Hi I made some small test pieces of copper clay (Creative brand) and fired them according to the manufacturers instructions (covered with kiln blanket and fired at 920C for 30 mins, then put in picklean for 10 mins) and they came out beautifully. But when I then fired some pendant shapes they turned black as soon as they were out of the kiln and were brittle and all broke apart after I took them out of the picklean. I tried again this time using the Bronze clay, and the same thing has happened

I'm not sure what's going wrong, considering the first batch were fine, and I'm using a brand new kiln (Paragon SC2). The only difference I can think of is that the test pieces were thicker - my pendant shapes were about 1.5mm thick.

Does anyone have any advice they could share, or insights as to what is happening. I'm afraid to fire any more unless more gets ruined...