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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.

Off-white

  • Oxides - TiO2

    Titania in significant amounts (+5%) will almost always contribute to a glaze opacity of mottled and variegated character.

Opacity

  • Materials - Rutile

    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 - Talc

    Talc is a refractory powder and can promote matteness and opacity when added to low-fire glazes.
  • Materials - Tricalcium Phosphate

    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.

Opacity and Matte

  • Materials - Magnesium Zirconium Silicate

    Produces opacity and a soft matte texture without the need for magnesia compounds which might devitrify.

White

  • Materials - Alumina Hydrate

    Alumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.
  • Materials - Arsenic Oxide

    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.
  • Materials - Barium Carbonate

    If available in sufficient amount, barium oxide will promote crystallization of a melt during cooling, thus imparting a measure of opacity.
  • Materials - Bone Ash

    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.
  • Oxides - Sb2O3

    Antimony works to a limited extent as an opacifier to cone 1.
  • Oxides - CeO2

    Used as an opacifier for special effects in the tile industry; as a replacement for tin opacifier in porcelain enamel.
  • Oxides - ZrO2

    Zirconium 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.
  • Materials - Tin Oxide

    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.
  • Materials - Titanium Dioxide

    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.
  • Materials - Zinc Oxide

    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.
  • Materials - Zircon

    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.
  • Materials - Zircopax

    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.
  • Materials - Zircopax Plus

    Superpax is a very fine grade zirconium silicate opacifier and will produce a whiter result than less expensive grades. See Zircopax.
  • Oxides - As2O3

    Arsenic can be used as an opacifier in glazes, although not as effectively as tin.
  • Oxides - TiO2

    Additions 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 - SnO2

    Tin 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.
  • Materials - Calcium Carbonate

    High calcium coupled with lower alumina will favour the formation of calcium silicate crystals during cooling. This mechanism promotes opacity.

White/Off-White

  • Oxides - ZnO

    In 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.

Out Bound Links

  • (Glossary) Opacifier, Opacification

    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 ...




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