I had a major problem of crazing in the last couple of kiln loads. The cracks didn't appear right away, but usually overnight or within 24 hours. Imagine my panic attacks when upon examining the pieces right out of the kiln, they all looked fine. I then notified the customers to tell them that their orders will be in the mail shortly. The next morning, I woke up early to do some serious packing to find most of the pieces have crazed overnight! For those of you who don't know what crazing is, it is a network of cracks in the glaze which occurs after firing.
What to do? It would take me at least another 2 to 3 weeks to remake everything. I went online to frantically learn as much as I can about crazing, and to understand what I did to cause the problem. Basically, crazing is caused by improper fit between the clay body and glaze. In other words, the glaze contracts or expands more than the clay body to which it was applied. Since the glaze is a thinner layer of the two, it will give and crack.
I've been using the same clay and glaze combination for over a year and have never had to deal with this issue until now. I know that glaze fit was not my issue. After further research and a test fire, I was able to figure out the problem. What was it? It had to do improper cooling after firing. I thought I could decrease the cooling period safely by unplugging the peep holes at about 600F, crack open the kiln lid around 350F - 375F, and start to unload at around 200F - 225F. I had to use thick gloves because the pieces, shelves, and columns were still too hot to touch with bare hands. I'm telling you this so that if you're thinking about doing what I did, DON'T, unless you intentionally want to craze your wares. Be patient, wait until the kiln is cool to the touch on the outside, then open the kiln. Don't unload the pieces until you can touch them with your own bare hands. They should feel warm but not hot.
Fortunately, I was able to recover most of the crazed pieces by re-firing them again and cooling them properly. You may find that by firing them the second time, a few will develop pin holes on the surface of the glaze. Well, that's another issue all together and will need a separate post of discussion. As for now, I can breathe a little easier and start packing boxes.
6 comments:
Oh, yeah. Got to peek at the kiln and then let it cool for 24 hours (or more!) before unloading. I've learned that the hard way. But it took me 5 years to figure it out.
This is so timely for me! In my last firing, I had what I thought was a great outcome... and then all my plates crazed. I did unload when they were still pretty hot as I had to use towels on my hands. They looked fine at the time and even a couple of hours later. But then it happened... I thought it was my switch from Aardvark Bee Mix to Laguna B-Mix. And that was a bummer because I have lots of the Laguna clay now... I will try refiring and NOT OPENING THE KILN. Restraint. Restraint. Thanks!
Yep - that'll do it every time! I am sometimes guilty of doing a fast cool when I wait too long to fire work before a deadline. Most of my glazes don't mind, but one of them is very temperamental and will craze unless it cools slowly and isn't removed before it's cool enough to handle with bare hands.
Do you ever have any problems with your porcelain plates cracking as they dry? I can only eem to salvage 2 for every 10 that I throw.
But as far as crazing, yep. You gotta let it cool properly.
Repeat after me...no instant gratification with ceramics...I still have a little burn on my finger from trying to pull my pendants out of the kiln while they were still toasty! I've fallen victim to crazing too because of it.
Love your work!
omg! what a labor of love, chi! ceramics-making is not for the faint of heart. {sigh}
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