YELLOWING AND BLEACHING OF PAINT FILMS
Henry W. Levison
AFTER THE FIRST CYCLE, when the Y.I. was only approximately two-thirds of the initial Y.I., it was thought that a trend of reduced yellowness had begun.12 That was dispelled, however, by the results in subsequent cycles. The age of the films of drying oils did not diminish the degree to which the films eventually became discolored. The bleaching appeared to depend on the intensity of the illumination or, more precisely, the net exposure (intensity times time) in a period of 30 days. The excellent window illumination following the greatest increase in yellowing (fourth cycle) bleached the panels close to the initial whiteness. This result particularly applies to the linseed oil white, which yellowed the most, but became visibly whiter than it was initially.
It is interesting to compare three mixtures that have the same proportion of linseed oil in their compositions, Nos. 2, 8 and 13. This allows comparison of the effect of run copal, maleated rosin ester (substitute for copal) and soya alkyd. The yellowing of the maleated rosin ester fell slightly below that of the copal mixture although it seems be slightly higher after 25 months in the dark. However, this vehicle bleached to the same Y.I. as the copal. The soya alkyd mixture, on the other hand, yellowed much less throughout and bleached to the same original whiteness, making it the formulation that exhibited the least tendency of these to yellow.
Undoubtedly, yellowing in the dark is the result of thermally induced oxidative reactions. Some years ago, Huey described a method of storing standard reference paint panels in the cold and under an inert atmosphere in order to minimize their discoloration in the dark.13 Museums and artists may not wish to store the paintings in their collections at particularly low temperatures or in an inert atmosphere, but they might consider storing them at a lower-than-normal temperature. Moreover, they may also wish to consider the centuries-old practice of exposing yellowed oils to indirect daylight or to filtered fluorescent lamplight for brief periods to reverse the yellowing of the vehicle which may have occurred over several years of storage in the dark. The present work has demonstrated that very brief exposure to indirect north light will almost completely bleach out the yellowing that may have been built up over several years' storage in the dark. Whether such a bleaching procedure would prove ultimately harmful to the paint, particularly if repeated often, is, of course, a matter that must be thoroughly investigated.