Tech Briefs

Cross-sectional specimens are prepared with wet grinding followed by dry polishing.

A technique similar to that of metallography has been devised for preparing cross-sectional micrographic specimens from small samples cut from oil paintings. Art experts at the Cleveland Museum of Art use the technique in their efforts to determine painters’ methods and to verify the authenticity of paintings. By implementing the technique with automated polishing equipment, they can prepare a cross-sectional specimen in 20 min, and a publication-quality photomicrograph (see figure) can be made from the specimen. In contrast, the prior manual preparation technique took about 4 h and yielded specimens that contained scratches and were not flat enough for viewing at higher magnifications.

Paint Layers Are Intact and clearly visible in a polarized-light micrograph of a cross-sectional specimen taken from an oil painting.
The technique is applied to a small (< 0.2 mm) sample that is removed from the painting with a scalpel. The sample is cast in polyester resin in a standard metallographic mount. The mount is then ground and polished in two stages to expose the desired cross section. The first stage involves traditional grinding with silicon carbide abrasive papers in water lubricant at a normal force of 200 N; a succession of four ever finer papers is used, starting with 320 grit and ending with 2,400 grit. The grinding time at each of the first three grits is automatically limited to 30 s. The grinding at 2,400 grit is stopped when the ground surface is within 10 μm of the sample, as measured by bright-field and polarized-light microscopy.

The second, more delicate, stage involves dry polishing; that is, dry grinding with a succession of even finer grits. First, silicon carbide in six steps of 1,500 through 6,000 grit is used, followed by aluminum oxide of 8,000, then 12,000 grit. Dry grinding must be used in exposing the cross section of the sample because some layers of the paint may be soluble in water, ethanol, kerosene, and other traditional grinding and polishing lubricants. A normal force of 10 N is applied in this stage. The grinding times are 10 s for each of the first three SiC grits, 15 s for each of the second three SiC grits, and 20 s for each of the Al2O3 grits. The resulting surface is highly polished and suitable for photomicrography.

This work was done by Todd Leonhardt of NYMA, Marcia Steele of the Cleveland Museum of Art, and William Waters of Waters and Associates for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to the Patent Counsel, Glenn Research Center; (216) 433-2320. Refer to LEW-16177.

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Preparing High-Quality Micrographic Samples of Oil Paintings (reference LEW-16177) is currently available for download from the TSP library.

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