Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms

Section: Glazes, Subsection: Thermal Expansion

Description

Band-aid solutions to crazing are often recommended by authors, but these do not get at the root cause of the problem, a thermal expansion mismatch between glaze and body.

Article

While potters can be very creative and inventive, they can definitely learn something from industry when it comes to dealing with crazing. First, you can't ignore the hygienic and strength issues associated with the problem. The second is that although a piece may not exhibit crazing out of the kiln, glaze fit may still be very poor, it may only be a matter of time until it shows up.

Crazing is due to a thermal expansion mismatch between body and glaze. As a piece of ware is heated and cooled during normal use, it expands and contracts. An incompatible clay and glaze usually means the glaze either immediately or eventually fails by crazing or shivering (the former being more common).

Thus since glaze and clay are inseparably joined they must be expansion-compatible, no amount of 'covering over' this incompatibility is going to make it disappear. You may well lengthen the time before crazing starts but it will not change the fact that it will happen. If crazing is the result of an expansion mismatch between body and glaze, then the only real solution is to adjust the expansion of one or both. If a suggested solution doesn't accommodate this, then you are treating the symptoms and not the cause.

In middle-fire and high-fire where glaze and interface development is good, the appearance of crazing suggests a huge incompatibility, not a small one. Strength tests have demonstrated up to a threefold difference in the strength of glaze test bars between high and low expansion versions of a glaze without visible crazing to suggest a problem. This means that by the time crazing shows up, there are already unreleased stresses that result in a big strength problem. To fix it, a significant change is needed, one that not only addresses fixing the crazing phenomenon but that restores the fired strength of the clay/glaze 'marriage'.

The bottom line is that crazing is best solved on the oxide level for the glaze and the material level for the body. That means body testing and glaze chemistry are involved in any real solution.

Often articles on how to deal with glaze crazing appear in ceramics journals. Unfortunately many continue to perpetuate a treat-the-symptoms approach rather than getting to the root of the problem. Consider some of the suggestions often given:

