More failed experimenting with base metal clay and my kiln

Did my first bronze/copper kiln test yesterday. For the most part, I followed Hadar’s way of testing (copper pieces, bronze pieces, copper in bronze pieces, bronze in copper pieces).

It was also the first time I used my new fiber firing bowl. I can’t tell you how FABULOUS it is to not have to vacuum out the inside of my kiln after each firing. (That black dust all over the kiln after firing base metal clay isn’t charcoal dust… it’s ash or firescale from the steel firing pan.)

The bad news, however, is that my bronze/copper pieces came out of the kiln seriously mess up.

If I’d known they were going to be that unrecognizable after the firing, I would have taken a photo of the pieces in their green state.

The bronze parts are all pitted, blistered, and misshapen. The copper looks best, but was only fired at 1470 external pyrometer reading (kiln setting 1370) so I’ll have to test them further to see if they’re fully sintered.

Aside from adjusting my kiln temperatures (see below), the only other changed factors were: new charcoal and a different firing bowl.

I placed ½ inch of carbon on the bottom of the firing pan, added my pieces, then covered them with 1½ inches of additional carbon. I did note that about ¾ an inch of the carbon had burned away during the second firing.

It is possible that things stayed hot for longer due to the new firing conditions, but I wouldn’t think maintaining the optimal temp could produce the effects of over-firing so severely.

Here was my set-up…
Media: bronze and copper separately and together, most pieces about 2mm thick
Kiln: top loading brick
Firing Container: round fiber firing container, no lid
Charcoal: coconut shell carbon, acid washed
Phase 1 Firing: full ramp to 1000 external pyrometer reading (kiln setting 870), hold for 1 hour (then opened the kiln and removed the bowl to cool)
Phase 2 Firing: full ramp to 1470 external pyrometer reading (kiln setting 1370), hold for 2 hours (did not open the kiln directly after firing)



The pitting makes me feel like I should place my coin-shaped pieces vertically rather than horizontally next time, but all the bronze pieces still look over-fired regardless.

So then I took one of the all copper pieces and started grinding it down, looking for signs of non-sintering.

The thing was sintered.

Extreme puzzlement.

Directions: copper and bronze both sinter at 1000, copper densifies at 1700 and bronze densifies at 1470.

Some of you know that I already suspected my kiln was running hot, so I recently purchased an external pyrometer (thermometer) and have been testing things out.

At lower temperatures (like 500 F), there is a 200 degree difference. At higher temperatures (like 1470) there is a 125 degree difference. This fluctuates, of course.

I will continue my tests, but if all else fails, I will swap out my kiln’s thermocouple and see if that makes a difference.

Here’s what bums me out (or confuses me, take your pick): The previous piece I fired was at a much hotter temp than yesterday’s test pieces.

It was in a stainless steel container and it was a much larger piece, but still… it was not fully sintered and had to be repaired and refired.

This was before I adjusted for my kiln’s over-firing, so it was being fired at much higher temperatures than my test pieces.

Can thinness/thickness have that much to do with temps? Can steel vs fiber container have that much to do with temps?

Okay, later today I’ll mix up some more text clay and see how it goes in round 2.

Stay tuned…