BRONZclay Fail

I went to the kiln this morning, hoping to “ooh” and “aah” over my BRONZclay pieces that fired last night.  Instead, this is what I fished out of the carbon.  Eight pieces, all cracked and split open.

I’ve only worked with this material one time before, in a workshop setting about a year ago. My piece didn’t sinter properly and crumbled in my hands. After that experience, I trained my focus on COPPRclay, and of course, the ever-reliable silver clay. I found a firing schedule for COPPRclay that works well for me every time and at any thickness. But, I really wanted to create in bronze and I had 200 grams of BRONZclay on hand. The variety of firing schedules on the web was a bit overwhelming, so I reached out to two metal clay friends who work in this material and asked them what firing schedule they use. Their answers were very similar, so I figured I was on the right track.

Here’s what I did:

• Conditioned the clay according to the info on Margaret Schindel’s BRONZclay Squidoo Lens
• Rolled to 6 cards, then rolled over my texture plate at 4 cards
• Clay was totally dry as it had been in greenware stage for over 24 hours
• Buried in coal carbon in stainless steel firing pan (carbon purchased from WLW and never used before)
• Ramped at 250 per hour to 1516, held for 3.5 hours
• Firing pan pulled out of the kiln when still hot and allowed to cool at room temperature (kiln was not red-hot, but still quite warm – I didn’t note the temperature)
• Kiln is a Paragon SC2

I have a few ideas for what might have gone wrong:

• Maybe I needed to let the pan cool in the kiln, rather than pulling it out
• The lid on my stainless steel firing pan is so warped that it barely connects with the pan.  It’s almost like having the pan uncovered.  this hasn’t been a problem with firing COPPRclay, in fact I have stopped using the lid altogether for that purpose, but maybe the BRONZclay is more particular about oxygen exposure.
• I used SLIK as a release agent, rather than olive oil
• For the holes that split open, I probably need to leave a wider strip of clay on the skinny side of the donut

I think it’s significant that the pieces appear to be fully sintered.  I whacked on them with a hammer, and flattened out some of the domed pieces.  They are quite strong and even marred the face of my hammer (darn it).  To me, this points to a temperature shock from pulling the pan out of the kiln too fast.

Perhaps someone out there can provide some advice. I’ll also shoot this out to the MC forum for more insight.  I really want to create in bronze and I’m looking forward to trying Hadar’s formula and Bill’s new Fast Fire formula, which is coming out later this month.  Hopefully, I can figure out how to make something beautiful from the clay I have on hand.  I’ll keep trying based on what I learn from this experience.