Methodologies for Instrumental & Analytical Characterization

This component encompasses both qualitative and quantitative analysis of photographs and photographic material in order to facilitate identification of photographic processes, and to develop a quantitative methodology for analysis of photographic material. The analytical techniques currently used in this research are:

• X-ray fluorescence spectrometry (XRF)
• Fourier-transform infrared spectrometry (FTIR)
• enzyme-linked immunosorbent assay (ELISA)
• inductively coupled plasmas mass spectrometry (ICP–MS)
• environmental scanning electron microscopy (ESEM)
• neutron activation analysis (NAA)

The project team is also working on the development of a series of other scientific methodologies that can be used to verify or cross-reference findings from the non-destructive XRF and FTIR analytical methodologies. They are testing all prospective and emerging analytical techniques that might benefit and strengthen their research.

Optical microscopic methods of identification of photographic processes—based on microscopic characteristics of studied photographs—do not conclusively identify which photographic process or process variant was used to create the photograph under study. For example, a palladiotype whose tonality was modified using mercury chloride might have the same tonality and microscopic signatures as a platinotype. In general, many photographic processes and process variants differ in the main image-forming element or elemental composition imparted on the photograph during processing or postprocessing treatment. Only chemical analysis of the image-forming metals can provide the information needed to complete the identification of the photographic process.

To facilitate identification of photographic processes, the project team is engaged in both qualitative and quantitative analysis of photographic material utilizing XRF spectrometry—a nondestructive technique used to analyze many art objects, including photographs.

Work undertaken

• Optimization and standardization of experimental conditions for development of a quantitative methodology for XRF analysis of photographs
• Preparation and testing of a series of well-defined quantitative thin film standards
• Studies of detection limits of various metals, thin film criteria for elements in metals expected to be found in photographic material, and concentration ranges of image-forming elements in current and historical photographs, using both calibrated vacuum-deposited thin metal standards and metal compound loaded filter standards
• Development of an analytical methodology for a study of the baryta layer in both historical and modern photographic paper (preliminary results indicate a great variety in both qualitative and quantitative composition of the baryta layer of photographic papers from different manufacturers, and changes of baryta layer composition over time)
• Preparation of a case study using the work of photographer Henri Cartier-Bresson to determine the applicability of quantitative baryta layer analysis in authenticating and provenancing black-and-white, fiber-based photographs
• Recording of qualitative signatures of all major monochrome photographic processes
• Analysis of photographs to identify all major, minor, and in some cases trace elements typical of various photographic processes and some important process variants
• Determination of concentration ranges of image-forming elements in a series of test photographs from the Conservation Institute’s photographic study collection
• Recording of analytical signatures of photographic processes included in the collection
• Further analytical work on the photographic paper database; inclusion of additional chemical and physical signatures of photographic papers; and development of the chemometrics methodology for dealing with multidimensional data sets

Fourier-transform infrared spectroscopy (FTIR) is an important analytical tool used in the examination of historical material. The technique produces a spectrum that provides intrinsic details about bonding features between atoms or characteristic functional groups in a molecule. It also provides information regarding chemical changes due to chemical treatment or aging, based on appearance of a new band, band shift, or intensity change of individual bands. One of the important advantages of using this technique is the ability to perform non-destructive analysis on historical objects.

FTIR spectrometry complements XRF analysis of photographic material. FTIR analysis provides information on organic materials in photographic support, the binder in the image layer, and coatings and varnishes applied to processed photographs, while XRF analysis provides information about inorganic components of photographic images.

Work undertaken

• Development of a methodology involving both reflection analysis and ATR analysis of photographic images
• Analysis of photographic images from the Institute’s reference collection for variations of the FTIR spectral signatures of photographs and photographic coatings, as a result of natural aging (the collection contains a large number of organic resins, balsams, gums, and protein-based materials as well as pigments and organic dyes that have found a broad range of applications in production, protection, and mounting of photographic images)
• Development of a motorized linear movement stage mounted on a hydraulic table to facilitate the nondestructive analysis of large and matted photographs that cannot be accommodated on the FTIR microscope stage
• Recording ATR–FTIR analytical signatures of various photographic processes included in the Institute's reference collection of photographic material
• Artificial aging of a selection of organic precursors of organic coatings and varnishes used to treat photographs and photographic negatives
• Preparation of a series of uniform thin films of organic photographic binders, coatings, and varnishes using the spin-coating technology, and study of detection limits (ug/cm²) for analysis of single-component organic materials
• Qualitative and quantitative study of multicomponent organic coatings and varnishes
• Analysis of binary-layer organic coatings using angle-resolved ATR-FTIR spectrometry
• Analysis of photographic coatings and varnishes using photoacoustic FTIR spectrometry

Enzyme-linked immunosorbent assay (ELISA) is an analytical technique capable of identifying and quantifying specific molecules (proteins, antibodies, hormones, etc.). The technique requires the removal of a small amount of material, but due to its high sensitivity (detection limits down to the nanogram-per-milliliter level in most cases) this can often be done without leaving any visual evidence of sampling.

The technique relies on using antibodies, specific to the molecule of interest, which bind to the molecule and induce a color change in the solution. The color of the solution is then measured at a particular wavelength to identify the presence or absence of the molecule. The amount of the molecule present in the sample can be determined by analyzing a series of standards of the molecule’s known concentrations. The ELISA technique relies on the availability of antibodies specific to the molecule in question, thereby limiting the range of materials that can currently be identified.

