Protection of Works of Art From Damage Due to Atmospheric Nitric Acid

Lynn G. Salmon, Glen R. Cass, Daniel Grosjean, Michael C. Jones, Mary P. Ligocki, and William W. Nazaroff
GCI Scientific Program Report, May 1992


Preliminary tests suggest that nitric acid vapor is capable of inducing rapid fading and color shifts in a variety of important artists' colorants. A study was pursued to determine whether or not nitric acid at the levels found to be drawn into museums from outdoors is capable of causing damage to the colorants used in works of art. The research effort consisted of several parts. First, chamber studies of the fading and color changes observed in artists' pigments upon exposure to ppb levels of HNO3 in purified air was conducted to identify those pigments that are susceptible to HNO3 damage. An investigation of the chemical mechanisms by which HNO3 attacks these susceptible colorants was conducted to confirm that HNO3 is indeed responsible for the observed color changes. Existing experimental data on indoor and outdoor HNO3 levels measured in five Los Angeles area museums were evaluated to determine the HNO3 flux to museum collection surfaces over time. Finally, advice was provided on methods for protecting museum collections from damage due to HNO3 vapor exposure.


The objectives of the present research project are to assess the effect of nitric acid on the fading of artists' colorants, determine the levels of nitric acid commonly encountered in museums, and suggest methods that can be used to protect works of art from damage due to atmospheric nitric acid. In Chapter 2 of this report, the fading of artists' colorants exposed to atmospheric nitric acid is studied using seventy-nine colorants, drawing on a collection of natural organic pigment samples from the Forbes collection at Harvard, traditional Japanese colorants on silk cloths, and modern synthetic organic and inorganic colorants. Chapter 3 examines the reaction products and mechanisms of nitric acid interaction with 9 organic artists' colorants using chemical ionization mass spectrometry. The potential threat nitric acid poses for works of art is examined in Chapter 4 which quantifies the nitric acid concentrations found in the atmosphere inside Southern California museums. Chapter 5 details procedures for protecting cultural artifacts from damage due to atmospheric nitric acid.

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