MEASURING THE RATE OF ATMOSPHERIC CORROSION IN MICROCLIMATES
ENVIRONMENTAL CONDITIONS affect the deterioration rate of artifacts. To know how much, one needs a measure of that rate. Many effects of environmental conditions on artifacts have been discussed qualitatively in the literature (e.g., Thomson 1978). Assessment of the chemical composition of museum environments is a topic of current conservation research (e.g., G.C.I. Newsletter 1988). Interest has focussed on the microclimate created in storage and display containers, partly because it offers good prospects for tight control of conditions, and partly because practical experience showed this to be a significant problem area (Oddy 1973, 1975; Blackshaw and Daniels 1978; Blackshaw 1982; Padfield et al. 1982; G.C.I. Newsletter 1987).
The corrosiveness of microclimates in the vicinity of wood and wood products has long been known and frequently discussed in the literature (Oddy 1973, 1975; Blackshaw and Daniels 1978; Blackshaw 1982; Padfield et al. 1982; FitzHugh and Gettens 1971; Arni et al. 1965a, 1965b; Packman 1960; Clarke and Longhurst 1961; Packman 1957; Donovan and Stringer 1969, 1971). The deleterious agents in these microclimates are organic acids. Acetic acid is the main agent (FitzHugh and Gettens 1971; Arni et al. 1965a, 1965b; Packman 1960; Clarke and Longhurst 1961; Packman 1957; Donovan and Stringer 1969, 1971), but formic, butyric and propionic acid have also been detected (Arni et al. 1965b; Donovan and Stringer 1969, 1971). Formaldehyde also is present over wood-based materials, as well as over many paints and varnishes. Aldehydes are easily oxidized to the corresponding organic acids on metal surfaces by atmospheric oxygen, thereby adding to the overall organic acid concentration. The deleterious effects of formaldehyde have received significant attention in the conservation literature (Carpenter and Hatchfield 1985; Hatchfield and Carpenter 1986; Leveque 1986; Conolly 1987).
Two approaches to quantifying atmospheric corrosiveness are possible: 1) determination of atmospheric composition, and 2) measurement of the corrosion rate caused by the atmosphere of interest. The first approach clarifies the reasons for atmospheric corrosiveness; the second quantifies the rate of the deterioration process. Results obtained by the second approach are directly related to the atmospheric composition, but the forms of the relations have to be determined experimentally. Research into the analysis of microclimate composition is under way elsewhere (G.C.I. Newsletter 1988, 1987). The topic of this article is the determination of atmospheric corrosion rates in microclimates dominated by the presence of wood or wood products, closely following ASTM standards G50–76 and G1-81 (1981).