There can be two reasons for this. The temperature in your kiln may be just high enough for bronze, but not for copper. If you want to test it, raise the temperature of the second phase by 100-150F. If the copper sinters and the bronze blisters, it means that your controller doesn’t display the actual temperature and you may need to replace the thermocouple.

The other reason could be that you had too many hollow forms in the kiln. Many hollow forms mean a lot of oxygen present. I try to fire no more than 3 small hollow forms at once, together with solid forms. Big hollow forms I fill halfway with carbon.

Why don’t you try this first, firing just 3 copper hollow pieces at the regular temperture. But make new ones, since those that you fired twice may be seriously oxidized by now.

Hadar

I filled the pan half full with carbon, put the pieces in the back and sides, 1/2″ apart, and covered them with 1/2″ of carbon. I fired them following the instructions in the manual, but after the 2nd firing when I pulled them out the copper pieces did not look properly fired. They were dark brown and looked like the binder had not fired out. I fired them again using the 2nd firing schedule, but they crumbled in my hand when I took them out.

Looking at your manual, it appears that something went wrong in the first firing. Should I fire them hotter than 1100 or longer than 30 minutes? Or both? I fired them high in the kiln, and as far back as the pan would go without hitting the thermacouple.

Thanks! I really love copper, and want to succeed with it.