academies. They oversaw the practice of restoration of public patrimony and also prevented radical cleaning of outdoor sculptures—for example, the Trinity Column monument in Vienna in 1776. New understandings of architectural surfaces were revealed in the debates about polychromy in antiquity. Thprotagonists—architects Leo von Klenze and Gottfried Semper in Germany and Jacques Ignace Hittorff in France—approved of the presence of color, as opposed to the neoclassic ideal of the pure material devoid of color.

In the middle of the 19th century, with mainly medieval architecture in mind, John Ruskin was the first to condemn all attempts at uncovering and radical cleaning as destruction of what should have been preserved. "The whole finish of the work was in the half inch that is gone," he wrote. "Take proper care of your monuments, and you will not need to restore them." This admirer of the "stones of Venice" was closely followed by Camillo Boito, who, on behalf of the cathedral at Murano, fought "against the cleaning, the washing, and the renovation which destroys stains and colors produced by the great Time."

By 1900, the call for "conservation instead of restoration" had grown strong in many European countries. In 1903, Austrian art historian Alois Riegl published his fundamental thoughts regarding the values of the past (age, history, and memory) and the present (use, artistry, and novelty). Behind his "value of age" was his belief that all works are subject to decay as part of their history. Their present appearance, he believed, should not deny or hide their fate.

Riegl's values are based on a historical and humanistic approach but also include the practical functions of any intervention. The opposite approach relied mainly on the results of scientific examination of materials and techniques. A scientific role in conservation interventions began to increase in the first part of the 19th century, though as early as 1795 in Paris, artist and dealer Jean-Baptiste-Pierre le Brun invited chemists to assist in the repair of a gallery in the national museum. During the 20th century, scientific research on the technology of art was firmly established. The first activity in the Anglo-American countries focused on the 1947 exhibition of cleaned paintings in the National Gallery in London, which was followed by very fertile discussions of the "cleaning controversy."

At present—with the intense cooperation between conservators, art historians, and scientists—a balanced and complementary understanding of the problems of cleaning finally seems to have been reached. This understanding generally relies on common agreement of the historical uniqueness of every artistic or cultural relic as an authentic document in all its individual aspects—an agreement that has been codified in the various charters for the restoration of art, architecture, and archaeology since 1931.

A Brief History of Cleaning

In antiquity, the application of lye (followed by polishing) was reportedly used for washing statues. The Romans valued and retained metallic patinas, mainly on ancient Greek works in bronze. For facades and interior rooms, since medieval times, periodic cleaning mostly meant new paints in different colors to give a new interpretation. Similarly, repainting and alteration were performed for many polychrome sculptures and mural and easel paintings. This making clean and looking new with repaintings—and sometimes gilding—for religious sculptures or church furnishings lasted until the 19th century. Regarding the general idea that cleaning should help make things look fresh and new, we should recall the double meaning of the Italian pulire as cleaning and polishing, as defined by Baldinucci. Old sourcebooks indicate an early practical knowledge about cleaning agents and their connection with revarnishing.

	William Hogarth's 1761 engraving Time Smoking a Picture, the artist's commentary on the monetary value many in his time placed on items that showed evidence of age and decay. Photo: Courtesy the Huntington Library.

Until the late 19th century, restoration was usually performed by artists, often as part of their function as keepers of collections. Only slowly did restoration become a special profession of art—a development that emerged first with paintings. The cleaning of a picture apparently was judged to be a task of art. Even Baroque painter-restorers, however, were aware of the need to document the fact that restoration had been done. For example, in his 1702-3 restoration of the wall paintings by Raphael in the Vatican, Carlo Maratta, head of the Academy in Rome, left an uncleaned area down in the School of Athens. The restorer in habit of an artist can be seen in Jan ten Compe's 1754 portrait of Jan van Dijk (the painting restorer for the City of Amsterdam) cleaning a landscape painting. Van Dijk, a cotton swab in his left hand, sits in front of his easel, dishes and bottles for liquids on a small board to the side. Yellowed varnish has been cleaned from the upper right part of the landscape painting.

Just four years later, Robert Dossie provided detailed information about cleaning practice for painted surfaces in his Handmaid to the Arts. He noted that "the art of cleaning pictures and paintings is of great consequence in preserving valuable works of that kind, but has been very little understood even by those who profess to practice it." Dossie criticized situations in which no thought was given to the different circumstances and to the effect of various solvents. He emphasized the need to retain the varnish when its removal was not necessary or when cleaning an oil painting would be a risk, while at the same time he advised "the taking off any foulness . . . by dissolving . . . the matter that constitutes it." He systematically listed a range of cleaning mediums from low to high strength, including water, olive oil, butter, wood or pearl shell ash, soap, spirit of wine, oil of turpentine, and essence of lemons.

	The section on the cleaning of pictures and paintings in Robert Dossie's mid-18th-century book Handmaid to the Arts. Dossie offered specific cleaning techniques for painted surfaces. Photo: Courtesy Special Collections, Getty Research Library.

