Crazed pottery or ceramics is not food-safe as the cracks can harbour bacteria or germs.
Crazing is a network of fine hairline cracks that appear on the glaze surface of a fired piece.
There are two types of crazing - immediate and delayed:
Generally, crazing is considered a glaze defect (e.g the glaze is weaker than the un-crazed clay body surface).
Most things expand when hot and shrink/contract when cool. A clay body and glaze fuse together during firing. The aim is to get the right combination of expansion and contraction for a good glaze fit.
Interestingly, the time and temperature of the firing is extremely important to the process and will affect the type of crazing pattern that occurs.
Another potential reason for crazing can happen after the ware has been put into use. If the clay body is porous (e.g earthenware clay) the fired item may absorb enough moisture to expand slightly. That expansion can be enough to upset the glaze-clay body fit and cause the glaze to craze. Continued heating and cooling of a piece e.g. ceramic tiles that are outside in all elements and mugs, bowls, teapots that are repeatedly used in the microwave, dishwasher or oven.
Under-fired or Over-Fired Earthenware Bisque
Sometimes under-fired bisque can cause crazing. Tests to determine whether earthenware is under-fired can often be inconclusive. However, a starting point (but not proof) may be the "tongue test". To do this, simply touch the bisque with your tongue. If the tongue does not stick to the bisque pottery then it is likely that the pottery has been OVER-FIRED. However, if your tongue sticks to the bisque and has a strong pinch, then the pottery has possibly been UNDER-FIRED. Please note this is not an accurate test - it is merely a guideline. Under-fired bisque can be repaired by re-firing the pottery to the correct programme (ramp time, soak and top temperature).
Sudden Temperature Changes and Prolonged Use
DO remember that our bisque is ceramic earthenware. Earthenware is porous and it will trap moisture. Therefore, extreme care should be taken as the pottery can get VERY HOT.
DO allow pottery to heat up gradually i.e. warm up teapots prior to use, warm pieces to room temperature after removing from refrigerator, don’t place cold wares in hot water.
DO allow hot pottery to cool down slowly i.e. don’t place hot wares in cold water.
DO NOT use domestic appliances if the piece is precious. Continual expansion and contraction of the piece will weaken the pottery over time and may cause cracking.
Glaze defeats and how these occur can be explained by Orton Ceramics in their helpful PDF document CLICK HERE to view.