Devitrification, Crystallization
Crystals can grow in cooling glaze melts if one or more of the following conditions are present: the glaze melt is fluid, cooling rate is slow, oxides that like to form crystals are present (e.g. ZnO, TiO), oxides that can form crystals are present in high proportions (e.g. CaO), oxides that stiffen the melt are not present or present in low percentages (e.g. Al2O3, Zr, MgO). Crystals are normally silicate or borate compounds, thus SiO2 and B2O3 need to be present in significant amounts. Crystals can be seeded by incorporating them in a glaze batch. Glazes not normally prone to crystallization can sometimes be partially crystallized by slow cooling and glazes prone to crystallization can often be quickly cooled to prevent it.
Crystallization can be highly decorative but is difficult to maintain consistency and is only used in one-of-a-kind artware. Unwanted crystallization occurring in a glaze during cool-down in the firing is called devitrification, it spoils gloss surfaces and can be a real plague to industry. It can be dealt with by faster cooling, higher Al2O3, switching some CaO for MgO, reducing B2O3.
Very glossy or well-melted glazes can be subject to this because they likely either contain a lot of SiO2 (which combines with other oxides to form silicate crystals) or have a very fluid melt (which enables crystals greater freedom to form). When devitrification is desired it is simply called crystallization. The chemistry of the host glaze is the key factor since it determines the amount of melting and the presence of oxides that can impede crystallization (e.g. Al2O3, MgO). Many mattes are simply glossy glazes in which the entire surface has been invaded by micro-crystals. Purely decorative highly crystalline glazes are high in Na2O and thus almost always craze badly.
A dramatic example of crystallization can be demonstrated by melting (and cooling) a powdered mix of 50:50 Ferro frit 3134 and cobalt oxide in a crucible at cone 6. The frit is a very active melter (it contains no alumina) and the cobalt is also an active melter, together they can work real magic! Out Bound Links
In Bound Links
Pictures
The variegating effect of a thin layer of titanium dioxide (outside of bowl) on GA6-D Alberta Slip glaze at cone 6
Example of variegation and phase separation with about 5% rutile added to a dolomite matte cone 10R glaze.
Crystallization in a high MgO matte at cone 10R
Crystallization of rutile is completely subdued using Ferro frit 3249 (20% with Alberta Slip) on the right (the left is frit 3134 20%)
crystalline and vitreous silica matrix
Example of a crystalline glaze
This high boron cone 04 glaze is generating calcium-borate crystals during cool down
An example of the same cone 6 high iron (9%) glossy borate glaze slow cooled (right) and free-fall cooled (left). Notice how the iron silicate crystals have invaded the surface.
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
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