Glaze Crazing

Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.

Details

The fired glaze exhibits a network of fine cracks. These may be visible after firing or may need enhancement with ink. Crazing may also appear after some time or after ware has been exposed to thermal shock.

Crazing profoundly affects ware strength. Ceramics are brittle, when cracks start they propagate, especially in vitreous ware. Crazed ware, out of the kiln, is thus pre-cracked and ready to fail. This is an inherent weakness because the cracks provide failure initiation sites, when ware is subjected to stress they jump into action. The author has measured 300-400% reductions in the strength of freshly fired ware with crazed glazes. A cone 01 piece having a well-fitted glaze will be stronger than a cone 10 one with a poorly fitted glaze!

The chemistry profile that creates the most crazing can also be inherently more leachable (e.g. the high KNaO and very low Al₂O₃ common in crackle glazes make them susceptible to leaching of added metallic colorants).

If the clay under a crazing glaze is porous and soaks up water then safety could also be a concern. The cracks and their access to a possibly porous body below can create a bacteria zoo, requiring extra care in cleaning and sterilizing ware. Containers used to store food are a special concern since a small colony in a crack can become a large culture in the food. If you have any doubt whether this is an important issue ask a commercial food service inspector about the subject. Consider also that the chemistry profile that creates the most crazing is also inherently more leachable (e.g. the high KNaO and very low Al₂O₃). While most crazed glazes are white or clear, it is common in crackle to add metallic colorants to them, these would be released into food and drink.

Is the crazing due to a thermal expansion mismatch between body and glaze?

This is by far the most common cause of crazing and solution strategies are case studies in glaze chemistry. Often by just looking at a recipe it is obvious it will craze (because it violates common sense recipe limits). Fired ceramic expands and contracts as it is heated. If the fired glaze has a significantly higher coefficient of expansion than the body then no amount of careful firing or thin glazing will avoid the inevitable crazing. If even only one piece crazes (assuming it has reached maturity) it is often a sign that all the other ware in that kiln will eventually craze. Such glazes usually need drastic changes since crazing is a visible manifestation of a fit problem that has already greatly reduced ware strength. Lower temperatures are more sensitive in this respect - there is a narrower range within which a glaze and body will be compatible.

Options to improve glaze fit that do not involve knowing about chemistry.

Perhaps the best option is to switch to 325 mesh silica, it dissolves into the melt much better than 200# silica (this can be surprisingly effective when the percentage of silica in the recipe is significant e.g. 20-35%).
Transplant the color/opacity/variegator mechanisms of the glaze into a base recipe that already works on your clay bodies. This most often works, but when not it is because the mechanism percentage is high enough to contribute to crazing or it is not compatible with the chemistry of the new host
Adjust the clay body to give it higher expansion and thereby greater contraction, thus putting glazes under compression. At low temperatures, this can be done by adding talc or dolomite, but it takes a lot. The former has health implications, the latter greatly increases porosity. At high fire, adding silica works but this is tricky because more feldspar is required to maintain maturity. This brings a bigger problem: Those increases necessitate a drop in clay content, thus less plasticity, possibly much less. Another side effect: Greater susceptibility to cracking when heat-shocked by hot liquids in use. So this approach is often a non-starter.
If a glaze is melting well and it is a gloss increasing the silica by 5 or 10% might help (this will only assist if the craze lines are far apart or crazing is delayed).
Zirconium opacifiers are also useful in transparent glazes; they have a threshold amount under which they do not normally opacify. Thus it might be possible to add as much as 3% to make the glaze both more durable and reduce its expansion a little.
Substituting a frit for one of lower expansion can be very effective, especially if the frit percentage is significant (e.g. 30%+). This is best for transparent, opacified white or stained glazes (since effects on color, texture and variegation are less likely). But for mattes and glazes have mechanisms involving rutile or titanium, frit changes are highly likely to change visual appearance.

Glaze thermal expansion is a product of its chemistry (provided it is completely melted). So being able to do the chemistry enables employing by far the most effective method to adjust expansion:

Reduce the amount of high-expansion oxides (like sodium and potassium) and replace them with oxides having similar function but of lower thermal expansion (e.g. MgO, CaO, ZnO).
Introduce boron at the expense of some of the flux (since B₂O₃ contributes to both glass development and melting).
If a glaze is glossy and well-melted: Increase the SiO₂.
The glaze is matte consider its mechanism and adjust with that in mind. For example, MgO mattes can often tolerate more MgO (which reduces expansion). Or CaO mattes may tolerate adding MgO. Alumina mattes, if well melted, might tolerate more Al₂O₃.
Add more flux to enable adding more SiO₂ and Al₂O₃ (the latter are both very low expansion). A small Li₂O, ZnO or B₂O₃ addition is an example.

Consider also the elasticity of the glaze as even relatively well-fitted ones can craze if exposed to radical temperature changes. High levels of sodium, potassium and calcium can make the glaze more brittle (the former also increase thermal expansion). Boric oxide is known to improve elasticity.

If the body expansion is too low (i.e. ovenware and flameware bodies) it can be very difficult to fit a glaze that has the desired visual characteristics. Lithium can dramatically reduce the thermal expansion of glazes, but its use requires a lot of testing since its contribution is not linear and it introduces other dynamics that must be considered.

Is ware crazing days or even months after firing?

If you are cooling your kiln very slowly to prevent ware from crazing it is likely the glaze does not fit. While it may be true that slower firing seems to solve the problem, time will bring out the crazing that the kiln did not. In fact, if you must slow cool to prevent crazing it is a virtual certainty that your glaze needs to have its thermal expansion reduced.

Could the coloring oxides in the glaze be involved?

Generally increased additions of iron and copper oxide to a glaze will reduce crazing (e.g. beyond 1 or 2 percent). Cobalt could have a moderate lowering effect, but since so little is typically used in glazes it will not be significant.

Is the crazing a result an under-fired body?

Underfired low-temperature bodies may contain uncombined alkali or alkaline earths that can react with water and cause the body to expand slightly. Check if crazing occurs in a glazed sample after several hours in a pressure cooker (or put a shard into an autoclave). Calcium carbonate is added to talc bodies to minimize moisture expansion. To avoid this it is best to leave a minimum of unglazed body surface and plug that with silicone sealant. Body formulations also often include a small percentage of calcium carbonate which is said to help prevent this phenomenon. Porcelain bodies also often craze glazes if not vitrified enough.

Is the crazing a result of sloppy manufacture?

Normally a glaze/body combination with compatible expansion characteristics will withstand considerable firing and usage abuse without displaying signs of crazing. However, in some cases, a glaze that otherwise 'fits' will craze if applied very thick.

Also, if the kiln is cooled very quickly or unevenly, especially if ware is thicker, the severe stresses can produce crazing. However, remember that a glaze's ability to withstand normal or even quick kiln cooling is an indicator of its ability to resist crazing in normal use.

Is it a result of mistreatment of ware during use?

If pieces must survive considerable thermal shock during use, then both ware and glaze need to have a low and linear thermal expansion curve and they must be compatible. This is difficult to achieve in low-fire ware because little mullite or other low-expansion silicate minerals develop during firing. If your low-fire body contains significant talc, reduce or eliminate it (also adjust glazes to have a lower expansion so they continue to fit the body). If your high-fire body develops non-linear expanding cristobalite during firing, find a way to reduce this.

Is it a result of inappropriate choice of manufacturing method or materials?

High-temperature firing is by far the best for the production of low-expansion ware. Many more minerals are available for both body and glaze mixes and higher temperatures produce low-expansion silicates and aluminates that give tough glaze and body matrixes capable of withstanding forces that might otherwise cause crazing.