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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!

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.

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.

Reagents for Microchemical Analysis

A future column will detail a number of microchemical tests specifically suitable for conservation microscopy. In the meantime, Cargille™ has assembled two microchemical reagent sets (Set 1 and Set 2) that contain all the reagents you will need except for the acids, bases, solvents, and other liquids, which you will need to get from a chemical supply house. Set 2 coincides with Chamot and Mason, Handbook of Chemical Microscopy Volume 2, one of the most important single sources for microchemical test protocols; Set 1 coincides with Volume 1 in the set, and, while interesting, is less critical (Figure 4).

An easy test for Prussian blue is illustrated in Figures 5A and 5B, and Figures 6A and 6B show the results from two separate tests for lead white.

Education

Campbell Center for Historic Preservation Studies

203 East Seminary

Mt. Carroll, IL 61053

815-244-1173

www.campbellcenter@internetni.com

Classes in many aspects of conservation. I will be teaching a class in pigment identification there in September 2004.

International Academic Projects

6 Fitzroy Square

London W1T 5HJ

England

Tel 44 207 380 0800

info@academicprojects.co.uk

These folks conduct a number of conservation related workshops in the microscopical identification of pigments and other museum subjects. Peter MacTaggart and Dr. Nicholas Eastaugh are the principal instructors. An excellent resource for European and other Eastern Hemispheric residents.

College of Microscopy

850 Pasquinelli Drive

Westmont, IL  60559

www.collegeofmicroscopy.com and contact at courses@collegeofmicroscopy.com

 Good mix of theory and practical instruction in scanning electron microscopy, infrared spectroscopy and small particle handling – specimen preparation techniques. A class on conservation microscopy is in the planning stages, probably for spring of 2005.

Instrumentation

All polarizing microscopes will be suitable, and even older, used microscopes, if properly maintained, will provide many decades of service. Leica, Zeiss, Olympus, Nikon, and LOMO all make research-grade polarizing microscopes. Student microscopes are less expensive but may lack important features. Take the time to carefully evaluate a microscope for your laboratory, as a microscope is a lifetime investment.

Papers of Importance

McCrone, Walter C., “The Microscopical Identification of Artists’ Pigments”, Journal of the International Institute for Conservation (J.IIC-CG), Vol. 7, Nos. 1 & 2. Contact us at mccrone@mccrone.com for a reprint of this most important paper. This paper is the clearest, most succinct, and generally most useful piece in the literature on pigment identification. It includes a table summarizing the optical characteristics of most major pigments and decision trees that help narrow down the list of possibilities. The paper also includes important tips on sampling, sample preparation and the like.

Reference Books (Number of stars indicates its importance)

The first four (Gettens & Stout, and the three volumes of the Artists’ Pigments Handbooks) are essential for any serious work in art materials and paint microscopy. For a good general introduction to polarized light microscopy, I recommend McCrone, McCrone and Delly’s Polarized Light Microscopy.

***Gettens, R. J., & Stout, G. L., (1966) Painting Materials, A Short Encyclopedia, Dover, New York.

***Feller, Robert L. (Ed.), (1986) Artists’ Pigments, A Handbook of Their History and Characteristics, Volume 1, National Gallery of Art, Washington D.C.

***Roy, Ashok (Ed.), (1993) Artists’ Pigments, A Handbook of Their History and Characteristics, Volume 2, National Gallery of Art, Washington D.C.

***Fitzhugh, Elisabeth West (Ed.), (1997) Artists’ Pigments, A Handbook of Their History and Characteristics, Volume 3, National Gallery of Art, Washington D.C.

***McCrone, Walter C., McCrone, Lucy B., and Delly, John G., (1987) Polarized Light Microscopy, McCrone Research Institute, Chicago, IL.  Possibly the best single-volume introduction to the field.

***Bloss, F. Donald, (1999) Optical Crystallography. Mineralogical Society of America, Washington D.C. This is an updated and beautifully printed treatment of the subject to which he has contributed generously over the years. He treats a difficult subject with clarity and thoroughness.

**Bloss, F. Donald, (1971 and 1994) Crystallography and Crystal Chemistry. Mineralogical Society of America, Washington D.C. This is a more classical treatment on crystallography. Very readable.

Ehlers, Ernest G. (1987) Optical Mineralogy, Theory and Techniques, 2 Volumes. Blackwell, Palo Alto, Oxford, London.

**El-Hinnawi, Essam E., (1966) Methods in Chemical and Mineral Microscopy, Elsevier Publishing Company, Amsterdam.

Nesse, William D., (1991) Introduction to Optical Mineralogy, 2^nd Edition, Oxford University Press, New York, Oxford.

Shelley, David, (1985) Optical Mineralogy, 2^nd Edition, Elsevier; New York, Amsterdam, Oxford.

**Stoiber, Richard E. and Morse, Stearns A.  (1994). Crystal Identification with the Polarizing Microscope, Chapman & Hall, New York, London. Not a book I have yet read but highly recommended by colleagues.

Tröger, W.E., (1962/1967) Optische Bestimmung der Gesteinsbildenden Minerale, 2 Volumes, about 1000 pages. Highly recommended by a German colleague as the most important standard work in that language. Part 1 of the fourth German edition has been translated into English as Optical Determination of Rock-Forming Minerals: Part 1 Determinative Tables.

E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart (1979).

Microchemical Test Methods

**Crown, David A., (1968) The Forensic Examination of Paints and Pigments, Thomas, Springfield, IL. A very useful listing of pigments, including organics, along with many of their characteristics. Includes many organic pigments not described in other manuals.

Grey, Egerton C., (1925) Practical Chemistry by Micro-Methods. W. Heffer & Sons, Cambridge, England. Dr. Grey was Professor of Chemistry at the Government Medical School in Cairo.

**Feigl,  Fritz, (1972) Spot Tests in Inorganic Analysis, 6^th Edition,  Elsevier Publishing Co.,  Amsterdam, London, New York, Princeton, NJ.

**Feigl,  Fritz, (1966) Spot Tests in Organic Analysis, 7^th Edition,  Elsevier Publishing Co.,  Amsterdam, London, New York, Princeton, NJ.

***Chamot, Emile M. and Mason, Clyde W., (1989) Handbook of Chemical Microscopy, 2^nd Edition, Republication by the McCrone Research Institute, Chicago. This is the most important book on chemical microscopy and coincides with Cargille’s Microchemical Reagent Set 2.

***Odegaard, Nancy, Carroll, Scott and Zimmt, Werner S., (2000) Material Characterization Tests for Objects of Art and Archaeology, Archetype Publications, London.  Very up-to-date. The approach is more for small rather than micro samples; spot test papers (very convenient!) also work well with pigments.

****Schramm, Hans Peter and Hering, Bernd, (1999) Historische Malmaterialien und ihre Identifizierung, E. E. Seemann, Leipzig. (Historical Paint Materials and their Identification) For the German reader, this new book is an up-to-date compendium of analytical tests for paint materials.