The Instruction Manual for Hadar’s clay is now updated and marked May 15, 2013.
Besides adding instructions for firing Smart Bronze, changes have been introduced throughout the Instruction Manual. I recommend reading it through when you get a chance, since it may shed some light on your personal experience.
I would like to share with you my experimentation with pre-firing.
First, not all clays require pre-firing. White Bronze, Smart Bronze (up to a certain thickness) and Low-shrinkage Steel XT do not require pre-firing. Steel, decorated with copper and/or Bronze XT, also does not require pre-firing.
Copper and the rest of the bronzes do require pre-firing. Firing longer hours and/or ramping slower has never worked for me. Recently I have done some more experiments with pre-firing in the kiln instead of on the stove-top. Here is what I did:
Brick top-loader kiln
I placed a bowl inside the kiln with copper and bronze pieces resting on top of carbon. Set the kiln to mid-fire schedule.
As the kiln reached 500°F I opened the lid to check on the pieces. I kept checking every 100°F. At 800°F pieces started to smoke and turn black.
At 1000°F the smoke was gone and the pieces were all black. I carefully turned the pieces over with a spoon to see if they are black on the other side.
They were not, but they turned black within seconds. I covered them with carbon, closed the door, and let the kiln complete its cycle. Worked great. 2:45 minutes from beginning to end. If the pre-firing is done on a stove-top, this is the time it takes for the second phase alone. So firing this way is shorter and easier than on a stove top.
If you happen to walk away while the kiln is ramping and come back after it reached 100°F, no problem. Pieces are probably all smoked up and ready for more carbon. Oxidation will not happen if it’s under 1:00 hour, and even if it does, it will be reversed while the pieces are fired inside carbon.
Turning the pieces over is a good idea. Also, it doesn’t matter if the vent hole is open or closed.
Muffle, front-loader kiln
I tried the same thing in a muffle kiln. Surprisingly, the pieces did not show a sign of smoke until the kiln reached 1200°F. At this point the carbon was already on fire. Since it was a front loader, I had to take the bowl out of the kiln to cover the pieces with carbon, which was awkward at this high temperature. And after the second phase, the pieces were not sintered.
I am guessing the reason is the fast ramp of this type of kiln. The chamber got hot, the outside of the pieces got warm, but the inside needed more time. So slowing the ramp may be the answer.
But then I found out something else. I know that firing twice always works, so I tried it again, but with less time. I fired the pieces in carbon at 1510°F for 1:00 hour. It took 1:20 minutes. Took the bowl out of the kiln and cooled the bowl and the kiln under 100°F. Put it back for another hour, and it worked. Another 1:20 hours, plus cooling time. A little longer, but less messy and more successful.
In fact, the easiest way for me to fire in a front loader was to fire 1:00 hour before I went to sleep. In the morning the kiln was cold so it wasn’t necessary to take the bowl out. Didn’t even open the kiln. Just fired one more hour.