The fading of artists' colorants by exposure to atmospheric nitrogen dioxide

Nitrogen dioxide is a common air pollutant, formed in the atmosphere from the nitric oxide emissions from fuel combustion sources. Samples of organic and inorganic pigments, iron inks on paper, and traditional Japanese textile dyes mordanted onto silk were exposed to O·50ppm NO_2 in air for 12 weeks. Ten of the traditional natural organic colorant samples on paper tested showed measurable color changes (∆E > 2) as a result of the NO_2 exposure, as did several of the anthraquinone-based synthetic organic pigments. The arsenic sulfide pigments orpiment and realgar and the iron inks tested showed considerable color changes (∆E > 5) over the 12-week period. Nine of the 23 Japanese dyed silk cloths changed color by at least two ∆E units, including an enju sample (derived from the Japanese pagoda tree) that faded by more than seven ∆E units An NO_2 dose (concentration times duration of exposure) of the magnitude employed in this experiment would be experienced inside an unprotected museum in downtown Los Angeles over about a two-year period, and in many other cities within a period of five to six years