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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
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.
''Frosting''
''Iridescence'' is caused by continuous flaking of a glass surface causing a multi-colored sheen to appear on the surface of formerly transparent glass.〔
''Milky''
''Enamel-like''
Devitrification
(詳細は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 creatiion of a silica gel, indicating the change in the glass's chemical composition.
Crizzling
(詳細は''Stage Two''
:"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.:〔
''Stage Three''
:"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.:〔
''Stage Four''
:"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.:〔〔
''Stage Five''
: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.:〔
Brittleness
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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