DARKENING OF PAPER FOLLOWING EXPOSURE TO VISIBLE AND NEAR-ULTRAVIOLET RADIATION
S. B. Lee, J. Bogaard, & R. L. Feller
FOR MANY YEARS, it has been known that exposure of paper to very short-wavelength ultraviolet, such as 254 nm radiation, will induce post-irradiation effects, the specific results of which are influenced by both internal and external factors.1–4 External factors during irradiation and subsequent storage include wavelength of irradiation, temperature, humidity, and atmospheric conditions. Internal factors relate to chemical composition of the paper: acidity, carbonyl groups, and the content of hemicellulose, extractives, and lignin. It is also known that exposure to visible and ultraviolet radiation results in complex discoloration effects both during and after exposure.5 Nonetheless, the fact remains that few studies have been carried out specifically to elucidate the behavior of papers subsequent to exposure to the wavelengths of visible and near-ultraviolet radiation normally involved in the display of works on paper, approximately 320 to 760 nm.
Launer and Wilson found that soda-sulfite paper irradiated under conditions that resulted in bleaching (330-440 nm radiation; sheet temperature 30°C) yellowed during a subsequent storage period of 15 months.6 MacMillan reported the sensitivity to thermally-induced aging following exposure of test papers in the Fade-ometer¯ to be a function of the aldehyde-group content in the cellulose molecules.7 This particular observation suggests that the observed post-irradiation darkening under visible and near-ultraviolet radiation may essentially be the result of thermal-aging processes even though the level of temperature of the paper when stored away from the light may not be far from normal room conditions. This view was expressed by Launer and Wilson.
One may reasonably regard the widely reported post-irradiation effects that occur as a result of exposure to short-wavelength radiation (below 280 nm) to have only minor relevance to museum and library problems. It is far more important for conservators to increase their knowledge of the effects of exposure to wavelengths of near ultraviolet and visible radiation. Reported herewith are data on the effects of different (a) exposure dosages, (b) initial pH values in the sheet, and (c) levels of hot-alkali-soluble matter, gamma cellulose and lignin upon the discoloration that occurs during storage at room temperature subsequent to exposure of test sheets to radiation in the visible and near-ultraviolet region.
The primary objectives of the studies reported here are to determine (1) whether paper exposed to these wavelengths will undergo post-irradiation darkening under rather ordinary conditions of storage away from the light, (2) whether the pH of the paper has an influence on post-irradiation darkening under these circumstances, and (3) whether the content of lignin, rather than hot-alkali-soluble (HAS) matter, has a marked influence on the phenomenon. The answer to all three questions is affirmative. The information is of value with respect to the behavior that may be expected as the result of the display of works on paper. The results are not intended to be informative with respect to the practice of intentionally bleaching by light while discolored papers are immersed in an aqueous bath. That will be the subject of a publication to appear elsewhere.