SPECIAL REQUIREMENTS FOR EXHIBITING ENAMELS

Autor: Barbara H. Berrie, Shelley Sturman, Mervin Richard
Rok vydání: 1994
Předmět:
Zdroj: Studies in Conservation. 39:3-3
ISSN: 2047-0584
0039-3630
DOI: 10.1179/sic.1994.003
Popis: The effect of lighting on the environment inside exhibition display cases has been known for many years and has been compared to the 'greenhouse effect'. While careful attention is often given to case environments when exhibiting hygroscopic materials such as ivory or wood, the specific environmental requirements for objects made of glass and enamel are less commonly addressed. Several mediaeval enamelled objects in the collections of the National Gallery of Art, Washington, were displayed in a peninsular acrylic vitrine, I07cm long, 56cm wide and 76cm high. This case was constructed from plywood and the interior was covered with fabric. The case was externally lit by 10 General Electric PAR 36 spotlamps (25W): six mounted at a distance of 4.7m, two at 6m, and two at 3.8m. In addition, there was one PAR 38 (65W) flood 3.7m from the case. The gallery temperature was c. 21°C and the relative humidity (RH) was 50 ± 50/0. A Spanish champleve enamelled ciborium that was among the objects in the case developed green and blue copper corrosion while on view. Analysis of the components of the ciborium showed copper corrosion was occurring at the junction between the metal substrate and one color of enamel on the object. This enamel was shown to be a low lead, low calcium potash glass colored by cobalt [I]. Low lead potash glasses have been recognized as particularly unstable. Conditions required for the safe exhibition and storage of unstable glass have been previously defined [2]. The temperature should be stable and moderate; the relative humidity should be in the range 40-60%. Below about 40% RH, hydrated glass loses water and begins to crizzle. The large size of the case, the buffering effect of the fabric and plywood of the case, the stable gallery environment and the low light levels in the gallery led us to believe that the environment inside the case would be maintained within acceptable temperature and RH ranges for the stability of sensitive glass. The development of corrosion on the object suggested otherwise. The environment inside the case was monitored for one month using a temperature and RH data-logger (ACR System Inc., British Columbia, Canada). Points were collected every 10 minutes. The air temperature and humidity cycled daily with the on-off cycle of the lights. Peak air temperature inside the case was 27°C; the fluctuation averaged 3.8°C. There was a concomitant daily small fluctuation in RH of I%. Over time the average RH inside the case slowly decreased. Equilibrium RH was more than 10% below ambient, that is, below 40%, owing to the high average temperature inside the case. In effect, the heat from the lights dried out the case. The low RH inside the case
Databáze: OpenAIRE
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