The fading of artists' colorants by exposure to atmospheric nitric acid

Experiments show that atmospheric nitric acid vapor is capable of inducing fading and color shifts in a variety of important artists' colorants. Seventy-nine samples were exposed to 12 and 40ppb nitric acid in air at 22°C and 50% RH in two consecutive experiments, each of 12 weeks duration. About half (27 from 52) of the colorants applied to paper showed sensitivity to nitric acid and most (22 from 27) of the Japanese dyed silk cloths showed some reactivity toward nitric acid at the higher concentration. At a concentration of 12ppb for 12 weeks, the nitric acid dose (concentration multiplied by duration of exposure) employed in this study is equivalent to the dose inside an unprotected museum in central Los Angeles over a seven-year period.