modernmicroscopy : columns : true colors

True Colors
The Microscope in Art Conservation and Authentication Studies
by  Joe G. Barabe, McCrone Associates, Westmont, IL

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Part 2

Protocols for Painting Analysis


Our analyses are directed to determining the earliest possible date of creation of the painting, based on our ability to identify materials constituting the work and our knowledge about their dates of discovery or invention.


The opaque white pigments provide a clear example of the workings of the protocol. If a painting contains lead white, a pigment in use from Roman times and even before, it could be Roman, Medieval, Renaissance, Impressionist, Modern or Contemporary – it is still manufactured as an artist’s pigment, although its use has declined considerably in recent years due to the pigment’s toxicity and the availability of suitable non-toxic substitutes. If the paint contains zinc white, it was created some time after 1825, when the pigment was first introduced, and if the paint includes titanium white, we know that the painting was made sometime after about 1919 or so. In actuality, the titanium whites have a complex history and provide us with about a half dozen dates ranging from 1916 to 1943, depending on purity and the crystal form of the pigment.


The Analytical Process


The analysis of a painting progresses in a number of discreet stages:

1.      Examination. All studies begin with a careful examination of the painting using a variety of light sources, from white light to ultraviolet, both with and without supplementary magnification. A general assessment is made as to the condition, and characteristics such as size, substrate, conservation history and so forth. If a signature is present, careful attention is given to the surrounding region, looking for signs of alteration or manipulation. Conservation history is particularly important, as a characterization of a 19th century restoration on a 16th century painting could be sadly misleading.


2.      Sampling. Small samples of every important color (ignoring obvious mixes) are taken using an extremely fine-pointed tungsten needle; the cosmetic result after such sampling is excellent (Figure 4).

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Paint microscopist Carol Injerd sampling a painting.


3.      Polarized light microscopy (PLM). In spite of a world-class array of analytical instrumentation, PLM remains our first and most important analytical technique. The analyst can see the individual particles through the microscope, even in complex mixtures, whereas bulk analysis can completely miss them. Even a little titanium white pigment, scattered throughout a work of art, points to post 1920s creation. The pigment tends to agglomerate, which is handy for analysis confirmation.


4.      Elemental analysis. We routinely run all samples through the scanning electron microscope (SEM) for energy dispersive x-ray spectrometry (EDS). This technique provides an elemental profile of each paint sample and is the perfect complement to PLM. It can confirm or deny the presence of certain pigments, and the printed spectra make great, easy-to-interpret figures for the report (Figure 5).

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EDS spectrum from a paint sample that included titanium dioxide and cadmum yellow.


5.      Infrared spectroscopy (IR) is used routinely for analysis of the medium. Anna Teetsov, our prize-winning specimen-preparation microscopist, has developed a number of methods to extract the different components from a paint mixture for individualized analysis. IR provides information based on the molecular vibrations of the molecule, and is the most important tool in identifying organic materials.


6.      Raman spectroscopy (Raman), another technique based on molecular vibrations, complements IR very nicely. Materials unidentifiable with IR can often be characterized with Raman, and vice versa. It also complements both PLM and EDS as well: a Raman spectrum can be obtained from a single particle on a PLM microscope slide, in mounting medium and under a cover slip. Also, it can further elucidate the nature of, say, titanium white by showing the crystal form of the material, anatase or rutile, each of which has different dates of availability in the 20th century (Figures 6A and 6B).

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Raman spectrum of anatase titanium dioxide.
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Raman spectrum of rutile titanium dioxide.


Coming Attractions


Future columns will discuss in greater detail the analytical tools and techniques used in the study of paintings and other kinds of cultural artifacts.

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