Posted 03 July 2012 - 01:48 AM
Other potters may have alternate opinions.
Posted 03 July 2012 - 02:06 AM
If it's thick and runny it could be running down the side of the bowl and pooling too thick at the bottom. break a bowl that bubbled and take a look - if this is indeed the case, the glaze will be very thin up top and very thick at the bottom. If it's like that you can thin it down a bit. more hold time could let all the gasses that want to bubble leave, but it could also encourage the running down by adding heat work. I've had this problem recently, bowls were the worst offenders because the shape funnels everything down into a narrow bottom, mugs were better. It was the dregs of that glaze bucket (very thick), now that it's been remade it's fine again.
Posted 03 July 2012 - 12:33 PM
Posted 04 July 2012 - 09:42 AM
As the kiln is heating up, the mass of wares needs to have time to absorb the heat energy from the flames and hot gases or the rediant electric elements. Objects that are more "massive" than others or are further away from the effects of heat transfer take longer than other areas to become hot.
The floor area of a bowl is "shadowed" from effective radiant heat transfer as well as is a bit of a stagnant zone for convective transfer. Add to this fact that for many of us, there is a tendency for bowls to be (ahemmmmmm ) a bit thicker there than what might be the case in the rest of the wall section of the bowl. Then add in the mass of the footring too. And the final and real contributor is the fact that the bowl footring is sitting on the very LARGE thermal mass of the kiln shelf.
So while the cones or the pyrometer is measureing the general temperature of the area near the probe or the heatwork where the cones are located, it is possible that the bottom area of the bowl is a bit behind the rest of the load whre these measurements are taking place.
This means that the glaze in that area might still be going through the "fining out" process... where gasses are still being evolved in the glass and are slowly moving through the mass of taffy-like glass toward the surface whre they eventually break and disappear. When the kiln shuts off you might be catching the glaze not quite to the maturity phase of things.
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art
Posted 04 July 2012 - 10:35 PM
Another thing to consider is that if your bottoms are real thick they will absorbe more water and have a thicker layer of glaze than the walls. As John said, the thermal mass of the thicker parts plus the thermal mass of the shelving (conducting away heat) lower the actual temperature of that part. There can be as much as 212 deg. difference between the bottom of a medium sized pot and its rim (unless you are using advancers, etc.).