Apply a thinner glaze coat
Either a glaze fits or it does not. Putting it on thinner does not change that. It could be argued that a greater percentage of the total glaze thickness is a part of the clay-glaze interface on a stoneware piece and thus a better fit results. Still, either it fits or it doesn't and if it doesn't then every time the glaze gets applied too thick it is going to craze. Also, glazing thinner might appear to prevent crazing, but time and delayed crazing will be the true indicator of whether the glaze really fits or not.
Add increasing amounts of silica
It is true that this does work in some cases of slight crazing. In more serious cases, it is surprising how much must be added to reduce expansion to any degree. Yes, silica is a low expansion oxide, but if the glaze is full of high expansion fluxes like sodium and potassium adding silica is a little like adding white paint to dilute the color of black. Additions of silica are often inappropriate for other reasons. The amount of silica in a glaze is a critical player in its appearance and surface texture; what is there, is usually what is required. Adding enough to simply hide out-of-the-kiln crazing does not deal with the deeper fit problems, these weaken the ware and will eventually craze it anyway. Adding more can introduce unwanted gloss, higher melting temperatures, and change in surface character. Since mattes, which usually maintain a critical balance of these very factors, are the ones that often craze, adding silica is not a solution. In addition, highly melted special-purpose glazes typically depend on low silica for their unique character. What about functional glossy glazes? Many will tolerate silica additions, but again, to put enough in to solve the problem will normally detrimentally affected melting.
Firing to correct cone over a longer time.
The contention here is that the clay will tighten and have a better interface with the glaze, this better interface will produce a better the fit. But a better interface for a crazing glaze does not mean a better fit, it means a forced fit. Firing to improve the clay-glaze interface to solve a clay-glaze expansion mismatch is misdirected. If the expansion is wrong, then this is what should be changed. The interface is conceptually the 'glue' that holds the glaze on. This approach is like saying; "If they are not compatible, then glue them together more strongly". Surely, it is best to make them more compatible. Again, time will be the true test of compatibility.
Fire higher, if glaze can take it, and they 'might fit better', 'what you are trying to do is change the range shrinkage of either the clay body, the glaze, or both in order to bring the glaze under slight compression'.
Firing a glaze higher on the chance that it 'might' fit better is not really a methodical approach. Incompatibilities don't normally just "go away". Firing higher is likely to introduce new problems with vitrified bodies or visual differences in the glaze. In fact, firing higher could enable the feldspar to take more body quartz into glass solution, reducing the body expansion and making crazing worse. Firing higher will work in one situation: An under fired body will absorb water and the moisture expansion will craze the glaze, firing it higher will make it less absorbent.
Add increments of 5% silica to the clay body. It remains crystalline.
True, adding coarser sizes of high-expansion silica has the effect of compressing the glaze (by virtue of its 1% contraction going down through quartz inversion at 1000F). However, it is not the whole matrix that contracts, rather it is each of those little independent crystals. If temperature gradients exist in the cooling ware, waves of stress ripple through it. On the other hand, if the body is not vitreous and very fine silica is added, the extra will accelerate crystobalite formation (because of the lack of glass development to arrest the conversion). The result could be ware likely to crack or craze due to the 3% expansion associated with repeated heating of a crystobalite body through 220°C during common use. Many bodies are already optimized to have the maximum practical silica. Adding more has to be done at the expense of other materials in the mix. If the clay component is reduced, the body will be less plastic; if the feldspar is cut back, the result will likely be a less vitreous and weaker material. Adjusting body expansion is a complex process that must consider glass development, the size and availability of quartz, and the types of clay minerals employed. At any rate, it is not usually practical to adjust the body because most people use prepared clays.
Slow cool the glaze kiln, don't open it until it is below 200 °C (390 °F)
If the glaze can barely withstand a normal cool cycle after firing, then how will it take the thermal punishment of the dish washer or the normal hot-cold shocks of everyday use? It is possible that a glaze under tension can be eased down and may well appear to be OK when the kiln is opened. But the first thermal stresses it undergoes in use will reveal what the slow cool temporarily covered up.
Bisque higher if low fire glaze is not fitting
Making a glaze fit on a low temperature body where there is a lack of clay-glaze interface is much more difficult. Band-aid approaches just won't work, the thermal expansion of the clay and glaze must match closely. It is true that crazing will be evident at low-fire with a much smaller difference between clay and glaze expansions than with stoneware. Thus smaller changes have a bigger effect. It is true that firing higher can change a clay body's expansion. However, the above approach will only work if the increased temperature lowers the expansion and if the glaze firing is lower than the bisque. However, adjusting low fire glazes with boron sourcing materials (i.e. frits) is much more practical. Low expansion frits, specifically targeted as additions to reduce crazing, are available.
Change to a frit of lower expansion
This suggestion goes in the right direction as it proposes to change the expansion. However, few frits are used in stoneware glazes (the few that are are normally there for a very specific purpose). Thus simply changing to a frit of different chemistry and lower expansion is likely to mean altered glaze properties unless you know the implications. In lower fire glazes, this approach is much more common since many boron frits are quite balanced, almost stand-alone glazes by themselves.

Rather than thinking of crazing as something that is "caused" by something else, it is better to think of crazing as evidence that a clay and glaze are not expansion-compatible. Viewing crazing as a material level problem might be OK for certain highly fritted low temperature glazes, but it does not work for stoneware glazes. It is oxides of high expansion that cause crazing, not materials. That is why crazing is a problem that is much more effectively solved on the oxide or formula level using simple calculations (e.g. substituting fluxing oxides of lower expansion for those with higher expansion).

In the end, while it may be feasible to increase the body expansion, most often you are going to reach the conclusion that controlling crazing is most easily done by adjusting the chemistry of the fired glaze in such a way that fired properties are maintained while expansion is decreased. Just remember, if a glaze crazes, it is a sign of big trouble; not the type of thing that small adjustments, material additions, or firing changes are going to easily solve.

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Authors

Tony Hansen (Owner)

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