SPECIAL REQUIREMENTS FOR EXHIBITING ENAMELS

Abstract

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, <40%, is below the range in which sensitive glasses are stable. To improve conditions in the case, all the lighting except the PAR 38 flood was replaced by lamps that have dichroic filters. Ten GE MR-16 VNSP-H narrow spot halogen bulbs were chosen (eight 20W and two 25W), providing greater illumination than the previous lamps. The light level at the case was approximately 800 lux. The internal environment was monitored for one month using the ACR data-logger. The change in lamps resulted in much smaller daily fluctuations inside the display case. The temperature was c. 22.8°C and the RH was 43 ± 1%. The use of dichroic filters on lamps significantly reduces the rise in temperature inside exhibition cases; in this instance, it reduced the maximum temperature inside the case as well as the size of the temperature fluctuation. However, despite the improvement in maintainance of temperature, it is clear that the RH can drop below the levels considered safe for a variety of materials, including certain glasses such as those on the ciborium. In addition to the use of dichroic lamps for lighting, which helps to maintain the temperature at appropriate values, it is necessary to buffer cases used for objects sensitive to low RH with conditioned silica gel. The authors are grateful to E.A. Murphy, G. Anson and F; Everly for their assistance. Authors' address: Conservation Division, National Gallery of Art, Washington, DC 20565, USA.