Shown above is the moisture test that Carole used. Fog on the mirror indicates that the clay is still releasing moisture.
A Firing Schedule for Clay Sculpture, by Carole Dwinell
Reader Response: Suggestion for removing bottle labels; praise for Petra Kaiser; safety glasses.
A Firing Schedule for Clay Sculpture
By Carole Dwinell of Martinez, California
My tree forest series clay sculptures are 16" to 22" tall with clay formed in sizes from tiny, thin leaves to thick trunks and bases.
I fire sculpture slowly with a five-ramp bisque firing. It takes 46 hours up in temperature and 32 hours down. I have not had ONE firing problem with that schedule. Period. Not a crumble, not a crack.
I initially thought this schedule was going to be super expensive in electricity, but that has not been the case. There has been only a slight rise in the monthly electric bill, about $22, and that was firing every week to be ready for a show.
The long soaks and the slow controlled cool-down keeps the temperature extremely even throughout the kiln. I put cones on all three shelves in one firing, and they were identical when I opened the kiln.
My firing schedule in degrees F for sculpture with thickness varying from 1/32” to more than 1”:
Ramp 1) Up at 40º/hr to 180º, soak for 12 hours
Ramp 2) Up at 60º/hr to 1000, soak for 3 hours, then shut vent
Ramp 3) Up at 60º/hr to 1200º, soak 1 hour
Ramp 4) Up at 80º/hr to 1888º, no soak
Ramp 5) The cool-down: Drop temperature 50º per hour to 300º before shutting off the kiln. Then let it go the rest of the way down on it's own.
At 900º to 1000º F (Ramp 2), I get quite a bit of moisture at the upper vent of the kiln (the lower intake still being open). At first I thought perhaps it was the weather, because I was positive that the 12-hour soak at 180º had dried any water out of the ware. But at approximately 1000º F, I was getting WAY MORE moisture than at the lower soak.
At the lower temperature (Ramp 1), the moisture barely even starts to fog the glass* I use. At the higher temperature (Ramp 2), my glass is thoroughly fogged for tests taken during the Ramp 2 first hour of soak before it gets less and less as that three-hour soak wraps up.
Everyone I've talked to who has had kiln problems seems to fire way too quickly. Why would one put so much work into a sculpture and then play kiln roulette by firing too fast? It is the same with drying. There are so many ways to get your pieces to dry evenly and completely, yet I see shortcuts all the time that lead to disaster.
*Note from Arnold: Carole is referring to the mirror or glass test. Hold a mirror above the lid or top peephole where hot air from the kiln will move across the mirror's surface. If the mirror fogs, the greenware is still releasing moisture. (If you are firing with a downdraft kiln vent, first turn off the vent. Then perform the mirror test.)
For this test to work, the mirror must be at room temperature. The mirror fogs when moisture in the hot air condenses on the cooler mirror. If you hold the mirror too long near the kiln, the mirror will heat up and will no longer fog when moisture hits it. So hold it at the lid for only several seconds at a time.
Last week’s Q&A section included information on scraping labels from champagne display bottles for glass slumping. Karen Hardy of Redondo Beach, California wrote, “I have a suggestion for removing labels from champagne or wine bottles. I use a product called Goo-Gone. You can find it in practically any hardware store and most grocery stores. Just spray or dab it on the label, let it soak in, and the label comes right off after a few minutes along with most of the gummy junk. Then you can use a paper towel with a bit more Goo-Gone sprayed on it to remove any leftover junk.
“The product is non-caustic, all organic, and has a wonderful orange smell. It's also really cheap, and you're not as likely to slice off your thumb as you would if you used a razor blade to scrape the label off (I'm accident prone).”
In last week’s Q&A section, I recommended the book "Introduction To Glass Fusing," by Petra Kaiser. Yvonne George of Cameron, North Carolina wrote, “I went to a bead show in Orlando and saw a short demo by Petra and Wolfgang. They were so giving, enthusiastic and informative. I bought some supplies and her EXCELLENT book. I've been to Petra’s home and studio. She is charming and so very knowledgeable. I would not be doing glass except for her and her encouragement...and the wonderful book. Highly recommended!”
Click here to read more about “Introduction to Glass Fusing.”
Kate Julian of Stilwell, Kansas wrote, “I love these pointers from you! My only comment though, shouldn't you practice what you preach? The woman in the photo is not wearing protective eyewear.”
Thanks, Kate, for the reminder!
Here is part of an interview with Angelica Pozo, author of “Making & Installing Handmade Tiles”:
Q. What are the most common mistakes of the typical beginning tile maker?
A. The most common mistakes start from making tile from unsuitable clay. A heavily grogged clay is the most forgiving for tile making. Some people make tiles from porcelain, but those require extreme care in forming and drying. Beginning tile makers would have a much easier time with the proper clay body.
Click here to read the full interview.
With best wishes,
Arnold Howard Paragon Industries, L.P. – Better Designed Kilns 2011 South Town East Blvd. Mesquite, TX 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 firstname.lastname@example.org / www.paragonweb.com
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