Saturday, July 6, 2013

Test Firing Copper and Bronze Metal Clay


The Process

Truth be told, I’ve never felt very confident about my firing schedules with metal clay. For starters, my kiln runs really hot (REALLY hot!) so I’m always in disbelief when I see how low my temp has to be compared with where most people fire at.

And since the current assignment involves clay other than steel, I decided to start at ground zero and do some testing.

I went with pieces that were 30mm across and 8 cards thick. I did sand both sides of each piece a little, so they were a bit thinner by the time they went into the kiln, but still significantly larger and thicker than previous test pieces (which was one reason I wanted to start my tests over).

Since I was starting at the beginner for all intents and purposes, I went ahead and used my fiber bowl instead of a steel bowl. As much as I love the steel bowls, I do worry about the residue that my vacuum can’t reach.

The recommended firing temp for mid-fire clay is 1460 so I decided to go with 1380 as my first test.

Here are the steps I did:
  • Traditional copper/bronze test pieces, 8 cards thick, then sanded a bit
  • Pre-firing in kiln (as part of phase 2 schedule), full ramp to 1380 with pieces on top of carbon in fiber bowl
  • At 800 (20 mins) pieces were black so I turned them over
  • Reverse sides were not black but blackened within seconds of being exposed to air
  • Continued to ramp (on top of carbon) for 9 more minutes
  • At 1000 degrees, covered with carbon and let cycle finish.=
  • 2:26 later (1380 hold for 2 hours) cycle was finished
  • I left everything until the kiln cooled to 940, then opened the lid
  • Half an hour later, I started to scoop the pieces but the carbon was still red so I left them to cool a bit more
  • No patience… I removed the pieces after only a 10 minute longer wait

BTW, all the info about test pieces and firing schedules (including the option for kiln pre-firing) is in Hadar’s blog and in her online instruction manual.

Here are the pieces straight out of the kiln.



Here they are after using the radial disks on them.



Then the big test… hardcore sanding to see if they’re sintered.



And then after adding the patina (to really bring out the contrast between the copper and the bronze).



Pretty cool, huh?

There is a bit of curvature of the copper-centered piece.



This makes me think that going higher in the temp, may produce even more warping, but I still may try 1390 tomorrow, just to confirm that 1380 is the ideal mid-fire temp for my kiln.

Tomorrow, I’ll do a second test (I already have the pieces made up since I made 4 total test pieces today). And while those are in the kiln, hopefully I’ll be able to finish up the rest of my assignment (which includes me trying the Mokume Gane techniques for the first time).

The Lighter Side




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