Kiln Connection: Glaze Defects Got you in the Pits?
Pitting of glazes is common. Pits in the glaze are small depressions and surface roughness, generally caused by glaze bubbles that have broken through the surface of the glaze, but which have not smoothed or healed. There are many causes, and likewise many solutions. Identifying where in the kiln it happens will help determine what to do about the problem.
Solving the ProblemTo discover the root cause of pitting, start by looking at the piece:
If the first case is true, it is likely that the pitting is caused by insufficient fluidity, or out-gassing of the base body (or glaze) at high temperature. Run a thermogravimetric analysis (TGA) on the body alone, then the body and glaze together to determine if off-gassing of the body (and hence loss of weight) is occurring at elevated temperatures. (The TGA will measure the rate at which the material changes weight on heating plotted against temperature.) Often CaCO3 will decompose and produce CO2, and these gas bubbles burden the glaze. Those bubbles that break the surface result in pits. In this case, consider altering the glaze or body constituents. Or, if warranted by the TGA curve, fire with a longer soak at lower temperature if this will prevent the off gassing at high temperatures. If no indication of out-gassing is obvious, consider altering the viscosity of the glaze at high temperature to obtain better flow and smoothing.
On the other hand, if the pitting is localized, look at the area where pitting occurs. Most likely, localized pitting occurs where there is excessive body thickness, double glazed sections, or any other combination of body/glaze design that hinders the removal of volatiles from the body. In this case, the root cause of pitting is trapped gases, and this typically has occurred early in the firing cycle. This has been the most frequent cause of pitting in cases I have worked on. Remember that evolved gases must pass through the glaze before the glaze softens. For this to happen a number of factors are vital: