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.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%).
|
