Crazing is the condition when the glaze on earthenware reveals spider like hairline cracks. Sometimes, small patches that cover an area where the glaze was too thick. Most often, crazing shows a network of hairlines that encompass to entire piece. These may be plainly visible after firing or may need enhancement with ink. Crazing may also appear after a period of time or after ware has been exposed to thermal shock. Fired strengths are directly related to crazing since ware strength is enhanced by having the glaze under slight compression whereas it is severely reduced when the glaze is under tension. If the underlying clay is porous, it tends to soak up water or beer. Then safety could be a concern with crazed ware since the cracks could be wide enough to provide a friendly breeding ground for colonies of bacteria. 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.
Is the crazing a result of the mistreatment of your beer mug?
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 crazing a result of poor craftsmanship?
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. If ceramic ware is porous it can soak up water that causes the ware to expand, thereby putting tension on the glaze and crazing it.
Is crazing due to a simple thermal expansion mismatch between body and glaze?
Fired ceramic expands and contracts as it is heated. If the fired glaze has a significantly higher co-efficient of expansion than the body then no amount of careful firing or thin glazing will avoid the inevitable crazing. This is by far the most common cause of crazing and solution strategies are case studies of applying ceramic calculations to solve problems. If even only one piece crazes 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 far more sensitive in this respect in that there is a much narrower range within which a glaze and body will be compatible. To improve glaze fit adjust the clay body to give it higher expansion and thereby the greater contraction that compresses glazes to prevent crazing (i.e. increase flint for high temperature bodies, talc at low fire). You can also adjust the glaze to reduce its expansion. There are many ways to do this. For example, if the glaze is melting well and it is not a matte, try increasing the silica. Or try introducing boron at the expense of some of the flux since B2O3 contributes to both glass development and melting. You can also introduce fluxing oxides of lower expansion at the expense of those with higher expansion in such a way that the fired properties are not changed too much; for example try adding CaO, MgO, or ZnO at the expense of Na2O and K2O (crazing is most serious with sodium and potassium glazes, to demonstrate mix nepheline syenite and water and apply as a glaze and fire at high temperature). If your glaze is opaque try using more low-expansion zirconium opacifier or use it instead of tin or titanium. 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 5% to make the glaze both more durable and reduce its expansion. 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.
Could pottery craze months or even days after firing?
Yes, as learned by the previous text, earthenware can start crazing before it is removed from the kiln. Keep in mind that there are ceramist among us that might speed up the kilns cool down time to induce crazing. Suddenly they have a mug that looks 40 years old. Fakes are a reality everything. Even American Indian arrowheads. Have some people no shame? Yet with that small percent of those with ill intent set aside, we can appreciate the skill and knowledge required to be a Master Ceramist.