Preventive Conservation
Environment
Optimum conditions for storage and display of stable glass objects are 40 - 60% relative humidity and a temperature range of 15 – 25 °C with maximum variations of 5 % and 4 °C respectively within any 24 hour period.
Relative humidity control is important as a moist environment will adversely affect any glass surface and a desiccating environment will cause further damage to weathered glass. In addition, adhesives used for repairs may ‘cold flow’ if exposed to high relative humidity or high temperatures.
High temperatures may damage painted, crizzled or weathered glass. After appropriate treatment, keep chemically unstable, crizzled and weeping glass at 42% relative humidity. This strict condition is necessary as higher relative humidity levels will dissolve salts and lead to their migration while lower relative humidity levels increase the risk of cracking. Use a sealed container and conditioned silica gel or an appropriate saturated salt solution to maintain this relative humidity level and seek advice from a conservator.
Light levels are not critical for most glass as fading is not usually a problem. Sunlight and other UV sources will affect decolourised glass, giving it a purple hue. Light levels of up to 300 lux, with a UV content of 75 µwatts/lumen (22,500 µwatts/m2) are acceptable for stable glass objects.
It is important to avoid heat build-up from lighting. Place display lighting outside showcases and if possible direct light onto glass objects by reflection.
Handling
Glass is a fragile material which is easily broken if not handled carefully. Take the following guidelines into account when handling glass objects:
- check for breakages or failing adhesive;
- remove any loose parts such as lids or stoppers before carrying;
- only carry one object at a time with the weight supported uniformly and one hand under the base;
- do not wear cotton gloves as glass surfaces are smooth and slippery. Instead handle objects with bare, clean hands or disposable rubber gloves;
- line trays used to carry objects with bubble wrap or cottonwool covered in tissue paper. Insert bubble wrap or crumpled tissue paper padding between objects; and
- keep handling of iridescent or weathered glass to a minimum.
Storage and Display
Do not store flat glass ‘face to face’ as trapped moisture will damage the glass surface. Use perforated acid-free board to separate the glass sheets and allow air exchange.
Store glass vessels in rigid cardboard boxes padded with inert polyethylene foam, bubble wrap or crumpled acid-free tissue paper. Use foam core board or ethafoam to build special cradles for top heavy or damaged objects.
Unless the surfaces are properly sealed (see the chapter Preventive Conservation: Agents of Decay), do not use wooden cupboards or showcases for storing or displaying glass objects. Acid vapours given off by hardwoods such as oak and birch and by composite boards are damaging to glass that has a high soda content. Enamelled or powder-coated metal cabinets or shelving are recommended. Line shelves with a non-slip material such as closed cell polyethylene foam. Attach this foam firmly to the shelving.