Listed below are some of the more common glaze defects and firing faults and possible ways to correct them.
This is caused by too much reduction in the early stages of the firing (usually from 600 to 1000 degrees celsius). Black coring makes the work brittle and easily broken. Evidence of black coring is a blackening in the centre of clay walls. To fix; keep the kiln oxidising until 1000 degrees, then go into reduction.
Blisters of glaze, or craters that are the remnants of burst bubbles, sometimes revealingCratering the bare clay underneath. are often a fault of over-firing or of firing too fast. If the blisters are more uniform and small then it can be a result of firing a high alkaline glaze too low. Glazes move through a boiling phase prior to smoothing out to a glossy glaze, then if heating continues they can start to boil again, depending on the materials used and the kinds of gasses released.
Bubbles appear inside the clay walls, making a dimpled appearance on the surface, or in extreme cases, large blister like bumps. This is a result of gasses being trapped inside the clay which has been taken to the point of vitrification. Reduction during the bisque firing leaves some carbon in the clay which causes bloating; always have an oxidising bisque firing. Some stoneware clays will bloat if fired too hot in an oxidising kiln, to fix this problem keep some reduction in the kiln throughout the last stages of firing.
The shrinkage of the glaze during the early stages of melting pulls the glaze off the pot leaving bare unglazed areas. Most often caused by dust, dirt or oil on the bisqued pots. Other causes are surface texture, very viscous glazes, too thick glaze slop, or pots that are damp. Applying more glaze to the affected area and re-firing usually saves the pot. Some special effect glazes use certain ingredients deliberately to create a crawling glaze.
A network of fine cracks through the glaze. Caused when a glaze contracts more than the clay during cooling. A large mismatch in shrinkages will result in fine crazing. Alkaline glazes or very fluid glazes tend to craze, even if not immediately obvious, over time craze lines will appear. Adjust the glaze by adding silica or alumina, or replace the alkaline fluxes (soda and potash) with alkaline earths (calcium, magnesia) or with boron frits.
A glaze with a dry surface texture can be the result of underfiring the glaze, too thin an application, or a glaze with too much alumina present.
Cracking in the fired pots that can often be attributed to uneven or too fast a cooling of the kiln. Large flat forms that have a lot of contact with the shelf should be cooled very slowly. A clay with too much silica in it will also cause dunting as the pot cools the last 600 degrees. A glaze that shrinks less than the clay can also break the pot when cooling, particularly if the glaze layer is thick on a thin walled pot.
Almost always a result of the pot being heated too fast to allow the water present to turn into steam and evaporate slowly. The pressure of steam builds up and shatters the pot. It is important to heat the kiln slowly for the first 300 degrees.
Usually the result of a slip or engobe being applied too thick. Slips should be applied to damp ware, engobes can be applied to bisque, but only with a thin coating.
Tiny holes in the surface of the glaze, sometimes penetrating down to the surface of the clay. Can be the first sign of blistering, or crawling. Soaking of the kiln at top temperatures for up to a hour can help the glaze to heal over. Some viscous glazes or those with fluorine in them will pinhole more easily.
The reverse of crazing, shivering is a result of a glaze that has a lower shrinkage than the clay. It can dunt a pot or flake slivers of glaze off rims or edges. Increase the alkaline content of the glaze at the expense of the alumina or silica.