POLARIZED LIGHT MICROSCOPY IN CONSERVATION: A PERSONAL PERSPECTIVE
WALTER C. McCRONE
Until the second half of this century the polarized light microscope held a dominant position in microanalysis. It was often the instrument of choice even when there was sufficient sample available for use of more macro methods of analysis (Schneider 1964; Benedetti-Pichler 1965).
There were many university professors of polarized light microscopy (PLM) in both chemistry and geology departments, notably E. M. Chamot and C. W. Mason at Cornell, N. F. Witt and C. F. Poe at Colorado, and M. L. Willard at Penn State. Shortly after World War II, however, transmission electron microscopy and infrared absorption spectroscopy were finding a place in industry. These were soon followed by the scanning electron microscope and the electron and ion microprobe analyzers. On the assumption that these high technology instruments rendered the polarized light microscope (PLM) obsolete, nearly all colleges and universities dropped their microscopy courses, and many industries stored their light microscopes and reassigned their light microscopists. Only a very few individuals and industries maintained their expertise in this nearly abandoned field.
Those microscopists who maintain their ability to apply PLM and its adjunct, optical crystallography, have continued because they are able to solve complex microanalytical problems quickly and with certainty. Unfortunately, nearly all have to operate in a world unaware and unappreciative of PLM. Often their results and conclusions are not accepted by the scientific world.
A prime example is research on the Turin shround (McCrone 1990b). From the evidence of PLM, this venerated cloth is a beautiful painting: red ochre and vermilion pigments in a collagen tempera medium on a 14th-century canvas. There is absolutely no doubt concerning this conclusion, yet it has been accepted only by a few individuals knowledgeable about PLM or by others who were convinced only by the carbon date of 1325 ± 65 years. Most others believe it to be the first-century shroud of Christ. Such reactions and receptions for microscopical results cause acute frustration among light microscopists.
The Vinland map (McCrone 1988) is another example. We reported this to be a 20th-century effort. Observations with the stereomicroscope at the time of sampling the ink indicated the map had to be a forgery. The usual yellow stain that forms over time along ancient black ink lines had been inked on. This proved an intentional fraud. Later, we found the yellow “stain” contained a yellow titanium white (TiO2) in synthetic pigment size and form, but characteristic of titanium white pigments as produced during the early 1920s before all of the ilmenite iron was removed from the TiO2. It was therefore, yellow, and it was sold only for off-white paints. This negative finding was accepted until another laboratory (Cahill 1987) reported the TiO2 is present in far too low a percentage to account for the amount we reported in the yellow ink. However, this laboratory's high-tech cyclotron procedure required a large fraction of a milligram sample, in which it found 0.0062% TiO2. The yellow TiO2 is not, however, dispersed throughout the parchment sample but concentrated in the very thin yellow ink borders, where it constitutes up to 45% of the yellow ink. These findings illustrate the difference between ultramicroanalysis of a small, highly concentrated sample and a trace analysis of a large sample with a very low concentration of the component in question. Such differences are evident to microscopists. Because my microscopical results are rejected by art scholars, I have recently decided to refuse to analyze the pigments in any painting where the objective is authentication.
The fact remains that PLM in the hands of trained microscopists solves many problems faced by art conservators quickly, confidently, and with a small capital investment.