The clay is so nice to work with that I ambitiously made a couple of larger pieces before I realized that bigger, thicker pieces need a different firing schedule. I found some iris pods in the backyard last fall and thought they would make a nice focal piece for a necklace. But the piece I lovingly created turned out to be 13mm thick in the center.
While there is a lot of information about firing this clay out there on the Web, I decided to contact the manufacturer himself for advice. Bill Struve was very helpful and patient with all my questions as we exchanged a few emails.
We decided that the slow ramp process Mardel Rein has devised, which takes into consideration the type of kiln, the thickness of the clay, and the target temperature, would be appropriate to try. So after an initial open air firing phase, ramping at 500° to 650° and holding for 45 minutes, I covered it with carbon and fired at 115° per hour to a target of 1490° and held for 3 hours. Yes, the piece was in the kiln all day.
One of the pods broke at the seams but otherwise the piece was intact. It was an easy fix. The second firing used the same first phase. But then buried in carbon it was a full ramp to 1510° with another 3 hour hold. Bill Struve had told me that the slow ramp on already fired items wasn't necessary.
Cleaned up and polished, this was the result.
While bronze clay is so much less expensive to purchase than fine silver and I feel compelled to create larger pieces, firing time can cancel out the savings, even when you have solar panels. So it's not something I will do often.
I'm thinking about digging out my husband's dehydrator. Some of my smaller/thinner pieces tended to crack or break but they always seemed to be pieces with more than 1 layer and cracks began in the layered area. It's possible that even pieces that sit around for awhile are not fully dry in the center of the thickest part. Pieces that sat for a week fired without problems. Those that were only a couple days old were the ones that cracked.
The shrinkage rate of Bronzclay makes it difficult to create my openwork/filigree designs. Even when my coils are thicker, they don't seem to stay together. So I may go back to Goldie Bronze for those designs despite its more grainy finish.
As I stated in an earlier post, bezels need to be attached to already fired pieces and the entire work fired.
again. Second firings can be at full ramp to target temperature.
Any pieces with nooks and crannies should be fired in a wire mesh cage. Because of the shrinkage rate of Bronzclay, this is particularly important. Small bits of coconut carbon can cause cracks in the design or become embedded in it.
My kiln is a Paragon Firefly and the bonding agent in this clay just would not burn out using the manufacturer instructions. I had better results using a first phase firing on a kiln shelf then burying the pieces for the second phase. I do this with both thin and thicker pieces.