Glaze opacity can be achieved using a number of different mechanisms (e.g. phase difference, crystallization, unmelted suspended particles, bubbles, matteness). Each mechanism or combination of mechanisms has advantages and disadvantages depending on the host glaze and process being used.Oxides - TiO2Titania in significant amounts (+5%) will almost always contribute to a glaze opacity of mottled and variegated character.
- As an opacifier rutile is economical compared with pure titanium and it is employed where white shades are not required. It is also used to stain pottery bodies and glazes (yellowish, orange and tan colors are most common because of its iron content).
Materials - TalcTalc is a refractory powder and can promote matteness and opacity when added to low-fire glazes.- Tricalcium phosphate has been used as an opacifier (to replace tin oxide) to give similar color, texture, and brilliance in leadless sanitaryware glazes above cone 8. Does not produce opacity at lower temperatures.
- Produces opacity and a soft matte texture without the need for magnesia compounds which might devitrify.
- Alumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.
- It can be used as an opacifier in glazes, although not as effective as tin. It creates a fine matrix of bubbles that impede light passage.
- If available in sufficient amount, barium oxide will promote crystallization of a melt during cooling, thus imparting a measure of opacity.
- Low temperature glazes sometimes employ bone ash for opacity because of the milky quality it produces. It can also be used to assist tin oxide where a less shiny surface can be tolerated.
- Antimony works to a limited extent as an opacifier to cone 1.
Oxides - CeO2Used as an opacifier for special effects in the tile industry; as a replacement for tin opacifier in porcelain enamel.Oxides - ZrO2Zirconium is an effective opacifier, especially in the Zirconium Silicate form. Materials of finer particle size are more effective. In lead glazes a cream tint is likely. Glazes high in boron or alkalis, or low in alumina and silica may not opacify well.- As little as 4-7% can produce brilliant white, although it is more typical to use 8-10% for full opacity. However, be aware that even tiny amounts of chrome in the kiln will volatilize and combine with the tin to produce pink shades.
- Titanium is a crystalline mineral and encourages crystal development during cooling and freezing of the glaze melt. This generally produces opacity. However, titanium opacified glazes have a much different character than zircon or tin types. The latter produces a much more even and bright white coloration. When used as an opacifier the batch amount can range to 10% or more of the recipe.
- Zinc oxide will produce opacity or whiteness, especially at low temperatures, if the calcium content is low. It does not opacify as well in boron glazes. It works well in combination with tin.
- Zirconium silicate is by far the most popular opacifier in glazes. It is available from many manufacturers. Opax, Ultrox, Zircopax and Superpax are examples of trade names of zirconium silicate and zirconium oxide products. The finer the milling of the material the better the opacity and the cleaner the color that will be produced.
- Zirconium silicates are used primarily as opacifiers in glazes at all temperatures. Although tin oxide is more effective, zirconium materials are much cheaper and are more stable in reduction and less reactive with some colorants (i.e. chrome). Although zirconium oxide is effective as an opacifier, zirconium silicates disperse better and are cheaper where the glaze can tolerate or be reformulated to tolerate the added silica.
- Superpax is a very fine grade zirconium silicate opacifier and will produce a whiter result than less expensive grades. See Zircopax.
- Arsenic can be used as an opacifier in glazes, although not as effectively as tin.
Oxides - TiO2Additions of 5-10% titanium to many types of glazes produces yellow and light tan coloration, the surface effect is usually crystalline in nature. Lead greatly enhances the yellow at low temperatures.Oxides - SnO2Tin is an effective opacifier to transform transparent glazes to white. The quality of color tends to be a 'soft-bluish white' compared to harsher effects with other oxides.- High calcium coupled with lower alumina will favour the formation of calcium silicate crystals during cooling. This mechanism promotes opacity.
Oxides - ZnOIn larger amounts ZnO can produce opacity or whiteness in glazes. It exhibits refractory properties and can contribute to the development of a crystal mesh surface.
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