"Speed is NOT your Friend."
A recent kiln blow-up in a glaze firing led us to some online searching to try to understand the cause. When I posted a query to Electric Cone 6 & Other Ways w/ Clay, one of the responders referenced the article "Speed is NOT your Friend" and posted its link at Dogwood Ceramic Supply.The article explains in detail why we don't want to dry pieces too quickly, fire pieces that aren't bone-dry, or open a kiln too soon after firing is complete. While some of the article gets a little complex (OK, a lot complex), it's worth a read--especially to understand the heating/cooling processes a kiln goes through.
We're still trying to figure out why several of the bisqued "cookies" shattered, damaging a few pieces (shards embedded in glazed surfaces). We know the kiln was fired at medium speed, not fast; the cookies had been used before so they weren't wet; there was no glazed greenware in the load.
We'll keep mulling this over.
Thanks so much Meredith - very good info. And definitely complex but in a good way - super interesting! Definitely I'll need to read it a couple of times (and again as I continue along the clay learning curve) but really cool. Thanks again.
ReplyDeleteThe last couple of times that I've fired a bisque, I've fired it on slow. Last time, since we had some newbie pieces from Deb's class, I fired it on slow with a two-hour preheat. It seems to have worked really well. I think that defaulting to "slow" for bisque kilns is a good idea from now on, unless we're in a hurry for a particular reason (lots of classes going on, etc). Firing a bisque at a medium speed would probably be fine, but as long as no one is in a rush then better safe than sorry.
ReplyDeleteSounds good. I've made a note in the notebook (mostly to remind ME :)
ReplyDeleteGosh Meredith, I wish you'd asked me what the kiln temperature read when we talked on the phone last Tuesday. I could've put your fears to rest. The temperature was 129 degrees. If I recall correctly, in the past, you and I have had a conversation about when is a good temp to open the kiln once it's firing is finished. If I remember correctly I'm more conservative than you are about temperature. What the University taught me is that with a bisque, it is okay( it's not great, but it's okay) to CRACK the kiln when the temp gets below 500, but that applies only to a bisque. With a glaze firing, 250 degrees to crack it; then open it when the temp drops below 200 degreed. What with the sheer amount of time involved in making just one piece, it serves no purpose whatsoever to open a kiln at a risky temperature. I'm quite sure your pieces are as important to you as my pieces are to me. Considering that is always someone else's pieces in a kiln, it is never okay to put someone else's work at risk, don't you agree?
ReplyDeleteMeredith, my eye just caught a very important distinction in your original post. The "cookies" we use are not bisque ware. If you don't realize what you've typed, your query/search may not answer your question. The cookies are fully fired. They don't have a glaze on them, but those cookies are not bisqueware, they are fully mature.
ReplyDeleteQuite right -- I was thinking only about their unglazed state & not the fact that they've been fired at glaze temps.
DeleteDid anything in that load blow up except for the cookies?
ReplyDelete"What with the sheer amount of time involved in making just one piece, it serves no purpose whatsoever to open a kiln at a risky temperature. I'm quite sure your pieces are as important to you as my pieces are to me. Considering that is always someone else's pieces in a kiln, it is never okay to put someone else's work at risk, don't you agree?"
ReplyDeleteSure do agree, Solveig, which is why I thought the article I linked to would be useful. ;)
As to cracking open a bisque kiln "below 500" (how much below?) or a glaze kiln at 250, I see that as rushing it, and I wouldn't do it. As the article points out in Section III, "The problem with accelerating the cooling of the kiln is that the core temperature of the ware is never accurately known. It is all a guess. From Dogwood Ceramic Supply's perspective we believe every kiln owner should just allow their kiln to cool naturally. Shaving a few minutes or maybe an hour or two of time is just not worth the risk to the wares."