ZrO2 (Zirconium Dioxide)

Data

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

Notes

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 Zircopax only whiten and opacify a clear glaze? No.

This GLFL test for melt flow demonstrates how zircon opacifies 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

All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).

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.

Adding Zircopax to G2571A dolomite matte

On the left: G2571A dolomite matte fired at cone 10R. 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.

Links

Materials	Zircon
Oxides	ZrO - Zirconium Oxide, Zirconia

Mechanisms

Glaze Opacifier	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.

By Tony Hansen
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