Identifying where in the kiln pitting happens will help determine what to do about the problem.
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 Problem
To discover the root cause of pitting, start by looking at the piece: Does the pitting occur all over the piece, more or less uniformly?Does the pitting occur in specific locations on the piece? For example is it more prevalent on double glazed surfaces, or thick/double walled sections? Is there any evidence that the glaze thickness is very high in the pitted areas?
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: The body must have enough open porosity for free gaseous flow. Excessively fine particles or sealing the surface of the pieces during drying can cause significant problems here.The kiln preheat oxygen level must be sufficient for oxidation of the volatile constituents-various compounds of carbon and hydrogen need oxygen to oxidize. A rule of thumb is that O2 levels must be greater than 10% up to 850°C.There must be sufficient time at elevated temperatures (between 600 and 800°C) for oxidation to take place, with good circulation.The sulfur content of the body must be low, since sulfur competes aggressively for available oxygen with C and H2.The glaze must not soften prematurely; if the glaze melts too soon, gases become trapped beneath the viscous glaze.The glaze thickness is also important. Since most manufacturers are processing as rapidly as possible, a slightly thicker glaze can create a longer path for gases to travel through, and the extra distance and time results in pits.
Testing Analytical data is most helpful to determine the cause of glaze pitting. The use of TGA on the body and thermodilatometric analysis (TDA, also known as dilatometry) curves for the glaze will indicate the amount of volatiles as well as the initial glaze softening point. (TDA measures the change in length [expansion or shrinkage] of a product as a function of temperature.) Simple in-plant tests are valuable, too. If you suspect low temperature bubble formation, test fire the glaze on a pre-fired, inert piece with no organics. This will provide an indication of the ultimate glaze smoothness potential. It might indicate that the solution is to increase high temperature glaze fluidity.
Summary Frequently, plants report that the degree of pitting they experience varies daily, making it a hard problem to solve. This makes sense; so many factors can alter the degree of pitting that very small differences in kiln operation, raw materials, glaze application, etc., can affect the final results. But employing the checklist above should enable you to find the predominant cause and fix it. And then, establishing firing parameters and controlling them should keep you out of the pits for good.