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Why does my glaze craze, and should I care?

It is important to remember that crazing (and its opposite fault, shivering/chittering) reflects mismatched clay and glaze combinations. So – the first solution may be to look at your material selection and the reasons for that selection.

There are possibly thousands of different glazes in use and in New Zealand, scores of different clay bodies. Each has a useful firing range within the material may give a satisfactory result but where the result will likely vary with changing temperature. Each material has a unique COE (coefficient of expansion), that is to say that each fired glaze or body expands and contracts at a rate that reflects its composition and the firing process.

At Decopot we are careful to align the expansion properties (at approx. 1180C) of Abbots Clear and our most popular white bodies: PW20, Midfire Porcelain and our RAM pressing body used to make the bisque that we sell.

Glaze fit is not an issue at high temperatures where the body may be softened, and the glaze is a fluid. As the piece is cooling in the kiln it will reach a temperature (different for all glazes) where it freezes or solidifies. At this point both glaze and body are solid and shrinking as they cool further in proportion to the COE. If the COE of the glaze is higher than the fired body the glaze will probably craze as it cools due to the stress between the two solid materials. A small COE differential may result in slight crazing (indicated by few, widely spaced crazing lines) and possibly only long after the kiln has cooled. Consider all the antique pottery with heavy crazing which has developed over years or decades. A large COE differential may result in a fine crackle, usually visible when the kiln is opened or soon after. Crackle glazes are examples of deliberately causing crazing with a high COE glaze. Even in this case the crazing effect will be different across a range of bodies.

Crazing is not the only fault that may be triggered by glaze fit issues. If the COE of the glaze is much lower than that of the clay body, then it is the body that will be under tensile stress on cooling as it shrinks more than the glaze. It is beneficial for the body to be under modest tensile stress as this keeps the glaze in slight compression, enhances the strength of the piece and resists crazing even when subject to rapid temperature changes. In severe cases where the body is under tension and the glaze under compression, glaze may chitter or shiver off the pot – usually on the edges, spouts, and handles. Less severe cases may be prone to dunting (cracking when exposed to thermal shock). Mugs prone to dunting may crack when filled with boiling water.

Whilst it is logical that these faults can occur, they can be managed. Your options are to review your material selection or to modify the materials. Unless you are using casting slip, there is no easy way for you to adjust the body, but you can adjust the glaze. Some visual glaze characteristics may change as the result of such adjustments.

Adjusting a glaze to reduce crazing:

If your glaze is crazing and if it is bright and glossy it will likely dissolve more low COE materials in the melt.

Silica: Increasing silica IN A GLAZE reduces the COE and crazing risk as long as it dissolves.

Kaolin: Increasing kaolin in a glaze will also reduce the COE. It may have the secondary benefit of improving suspension in the bucket.

Talc: Adding Talc to a glaze will usually lower the COE. In excess it may have a matting effect (at which stage it is developing crystals rather than dissolving) and the magnesium in talc can affect some colours.

Boron: Adding a high boron frit, usually as a minor component with the other materials mentioned can improve the solubility of those materials in the glaze. By themselves, most frits will increase the COE.

These three materials can, of course be added in combination with one another. Potters wishing to make these adjustments should do so incrementally and keep a careful record of both adjustments and outcomes.

Adjusting glazes to reduce/eliminate dunting:

Here we are looking for high COE materials that can be added to increase the overall COE of the glaze. Again, there may be impacts on the visual result. A benefit of this is that an adjusted glaze is uniquely yours. Again, the opportunity to make effective adjustments will depend on the ability of the glaze to dissolve or incorporate the modifiers.

Our best high-COE options (which may be used in combination) are:

Potash Feldspar

Nepheline Syenite

High alkaline frit – eg Frit 4110

Calcium Carbonate/whiting.

If you are experiencing crazing with your favourite glaze and clay body then it may be effective to perform the adjustments suggested, or even increasing the firing temp somewhat. If the glaze continued to craze – ask yourself if it really matters to you.