1.10 Protection of Works of Art from Damage Due to Atmospheric Nitric Acid

California Institute of Technology
Daniel Grosjean and Associates
Glen R. Cass
Lynn G. Salmon
William.W. Nazaroff
Mary P. Ligocki
Michael C. Jones
Daniel Grosjean
Department of Civil Engineering,
University of California,
633 Davis Hall, Berkeley, California 94720

Period of Activity: 7/89 to 1/90

Project Abstract
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.

Primary Publications
Salmon, L. G., W. W. Nazaroff, M. P. Ligocki, M. C. Jones, and G. R. Cass, "Nitric Acid Concentrations in Southern California Museums," Environmental Science and Technology, Vol. 24, Nē 7, 1990, pp. 1004-1013.

ABSTRACT-Measurements were made during two seasons at five Los Angeles area museums to determine the concentration of nitric acid in outdoor and indoor air. Mean seasonal indoor nitric acid concentrations ranged from <0.1 to 1.5 mg/m3 corresponding to less than 1% to 40% of the outdoor nitric acid concentration depending on building construction and ventilation system design. A mathematical model was applied to determine whether indoor/outdoor HNO3 concentration ratios can be predicted from data on building parameters and ventilation system design. Good agreement between predicted and measured values was found. The rates of deposition of total inorganic nitrate onto vertical surfaces due to gas-phase plus aerosol-phase pollutants were measured and found to vary from 0.2 to 5.8 ng m-2 s-1. Measurements indicate that nearly all of this deposition flux was delivered due to deposition of gas-phase species, but comparison to theoretical HNO3 transport calculations suggests that gaseous species in addition to HNO3 contribute to the observed accumulation in inorganic nitrate.

Salmon, L. G., G. R. Cass, D. Grosjean, M. C. Jones, M. P. Ligocki, and W. W. Nazaroff, "Protection of Works of Art From Atmospheric Nitric Acid," Final Report to the Getty Conservation Institute, May 1992, 100 pages.

ABSTRACT-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.

Salmon, L. G., and G. R. Cass, "The Fading of Artists' Colorants by Exposure to Atmospheric Nitric Acid," Studies in Conservation, Vol. 38, 1993, pp. 73-91.

ABSTRACT-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 ēC 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.