Dossie's approach was copied by many until the 19th century, and such practice continued even after 1900. The old idea of refreshing surfaces was given new scientific support by chemist Max von Pettenkofer with the use of solvent vapors for reforming the surfaces of degraded varnishes—a process that was patented in Munich in the mid-19th century. As a way to avoid the darkening of varnishes based on oil and/or mastic—and for ease of reversibility—Friedrich Lucanus introduced dammar resin into restoration practice in 1829. He also advised restorers to note, on the back of every picture that they restored, the materials they had used in their work.

In the field of mural painting, many now-famous pictures (including The Last Supper by Leonardo da Vinci) had been whitewashed years after their creation. Removing these superimposed layers became fashionable during the second half of the 19th century and was done mainly mechanically by scraping. Works on stone were treated even more severely. Crusts of deposits—together with the corroded original surface (including original tool marks and colorings)—were reduced by chisel and hammer, as was done by stonemasons for most medieval cathedrals in Europe after about 1850. In northern and western Europe in the later part of the 19th century, cleaning with hydrochloric acid and impregnation with sodium silicate caused other types of long-term damage to both painted walls and stone works. By then, sandblasting had come into use for cleaning works of stone and stucco.

The introduction of new and more efficient techniques for cleaning was mainly the fruit of closer collaboration between conservators and scientists after 1945. This development was supported by the establishment of interdisciplinary centers for conservation and research—the Istituto Centrale per il Restauro in Rome, the Doerner-Institut in Munich, and the Institut Royal du Patrimoine Artistique in Brussels, among them. With the start of professional conferences—for example, those organized since 1961 by the International Institute for the Conservation of Historic and Artistic Works, and since 1968 by the International Council of Museums Committee for Conservation—the international debate on all aspects of cleaning, from theory to practice, intensified and continues today.

Since the 1980s, totally new perspectives have been introduced into conservation, particularly in the treatment of surfaces of works of art and monuments. On the diagnostic level, the understanding and definition of the characteristics and the condition of surfaces have deepened, and new instruments, such as microtools and lasers, have offered ways of mechanical/physical cleaning previously not possible. On the chemical level, a wide range of newly tested products—including solvents, soaps, and enzymes—have come onto the market. But even more important has been the improvement in controlling solvent action through the use of poultices (with fillers) and pastes or gels for precise control of the area, penetration, and time.

In the field of stone, plaster, and wall paintings, many surfaces currently show severe damage from chemical transformation caused by external deposits or internal transport of acidic or basic salts. Readily soluble salts can be extracted, but to deal with some types of damage processes, several methods for chemical passivity have been developed over the last 20 years for specific cleaning needs. Similar to the approach of revarnishing painted surfaces after cleaning is the concept of "sacrificial layers"—a reversible protective coating and a buffer against weathering. This technique has proven successful for unpainted plaster and sandstone. The possibilities for applying new methods have substantially changed not only the quality but also the delicacy and accountability of conservators' choices of cleaning interventions on the inorganic surfaces of monuments and on organic substrates, such as paintings, paper, and textiles.

An Irreversible Intervention

	Detail of a half-cleaned 17th-century marble sculpture on the exterior of Salzburg Cathedral. Photo: Manfred Koller.

The surface of a work of art must be taken as an archive of its own history, from its creation to the present. In many cases, the real status of a piece, in terms of its origin and history, is little—if at all—known. This fact makes all the more valuable every artwork still left uncleaned. Its untouched integrity offers two opportunities—a sensitive one, for the evocation of hidden artistic values, and another one, for research regarding technology and history of the piece.

These two opportunities are often counter to each other. Moreover, if all later additions are removed in an attempt to return a work of art to its "original" status, inevitably the "archive" of time and history is destroyed. The "original" surface in reality no longer exists, having suffered several transformations through time. The idea of recovering any "true" original is therefore an unrealistic one.

Cleaning has implications both for conservation and for restoration. As it is an irreversible intervention, every decision and operation is one of major consequences. The methodology for undertaking this intervention was clearly put forth by historian and critic Cesare Brandi, who laid out the criteria for examination of a work of art: material and technique, history, and aesthetics. Together they form what Brandi calls "the potential unity of the work of art," and they must be considered for cleaning, as well as for retouching.

	A partially cleaned portion of an early 18th-century vault painting by Giovanni Antonio Pellegrini in the Salesianerinnenkirche in Vienna. Photo: Manfred Koller.

Paul Philippot has described the double reality of any object of art—that it contains both material/technical, and historical/ aesthetic aspects. The task for the conservator is to maintain the proper balance between these two. "From a critical point of view," he writes, "cleaning then becomes the search for an achievable equilibrium that will be most faithful to the original unity." When that equilibrium is achieved, the result could be seen less fatalistically than art historian Max J. Friedländer perceived the process nearly a half century ago: "The job of the restorer is a most thankless one. In the best case, no one is aware of him. . . . His mastery remains invisible, but his failing becomes evident. . . . Restoration is nevertheless a necessary bad thing."

Manfred Koller is the head of the restoration department of the Austrian Federal Office for Monuments (Bundesdenkmalamt), a lecturer at the Universities of Art and Science in Vienna, and coeditor of Restauratorenblätter, a periodical of the Austrian Group of the International Institute for Conservation of Historic and Artistic Works (IIC).