clay lover, on 13 July 2012 - 07:16 AM, said:
yedrow, on 12 July 2012 - 11:33 PM, said:
I too agree with Neil and Marcia. It seems odd since you fired it twice. I think too that you may see if you are firing your ware too fast. If you go through the 1100 to 1800 zone too fast you may not sufficiently outgas the volatiles. Also, if you go too fast the bubble with rise from the center of the clay, but if you are over-firing it is likely the bubble will rise from the surface of the clay.
Yedrow, I am very interested in your comment. Although I am not having bloating, I am interested in learning all I can about not firing any longer that necessary. Could you expound on what you feel is 'too fast'?
I am quite happy to take as long as it takes, but why take longer?
Currently for ^6 I am firing
200*/hr to 205,
4oo*/hour to 1900
110*/hr to 2195 with a 20 minute soak, then down firing slowly to off at 1600*
My cone are good and even through the kiln. Glazes look good.
Any suggestions for improved efficiency wqill be appreciated and considered.
Other than slight pinholing occationally, I am not having problems.
How slow to go?
First, I may have expertise at throwing pots, but I'm not one of the experts on firing kilns, my apologies if I sounded otherwise.
Anyway, I think firing somewhat slow is best. I bisque fire at about 180˚/hr. above 212˚. Clays are different and some seem to me to have more easily volatilized materials in them, sulfer, carbonates, detritus, etc. I think its a good idea get as much of that out as possible. Once the clay-glaze boundary is established, gasses won't easily escape. Then, when the clay softens as it vitrifies, the pressurized air can bulge up. Also, when the body is more open oxygen can get in and oxidize the volatiles. If the walls of the ware are thick (I throw about 1/8th inch, averaged top to bottom, but newer potters throw twice that thick or more) then the ware must be fired slower since there is more mass for volatiles and oxygen to move through. I glaze fire at about 300˚/hr with slow drop in heat edit:[during cool down] from 1900˚ to 1500˚. Note too, in an electric kiln, the heat is moved primarily by radiation and a quickly fired kill can have quite a lag between the ware that is close to the elements and that which is in the center, in the "shadow" of other ware. This is especially true in a wider kiln.
That being said, I think the problem is probably over-firing and that it may well be the supplier. I've only used Laguna clay once, and my results were pretty bad, and their customer service was worse. They appeared to have no idea why I was having the problem and appeared to have no interest in solving it. I've said elsewhere, Standard clay has the best clay I've ever thrown with, by far. And, your local supplier will probably be able to handle your needs at a good cost.