In contrast to the typical homogeneous surfaces of sanitaryware and most table ware, reactive glazes exhibit discontinuities in texture, matteness, reflectivity, color, etc that are called variegation. These variations are normally caused by crystallization, speckling and rivulets and opacity variations associated with phase separation. By understanding these mechanisms it is possible to produce glazes of more interesting appearance.
•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
Oxides - ZrOZrO can produce patterns of minute darker and lighter areas in an otherwise drab glaze surface. Significant amounts are needed (e.g. up to 15%).
Oxides - Li2OLithia contributes to mottled and flow effects when used in small amounts (-1%).
Oxides - CaOCaO can mottle glaze surfaces at high temperatures if significant amounts are present, it is a very strong flux and can contribute to phase separation.
- When used with tin and rutile (e.g. 4% of all three) iron oxide can produce attractive mottled browns in glossy glazes.
Oxides - P2O5Phosphorus can produce variegated and mottling effects in glazes (especially low fire) when used in small amounts (e.g. up to 2%). Bone ash is a source.
- Ilmenite can be used with rutile to produce classic rutile-breaking glazes. Use up to 2% ilmenite.
- Rutile has the unique property of breaking up and variegating the color and texture of glazes, it is quite popular in tile and art pottery for this reason. However this effect depends on the rutile powder being coarse enough to act as an adjunct (the finer grades disperse more readily into the glass matrix). The addition of 4-8% rutile to many stoneware glazes can make an otherwise drab or flat glaze become much more interesting. When used in combination with colorants, it can be very effective at improving the character, however it will affect the color. When used with added tin streaking and mottling effects can be enhanced, especially in lead glazes.
- Granular iron pyrite can added to glazes to produce speckle at any temperature. At higher temperatures the speckles will bleed, in reduction they will melt and blossom.
- Illmenite granular is available in a variety of particle sizes and can be employed to add speckle to glazes. Start with about 0.2%.
- Granular rutile is popular as a specking agent. Start with 0.2%. This material is heavy and will settle in glaze with a thin slurry.
- Tin reacts with iron in fluid glazes to produce variegated surfaces. A good example is the Albany Slip 85, Tin 4, Lithium 11 glaze for cone 6.
- Smaller amounts of titanium dioxide (i.e. 5%) added to colored or opacified recipes can variegate the surface and make it more interesting (e.g. it alters the shape of crystals, shade of colors).
Oxides - TiO2Titania is a classic addition to produce mottled and variegated effects in all sorts of glazes. The more you use the greater the effect (up to 10%).
Variegation and phase separation with about 5% rutile
The glaze is a dolomite matte fired to cone 10R. High fire reduction is among the best processes to exploit the variegating magic of rutile.
Out Bound Links
This is an overview of the various mechanisms you can employ to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.
Phase separation occurs when a glass melt separates into two or more liquids of slightly different chemistry (and therefore potentially different fired appearance and physical presence). The homogeneity of the fluidity of the melt can be disrupted late in the melting process or even in the cooling. ...
Crystals can form during cooling and solidification in many kinds of glazes and they can be microscopic or very large, widely scattered or completely covering. Matte glazes (e.g. high CaO) are often such because of a dense mesh of micro-crystals growing on the surface. Unwanted crystallization is ca...
When ceramic melts are cooled they prefer to solidify as an organized molecular structure. Given sufficient time and sympathetic chemistry, they will form a crystalline structure. But if cooling is faster they solidify as a glass.
Crystals can grow in cooling glaze melts if one or more of the fol...
A glaze additive that transforms an otherwise transparent glaze into an opaque one. Common opacifiers are tin oxide and zircon compounds. Opacifiers typically work by simply not dissolving into the melt, the white suspended particles thus reflect and scatter the light. Since they do not participate ...
Variegated, or mottled, glazes are those that do not have a homogeneous solid color or character (i.e. like a ceramic sink or toilet bowl). They are often called 'reactive glazes'. They contain higher percentages of fluxes and additions intended to produce one or more variegation mechanisms. Variati...
MgO - Magnesium Oxide, Magnesia
A kiln atmosphere which is deficient in free oxygen. In traditional ceramics, reduction firing requires a specially designed fuel fired kiln that restricts the flow of incoming air so there is enough to burn the fuel and no more (in some cases it is restricted so that is actually less than enough to...
Dolomite matte glazes are normally fired at cone 10 and often have a very pleasant-to-the-touch silky-feeling surface. This unique feel is a product of tiny discontinuities in the glaze melt (phase changes) that exhibit at the surface as tiny waves and ripples. This phenomenon is thought to be assoc...
In Bound Links
Identifying a mechanism means you identify the reason a glaze does something specific. Especially visual. Most glaze recipes can be separated into two parts: the base and the mechanism of the color, opacity and variegation. The base is likely just a transparent glossy or translucent matte (although ...