Digitalfire Ceramic Glossary



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Glaze fit


The relationship between the thermal expansion of body and glaze is called the "glaze fit". Glazes fit the clay body they are on when they do not form the familiar crack pattern of crazing when the ware is suddenly cooled or flake off on contours when ware is suddenly heated. Thermal expansion happens in thousandths of an inch as ware is heated during dish washing, in the oven, on the burner, etc. This sounds like a small amount of change, but ceramics are brittle and if the glaze is expanding or contracting more than the body to which it is attached, something has to give way. Thermal expansion and contraction are not to be confused with fired shrinkage that happens as a body densifies during heatup in the kiln.

Failure due to a poor fit glaze does not always happen right away, sometimes the glaze has enough elasticity to resist cracking or peeling, but repeated heatings and cooling will eventually reveal a poorly fit glaze. Ideally a glaze should have an expansion that is slightly lower than the body so that contraction during cooling puts the glaze under compression. This prevents crazing, the most common misfit.

Each clay:glaze combination needs to be considered on its own, tested as a unique instance. The 300F:Ice water test is a way to reveal the tendency of a glaze to craze. When people or factories make their own glazes they are in a position to adjust the chemistry to move the expansion up or down to deal with any misfit (lots of references are available on this site on how to do this). A less ideal solution is to change the recipe of the clay body, but few people are in a situation to do this. People who use commercial glazes are the most vulnerable, when these glazes fit their clay bodies it is actually just an accident.

Many suggestions can be found in books and on the internet on how to fix poor glaze fit. These often recommend firing or glaze thickness changes and use fancy words like "cristobalite inversion". But do not fall for band-aid fixes, if a glaze does not have a thermal expansion that is compatible with your clay body then the best approach is to change the expansion of one or the other. You can reason this out as follows: If a glaze is crazing it is stretched on the ware and has cracked to relieve the stress. The stretching happened because during cooling in the kiln the glaze contracted more than the body. So to fix the problem the glaze needs to have a lower contraction (and thus lower expansion). Or, the body needs to have a higher contraction (or higher expansion).

Pictures

Simple dilatometric curve produced by a dilatometer

Dialometric chart produced by a dilatometer. The curve represents the increase in thermal expansion that occurs as a glass is heated. Changes in the direction of the curve are interpreted as the transformation (or transition) temperature, set point and softening point (often quoted on frit data sheets). When the thermal expansion of a material is quoted as one number (on a data sheet), it is derived from this chart. Since the chart is almost never a straight line one can appreciate that the number is only an approximation of the thermal expansion profile of the material.

Why are these crazing lines dark like this?

This is an example of serious crazing in a glaze. The lines have gotten darker with use of the bowl! That means the color is organic, from food. This cannot be healthy.

Shivering on a transparent over an engobe

Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This is a low quartz cone 03 vitreous red body having a lower-than-typical thermal expansion. The white slip (or engobe) has a moderate amount of quartz and is thus put under some compression by the body. But the compression is not enough to shiver off (e.g. at the rim) when by itself. However the covering glaze has an even lower expansion exerting added compression on the slip. This causes a failure at the slip-body interface.

If you think one slip fits any body, think again

This flake shivered off the rim of a low fire terra cotta mug. It is Fishsauce slip. It is about 2 inches long and has razor sharp edges. This is not the sort of thing you would want to be falling into your coffee or food and then eating! This flake did give evidence that it was loosening so there was little danger of me consuming it, but smaller flakes can go unnoticed. Slips (or engobes) must be drying compatible, have the same firing shrinkage, the same thermal expansion and be quartz inversion compatible with the body. It is easy to ignore all that and pretend that it works, but the bond between engobe and body is fragile at low fire and easily compromised by the above incompatibilities. Slips must be fitted to the specific body, glaze and temperature; that involves a testing program and often a little chemistry. I have documented online how to I adapted this slip to Plainsman Terrastone 2 using my account at insight-live.com.

This glaze fits this clay better at cone 03 than it does at 04 or 02

This is a talc body (Plainsman L213, about 50:50 talc:ball clay). They are fired to cone 04 (left), 03 (center) and 02. The glaze is G2931F, it fires crystal clear. Each of these cups has been subjected to "boiling water to ice water to boiling water" immersions. The cone 04 one crazed. The cone 02 cup cracked (the denser matrix could not withstand the shock) but did not craze (although it showed a hint of shivering). The center cup, fired at cone 03, is perfect.

An extreme example of what happens when glaze is under excessive compression

This is a low temperature slip-cast vase. Only the outside of the vessel is glazed. It burst apart like this on its own after firing.

Out Bound Links

In Bound Links

  • (Recipes) G1916Q - Low Fire Frit 3195 Glossy Transparent
    An expansion-adjustable cone 04-02 transparent glaze made using three common Ferro frits (low and high expansion) and a suspension strategy that produces an easy-to-use slurry.
    2014-09-30 - This recipe contains a very high percentage of fri...

By Tony Hansen




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