Ceramic Oxides

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ZrO2 (Zirconium Dioxide)

COLE - Co-efficient of Linear Expansion 0.020
GSPT - Frit Softening Point 2700C


Zirconium oxide is extremely refractory, even more so than alumina. Although a variety of figures are quoted, around 2700C (it is difficult to accurately determine the exact melting temperature of pure oxides). Like alumina, it maintains its refractory character even within a mix of other oxides. It is primarily used as an opacifier in glazes, it does not easily go into solution in the glaze and so does not participate in the chemistry (see notes on Zircon about its opacification). It is normally used in the form of zirconium silicate.

It is beneficial to add zircon to transparent glazes in amounts up to 3% to improve hardness and duravility. This is because small amounts can be taken into solution and will therefore not opacify.

ZrO2 reduces the thermal expansion of a glaze (although not by the same mechanism as other oxides that impose their expansion by going into solution in the glaze melt). Thus its contribution to the calculated thermal expansion of a glaze may differ unexpectedly from what it actually does in the kiln.

ZrO2 reduces glaze melt fluidity, both because of its high melting point and high surface tension.

It is commonly used in stains to stabilize colors and as a mechanism to encapsulate otherwise volatile oxides.


Does Zircon only whiten and opacify a clear glaze? No.

This melt flow tester demonstrates how zircon opacifys but also stiffens a glaze melt at cone 6. Zircon also hardens many glazes, even if used in smaller amounts than will opacify.

Ceramic Oxide Periodic Table in SVG Format

The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.

How do you turn a transparent glaze into a white?

Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.

The action of Zircopax vs Tin Oxide at cone 10R

On Plainsman H443 iron stoneware in reduction firing. Notice Tin does not work. Also notice that between 7.5 and 10% Zircopax provides as much opacity as does 15% (Zircon is very expensive).

Adding an opacifier can produce cutlery marking!

G2934 cone 6 matte (left) with 10% zircopax (center), 4% tin oxide (right). Although the cutlery marks clean off all of them, clearly the zircopax version has the worst problem and is the most difficult to clean. To make the best possible quality white it is wise to line blend in a glossy glaze to create a compromise between the most matteness possible yet a surface that does not mark or stain.

Opacifying a reduction dolomite matte

Opacifying a cone 10 reduction magnesia matte glaze. On the left: G2571A dolomite matte, a popular recipe (from Tony Hansen). Right: 10% Zircopax has been added. Both are on a buff stoneware (H550 from Plainsman Clays).

Cutlery marking in commercial tableware

Even commercial dinnerware can suffer cutlery marking problems. This is a glossy glaze, yet has a severe case of this issue. Why? Likely the zircon opacifier grains are protruding from the surface and abrading metal that comes into contact with it.

Out Bound Links

  • (Materials - Material source) Zircon - ZrO2.SiO2

    Zirconium Silicate

  • (Oxides - Related) ZrO - Zirconium Oxide, Zirconia

By Tony Hansen

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