Work undertaken

• Detection and identification of gum arabic in photographs produced using the gum dichromate process
• Detection and identification of albumen in photographs produced using the albumen process
• Development of a methodology for the virtually noninvasive sampling of photographs and photographic materials for ELISA analysis
• Preparation and analysis of a series of quantitative standards for the determination of detection limits and assessment of quantitative analysis feasibility
• Testing of additional antibodies for the identification of organic components of photographic processes and materials

Identification of the process used to create a photographic image can often be accomplished by XFR analysis, but determining if a variant of a particular process was used can be more difficult—these variants are often very similar to one another chemically or to the original process. Identifying these variants involves looking into the material's trace components for unique chemical markers.

Inductively coupled plasma mass spectrometry (ICP–MS) is an analytical technique capable of giving elemental information about minor inorganic components in the material being analyzed. The technique is capable of identifying elements to parts-per-billion and -trillion levels, making it ideally suited for identifying many of the minor elements that may be part of a sample. The technique uses a high-temperature (6000°C) argon plasma to break down the molecules present in a sample into their constituent chemical ions. These ions are then transported through a vacuum into a quadrupole mass analyzer. The mass analyzer separates the ions based on their mass-to-charge ratio (m/z) by varying the voltage and radio frequency of the instrument to select for different ions. The instrument then generates a spectrum showing the amount of a given material at a certain mass-to-charge ratio.

One drawback of ICP-MS analysis is the need to remove a small sample of material from the object being analyzed, which will ultimately be destroyed. This is partially offset by the fact that this highly sensitive technique requires only a tiny amount of material.

ICP-MS complements XRF analysis by providing information on trace elements in photographic materials. When coupled with XRF analysis, the two techniques allow for the identification of most of the inorganic elements present in photographic materials—whether they are major or minor components.

Work undertaken

• Quantitative analysis of silver, platinum, and palladium step tablets
• Qualitative screening of a range of photographic papers along with semiquantitative and quantitative analysis of the papers
• Analysis of trace elements in barite minerals from various mines throughout the world
• Identification of a series of chemical markers that may be used to help identify various photographic papers
• Determination of a methodology for placing photographic materials containing large amounts of insoluble minerals into a solution for ICP-MS analysis
• Determination of trace and ultra-trace elements in various photographic materials that may aid in the identification of various types of photographic papers

The majority of analytical work has focused on identifying what major, minor, and trace elements, and in what quantities, are present in various photographic materials. Most analytical techniques can provide information on the chemical makeup of a sample, but they offer little information on the distribution of each element in the material. Understanding the chemical structure of the material is vital to the interpretation of analytical results obtained from various types of analysis.

Environmental scanning electron microscopy (ESEM) allows the viewing of small features of an object at a resolution that is much greater than that of a conventional optical microscope. To accomplish this, the ESEM uses an electron beam to create an image on a computer screen. The beam bombards the sample and causes secondary electrons to be ejected from the material. It is these secondary electrons, along with backscattered electrons, that are collected by a detector and used to generate an image. By utilizing electrons instead of conventional light, the ESEM achieves a much higher resolution, which allows the user to see much smaller details than would be possible with an optical microscope.

The ESEM can also be used to help determine the elements present in the material and their location in the sample. In order to do this, an energy-dispersive X-ray (EDX) analyzer is attached. The EDX can be used to detect the X-rays coming from the sample and to identify the elements present based on the X-ray energies. Using the EDX in conjunction with the ESEM yields information on the structure of the photographic material, its chemical makeup, and information about the distribution of the elements in the sample.

Work undertaken

• Determination of the placement of various elements through the analysis of a range of black-and-white fiber-based photographic papers
• Analysis of a series of gold-toned silver gelatin prints to identify the location of the gold and silver imaging metals
• Determination of the location of a range of elements in various photographic materials in order to better understand the materials' structure and their possible effects on various analytical techniques
• Determination of the location of possible chemical markers that can be used as aids to help identify different photographic papers

When performing analysis of materials, it is always preferable that they be in their original form without any additional chemical processing. Analysis of photographic materials, because of their complex structure, often involves either physical or chemical manipulation of the material, or both. Certain analytical techniques such as ICP-MS and ESEM are impossible to perform without altering the material. There are however other techniques such as neutron activation analysis that allow for analysis of photographic materials without alteration.

Neutron activation analysis (NAA) is a technique capable of providing qualitative and quantitative information on a broad range of elements. Its accuracy and reliability make it one of the preferred techniques for developing new procedures or standard reference materials. NAA requires no sample preparation—the material to be analyzed can be placed into the sample chamber as is.

NAA bombards a sample with a source of neutrons to generate the activated form of the material, which undergoes radioactive decay with the emission of a high-energy gamma ray(s). These gamma rays can be detected and sorted, based on their energies, using a gamma ray detector, multichannel analyzer, and computer. The resulting spectrum shows the distribution of gamma ray energies versus the number of gamma rays detected. The gamma ray energies give information about the particular element from which they emanate. While NAA analysis is a nondestructive technique, it does require the removal of a small amount of material for placement in the sample chamber. This material can be recovered after it has been irradiated.

NAA requires access to a neutron source, preferably a nuclear reactor with a high neutron flux, which limits its broad use. The project as taken advantage of the NAA technique to generate reliable quantitative standards relating to photographic materials analysis that can be used to cross-calibrate other analytical instruments and to confirm the presence of certain trace and ultra-trace-level elements that have been identified using one of the other analytical techniques.

Work undertaken

• Development of quantitative barium and strontium paper standards that can be used to cross-calibrate other analytical instruments
• Development of quantitative titanium and calcium paper standards to be used for cross-calibration of other analytical instruments
• Confirmation of the presence of certain trace and ultra-trace elements present in photographic materials
• Identification of chemical markers that may be used to help identify various photographic processes or materials