Electric bisque strategies

[Lee b]

So far, I've bisque d twice in my  Cress kiln sitter type electric kiln. Results? not so good. First effort, two levels, bottom pieces were good. Top level, half of the pieces shattered to crumbles. These were hand built sculptures, bone dry, cone 5 sculpture clay. I set for 8.5 hours with top peep hole out. Cones didn't bend, so I hit maybe o6, or o7. ...second firing had one large sculpture, which split the top off at the shoulder, and pulverized the base. Apparently, I blew through the first several hundred degrees too fast.  Fired for ten hours, bent the cones good for 05. Seems I need to "candle" the preheat on manual for a couple two or three hours, then turn the speed dial to slow for the rest. Does that sound right? Any ideas? 

 

[liambesaw]

Pieces blowing up in a kiln is almost always due to moisture.  If you have any thick pieces (more than half an inch), you'll need to candle for quite a while, maybe overnight.

I candle for 8 hours if there's anything questionable.

[Bill Kielb]

Just to add candle to under 200 degrees for time or very slowly get to 200 degrees so the moisture has time to leave gently. Steam = explosion. For large sculptures we check them with an IR camera else they feel bone dry but are not.

large sculpture below, almost dry and several days later much drier. Still candled for 8 hours though.

[oldlady]

8.5 hours seems way too short a bisque firing.   even if everything is totally dry and not sculpture.   but adding sculpture is definitely the wrong thing to do.   candle as in the above advice and if you have not done this type of firing in that kiln before, set the timer to the highest number after you let the kiln warm.   allow the sitter to operate the way it was intended.   you will then be able to judge the length of time it really takes.    sitters are not infallible but they do work well if you do not constrict them with a timer set for too short a time.