Devitrification
Encyclopedia
Devitrification is the opposite of vitrification
Vitrification
Vitrification is the transformation of a substance into a glass. Usually, it is achieved by rapidly cooling a liquid through the glass transition. Certain chemical reactions also result in glasses...

, i.e., the process of crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...

 in a formerly crystal-free (amorphous) glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

. The term is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 vitreus, meaning glassy and transparent.

Devitrification in glass art

Devitrification occurs in glass art
Glass art
Studio glass or glass sculpture is the modern use of glass as an artistic medium to produce sculptures or three-dimensional artworks. Specific approaches include working glass at room temperature cold working, stained glass, working glass in a torch flame , glass beadmaking, glass casting, glass...

 during the firing process of fused glass
Fused glass
Fused glass is a term used to describe glass that has been fired in a kiln at a range of high temperatures from to . There are 3 main distinctions for temperature application and the resulting effect on the glass....

 whereby the surface of the glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 develops a whitish scum, crazing
Crazing
Crazing is a network of fine cracks on the surface of a material, for example in a glaze layer.Crazing is a phenomenon that frequently precedes fracture in some glassy thermoplastic polymers. Crazing occurs in regions of high hydrostatic tension, or in regions of very localized yielding, which...

, or wrinkles instead of a smooth glossy shine, as the molecules in the glass change their structure into that of crystalline solids. While this condition is normally undesired in glass art
Glass art
Studio glass or glass sculpture is the modern use of glass as an artistic medium to produce sculptures or three-dimensional artworks. Specific approaches include working glass at room temperature cold working, stained glass, working glass in a torch flame , glass beadmaking, glass casting, glass...

, it is possible to use devitrification as a deliberate artistic technique.

Causes of devitrification, commonly referred to as "devit", can include holding a high temperature for too long, which causes the nucleation of crystals. The presence of foreign residue such as dust on the surface of the glass or inside the kiln prior to firing can be a key nucleation points where crystals can propagate easily. The chemical compositions of some glass can make them more vulnerable to devitrification than other glass, for example a high lime
Lime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...

 content can be factor in inducing this condition. In general opaque
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

 glass can devit easily as crystals are present in the glass to give its opaque appearance and thus the higher the chance it might devit. Consequently, clear uncolored glass is least likely to devit.

Techniques for avoiding devitrification include cleaning the glass surfaces of dust or unwanted residue, and allowing rapid cooling once the piece reaches the desired temperature, until the temperature approaches the annealing
Annealing (glass)
Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change...

 temperature. Devit spray can be purchased to apply to the surfaces of the glass pieces prior to firing which is supposed to help prevent devitrification, however there is disagreement over the long term effectiveness of this solution and whether it should be used as a substitute for proper firing techniques.

Once devit has occurred, there are techniques that can be attempted to fix it, with varying degrees of success. One technique is to cover the surface with a sheet of clear glass and refiring. Since devitrification can change the COE somewhat, and devitrified glass tends to be somewhat harder to melt again, there is the possibility of this technique resulting in a less stable piece, however it has also been used effectively with full-fused pieces with no apparent problems. Applying devit spray and refiring can also be effective. Alternatively, sandblasting, acid bath, or polishing with a pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...

stone or rotary brush can be used to remove the unwanted surface.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK