Conservation and restoration of glass objects
The conservation and restoration of glass objects is an activity dedicated to the preservation and protection of objects of historical and personal value made from glass. When applied to cultural heritage this activity is generally undertaken by a conservator-restorer.
Glass was discovered in the third millennium B.C.E. and is still a relatively new material. The art of glassworking did not occur until long after metalwork or pottery. The act of conservation and restoration strives to prevent and slow the deterioration of the object as well as protecting the object for future use. Water is the primary cause of deterioration in glass materials.
Methods of deterioration
Weathering is a broad definition for any form of deterioration which appears on the surface of the glass and is caused by exposure to adverse conditions. Below are the seven typical forms of weathering.
Dulling is the simplest form of weathering. It occurs when glass loses its clarity and transparency and becomes translucent.
Strain cracking describes the presence of minuscule cracks running through the glass in all directions, giving it a sugary appearance and causing the glass to disintegrate.
Devitrification is the process of converting glass into a crystalline substance by heating it to a temperature just below its melting point then cooled. By doing this, the randomly situated molecules in the glass, when cooled, will attempt to settle in a less random and more crystalline form. This technique is used to create an opaque appearance in the glass. Due to the high level of stress this technique places on the glass, it may cause the glass to deteriorate. For more information on devitrification as an artistic technique, see the main article above.
The term "devitrification" is used by archaeologists and art historians to refer to a glass object which has lost it vitreous nature, giving the glass a weathered appearance. This is first caused by the loss of alkali then followed by the loss of other materials of the glass, and finally the creation of a silica gel, indicating the change in the glass's chemical composition.
Atmospheric deterioration, commonly referred to as crizzling, weeping or glass disease is primarily caused by a chemical imbalance in specific types of glass, though it can occur in almost any glass. This form of deterioration is a basic defect in glass caused by incorrect proportions of ingredients, particularly an excess of alkali in the glass composite. Crizzling occurs in 5 main stages.
- The "initial stage" of crizzling occurs when alkali is present on the glass, giving the glass a cloudy or hazy appearance. If neglected, crizzling's initial stage may occur within five or 10 years of the glass's manufacturing. Indications of crizzling may be seen as tiny droplets in high (above 55%) humidity or as fine crystals in low (below 40%) relative humidity. A feeling of slippery or slimy glass is another possible indication of stage one crizzling. The initial stage is treatable. Washing the glass removes the alkali and will keep the glass looking new.:
- "Incipient crizzling" has similar characteristic symptoms as stage one, however, the haziness seen on the glass may not disappear entirely when washed and dried. When examined closely at an angle with a low light, fine cracks, which appear as tiny silvery lines or shimmering rays, may be visible. A microscope can confirm the presence of cracks. These cracks are the first sign of deterioration in the glass. The cracks are caused by a combination of the cycling of the glass through humid and dry environments and the loss of alkali, leaving microscopic voids in the glass.:
- "Full-blown crizzling" occurs when the fine cracks in the glass during stage two of deterioration have progressed and are visible to the naked eye. In some cases, the crazing also gains a more uniform appearance. The deeper cracks are indicative of higher amounts of alkali leaching from the glass composition. Crizzling may not be uniform due to the creation of micro-climates on the glass. Crizzling is far more likely to occur in areas with restricted air-flow. Symptoms of stage one and two may also be present on the glass if this advanced form of deterioration has not affected all of the glass's surface.:
- "Advanced crizzling" shows distinctive cracks on the surface of the glass. The fissures may be so deep that some surface material may be lost from flaking of chipping, this is referred to as spalling.:
- The "fragmentation stage" of crizzling is the final stage of deterioration and occurs when the structural integrity of the glass is no longer present causing the glass to separate into pieces.:
Storage and handling
Education and training
In France, conservators specialized in earthenware and glassware are trained at the Institut National du Patrimoine (The National Institute of Cultural Heritage). Their mission is to intervene when heritage resources are threatened or deteriorated for several reasons. The conservator prevents works of art from disappearing or loses its purpose whilst analyzing the complex stage of its material history and the cause of alteration.
Historic methods of glass cleaning
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- Conservation and restoration of metals
- Conservation and restoration of copper-based objects
- Conservation and restoration of ferrous objects
- Conservation and restoration of ivory objects
- Conservation and restoration of ceramic objects
- Conservation and restoration of silver objects
- Koob, Stephen (2006). Conservation and Care of Glass Objects. New York: Archetype Publications. ISBN 1-904982-08-5.
- Newton, Roy; Sandra Davison (1989). Conservation of Glass. Butterworths. ISBN 0-408-10623-9.
- Tennent, Norman, ed. (1999). The Conservation of Glass and Ceramics: Research, Practice and Training. London: James & James (Science Publishers) Ltd. ISBN 1-873936-18-4.
- "Crizzling". Corning Museum of Glass. April 26, 2012. Retrieved April 26, 2012.