I have been struggling to figure what the white spots are that surface in the pots after they come out of the fire. If the pots are just low fired, to bisque temperatures, or in other words, not vitrified, then the white spots usually do not surface, and the pots are fine. But I have been trying to reach high fired temperatures, so that the clay is vitrified. And with several of the individual clay bodies, as well as the blended body that I had high hopes for, the spots start to surface after a day or so out of the kiln, and eventually lead to the pot decomposing…literally falling apart over time.
So I made an appointment to meet with John Pacini, the clay manager at Laguna Clay. John knows more about clay than anybody I know. John Brooks, the owner of Laguna also joined the conversation.
John P. immediately identified the culprit as gypsum, or some other, similar form of calcium. He explained why the pots could sit unfired for a year and not have the calcium surface, but that once they were fired, the calcium appeared, up to a week after firing, and would eventually undermine the structural integrity of the pot, causing it to slowly turn to a pile of rubble.
Apparently, if the pot is unfired, or fired low enough, the gypsum can get all of the moisture that it needs from / through the clay. But once the clay is high fired, and has no moisture at all in it, the gypsum, in an attempt to rehydrate itself, migrates toward the surface of the pot in search of moisture. As it moves through the clay, it destroys its molecular and structural integrity, and the pot eventually falls apart. Which is so bizarre and fascinating and amazing to me. John suggested as a first course of action to re-seive the clay through a finer, 100 mesh, screen. It had previously been screened down to about 30 mesh. His thought was that the 100 mesh screen would catch most of the gypsum, and that if any gypsum remained behind, and if it fit through a 100 mesh screen, it might not cause any problems to the clay. Since getting this advice, we have re-screened the already blended clay body of clays 2,3,5. And we screened a batch of each of the three clays, 2, 3 and 5 individually as well. The blended body still shows signs of the gypsum when fired to cone 5. Clay 3 and 5 on their own are gypsum free now, and clay #2 seems to be the one with the serious gypsum problem, and the one that is contaminating the blended body. So for now, I will focus my energy on using clays 3 and 5, while continuing to try to fix clay #2.