For a microscopist, some of the most interesting subjects of inquiry are paint pigments. Their identification is often a crucial factor in establishing when a painting was made, which can shed light on its attribution or authenticity, or the nature of an architectural coating, which may affect treatment and restoration strategies. A third area, automobile paints, is a specialized field of forensic science and will not be discussed in this column at this time.

In order to successfully identify pigments from a painting, object or architectural site, the microscopist needs reference samples that adequately cover the range of possibilities, reference texts, the proper instrumentation, and, if possible, an experienced mentor. Here is a list of a few resources useful to learning and practicing pigment microscopy:

Reference Samples: Pigments

Comparison of the questioned material with known reference samples is absolutely critical, as microscopy is a highly visual analytical technique. Some of my favorites are the following:

Kremer Pigments ( www.kremer-pigmente.com ) is one of my favorite sources. They sell dry pigments without binding media (such as gum for watercolor, oil, casein, vinyl, acrylic, and so forth), which make them ideal for reference material. They sell them in bulk, which is good if you are sharing the material or need extra for other instrumental references. They also sell paint swatch samplers of their pigments, which is the easiest way to get a wide variety of pigments.  A full set costs about US$130, and you can purchase individual sets of selected colors for about US$10 each. The most important set is the chart of Historical Colors, 60 samples for US$18 (Figure 1). Wonderful! This is the best way to acquire a comprehensive collection for the best possible price. They also sell a full collection of bulk samples of the important historical pigments in an attractive case – very nice!

click image to enlarge (182K) Kremer Color Chart 001: Historical Colors Figure 1

Making reference microscope slides from the sample swatches is simple:

1. Scratch the surface of the swatch with a needle, covering a moderately broad area, to loosen a good number of particles.
2. Prepare a microscope slide by putting a drop of mounting medium (I use Cargille’s Meltmount™, refractive index 1.662) in the middle of a standard 1” x 3” (25 mm x 75 mm) microscope slide.
3. Warm the slide on a hotplate to spread and flatten the mounting medium.
4. Allow to cool.
5. Collect the loosened pigment particles by pressing the flattened drop repeatedly over scratched areas.
6. Coverslip the preparation and place onto the hotplate to melt the mounting medium.
7. Remove from the hotplate and press the coverslip to disperse the pigments and mounting medium.
8. Label the slide. Store the slides flat. Done!

The resultant slides are of very good quality, with the pigments (usually) very well dispersed. Figure 2, cobalt violet brilliant (PV49), taken with plane polarized light at 400X magnification in Meltmount ™ (1.662) is a typical preparation.

click image to enlarge (179K) Dispersion of Cobalt Violet Brilliant (PV49), Kremer number 45820, from the Kremer Chart006 Blue Pigments. Plane polarized light at 400X, in Meltmount™, refractive index 1.662 Figure 2

click image to enlarge (167K) The McCrone Paint Pigment Reference Set from McCrone Microscopes and Accessories Figure 3

Of course, the easiest way to get a high quality set of ready-made pigment reference slides is to purchase a Pigment Reference Set of microscope slides from McCrone Microscopes & Accessories (www.mccrone.com), which includes 50 of the most important pigments (Figure 3). McCrone Microscopes & Accessories is also an excellent source for all kinds of microscope supplies, such as microscope slides and coverslips, refractive index liquids, mounting media, hotplates, microscope labels and the like. Other scientific supply venders may also have these materials available.