ULTRAVIOLET-FLUORESCENCE MICROSCOPY OF PAINT CROSS SECTIONS:
JOHN M. MESSINGER
The results obtained with SB are fully consistent with that obtained by Johnson and Packard (1971). SB will stain layers containing acrylic emulsion a blue to black color. The results obtained with AB2 are fully consistent with those obtained by Martin (1977). It fails to stain acrylic emulsion layers to any significant degree.
The results obtained with RB and DCF are fully consistent with those obtained by Wolbers (1987);. Layers containing acrylic emulsions are stained varying shades of red to yellow by RB and yellow-green to green by DCF. It has been pointed out that these results—using knowns—seem more consistent that the results typically found when stains are sued with unknowns, especially from older paintings (Gifford 1991).
Both LISSA and FITC consistently yield a positive result when used to stain linseed oil containing certain pigments and with media based on acrylic emulsions. In fact, upon staining with FITC, a sample containing three layers: egg albumin over zinc white in oil, over acrylic gesso, left the albumin layer unstained, stained the zinc white layer bright green, and stained the acrylic gesso layer dim green. Similarly, LISSA consistently (tested three times with different lot numbers of the stain and delivery solvent) marked a sample very convincingly for protein. Yet this sample contained only linseed oil with whiting.
It was observed that the acetone delivery solvent used with LISSA and FITC would etch the surface of the young linseed oil samples used in this study. This fact may explain some of the discrepancy between these findings and those of Wolbers (1987, 1988). However, it cannot be the full reason as neither LISSA nor FITC would stain lead white or ultramarine containing oil layers. LISSA may have some value in detecting acrylic paints as it has consistently yielded a characteristic yellow-orange color with the acrylic paint samples analyzed in this study.
The fluorescent protein stains contain sulfonyl chlorides or isothiocyanates that are highly electrophilic functional groups. These materials react quickly and exothermically with nucleophiles such as hydroxyl groups (−OH), and in particular with primary amines (−NH2) and thiols (−SH) all of which are found in proteins. Sulfonyl chlorides react with water to form sulfonic acids and isothiocyanates react with water to form thiourethanes. One might conjecture that they would react with water that is adsorbed onto the surface of pigment particles and remain tightly bound to these particles in the same manner in which polar compounds are adsorbed onto the surface of the silica gel particles used in chromatography. It is interesting to note that manufacturers usually coat titanium oxide with silica and/or alumina to aid in its dispersion in the media and to improve the durability of the paint media (Whitehead 1978). The results of this study are consistent with the hypothesis that fluorochromes with electrophilic groups are reacting with water adsorved onto the surface of the polar micelles present in acrylic dispersion paints.
DC-C7A is the only protein-selective, fluorochrome stain in this study to stain casein-based paint media (see figs. 2–4). Although it contains a sulfonyl chloride as its protein reactive functional group, it does not yield as many false positives with oil-containing layers as do LISSA and FITC. It uses an aqueous, alkaline delivery system that causes excavation of gelatin containing layers. However, this alkaline delivery system is advantageous in certain circumstances, as it did not excavate the carbonate- or oil-containing layers examined in this study. It does not stain iron oxide pigment in either linseed oil or Elvacite 2044. DC-C7A appears to be far more reliable than the other fluorochrome stains, as it does not stain most linseed oil-containing layers to any significant degree. It would be prudent to confirm the presence of titanium oxide before drawing any conclusions as to the presence of protein or acrylic media. DC-C7A does not stain egg white to any significant degree.
The author wishes to acknowledge F. Christopher Tahk, Dan Kushel, and Christopher Augerson for their helpful suggestions and lively discussions.