Heating Up the Relationship: Guidelines For Firing Your Married Silver And Copper Clays, Part 2
Mary Ellin D’Agostino
This is the second part of my report on how to fire metal clay alloys made from silver and copper. For directions on how to make and use these alloys, see the post In Search of Married Metals: Alloying Silver and Copper Clays Project. In part one of Heating up the Relationship, I explained the method for firing alloyed silver and copper clay pieces made using any of the commercially available copper clays. In all tests in part 1, low fire silver clays were used. In part 2, I will discuss firing alternatives for quick-fire copper clays.
Open Shelf & Torch Tests:
Alas, no-carbon firing is not a good option for mixtures of silver and copper clays unless they contain only the smallest amount of silver or the alloyed clays. Torch firing produced a very weak product in all of the alloy mixtures. Paradoxically, in a complete turn around to alloys made from the first generation copper clays (Metal Adventures CopprClay and Hadar’s “traditional” copper clay), the sterling alloy was very weak and the shibuishi alloys were the strongest. None of the open shelf or torch firing tests resulted in a work product I would consider using in a final product. Even the strongest (the shibuishi alloys) broke when bent as little as 30°. Most of the test pieces snapped long before that point. This was true for pieces torch fired for a few minutes and those kiln fired for as long as 3 hours. The maximum firing temperature for these alloys is approximately 1450°F/788°C. If you exceed this temperature, the pieces melt. See Part one of this series for a discussion of firing and melting temperatures for silver-copper alloys.
An attempt at firing with a Speed-Fire Cone also resulted in failure, but may have been due to not keeping the pieces hot enough as the gas tank was a bit low. If the other results had been more promising, I might have rushed out and gotten a new tank of propane and tried again. Since all the torch and open shelf tests were dismal failures, I have not been in a rush to try again with the Speed-Fire Cone system. If anyone tries it and gets better results, let me know.
No-carbon firing is not recommended for alloyed copper and silver clays. However, it is possible that if you make the piece very thick and don’t need a really strong (bendable) end product, you might be able to use one of these methods. Hadar Jacobson reports some success with a shibuishi clay that she torch fired, but doesn’t go into detail as to how strong her piece was. Click here to go to her report.
Carbon in a Fiber Blanket Box Tests:
Again, both Art Clay and Hadar’s Clay brands of quick fire copper were tested. For a full description of the fiber-blanket box method, see Hadar’s blog on the subject. A box was made of fiber blanket and filled about ¾-1” with carbon. The test strips were placed in the carbon and covered with another ¾-1” of carbon. (All carbon tests have been done with the coconut based carbon).
In the first test (test series C5 & D1), the kiln was heated to 1450°F/788°C and held for 2 hours. Binders were not burned off in advance. Approximately half of the carbon burned away. Pieces came out black and the alloy strips with the most silver melted. Some bending was possible, but the maximum bends ranged from 45° to 90°. Bending beyond this point resulted in the test strips snapping. Results were similar for both brands of clay with one small difference. The Art Clay copper pieces with more than 25% silver melted, while only those with more than 50% silver melted in the pieces made with Hadar’s copper clay.
In the second test (test series D3 & C7), I reduced the temperature to 1425°F/774°C and held for 3 hours. Results were similar to the first. Alloys with more than 25% silver mixed with Art Clay brand copper melted while those with more that 50% silver mixed with Hadar’s brand copper melted. In all cases where the pieces didn’t melt, the metal failed at or before a 45° bend. Again, this firing schedule may be adequate if pieces are thick and are not likely to be subject to much stress.
One complicating factor is that in these final tests (C5-C7 and D1-D3), reconstituted silver clay was used that contained some PMC+ in addition to PMC3 silver clay. This was an economy measure on my part as these tests are getting expensive! If anyone gets better results using only low-fire silver, let me know.
Steel Box & Carbon Tests:
In these tests (test series D2 & C6), the pieces were placed on carbon in a 3.5” diameter stainless steel box, the binder torched off, covered with more carbon, then sealed with a stainless steel lid, heated at full ramp in an Evenheat E91 kiln to 1450°F/788°C and held for 6 hours. These tests also used the reconstituted clay rather than pure and fresh low-fire silver clay. Results are consistent with use of the first generation copper clays (Metal Adventures CopprClay and Hadar’s Traditional copper clay). In these cases, the pieces with more silver were the strongest while those with the most copper were somewhat weaker. Maximum bend achieved before breaking were 45°-90°. An 8 hour firing would probably improve the strength. Firing in a larger box and kiln might also improve matters as I achieved the best results in earlier tests using a large kiln with a significantly larger firing box with a lot more carbon to create a better reduction atmosphere. I will report more after further testing.
Tnanks go to Jackie Truty and Art Clay World for the Art Clay Copper sample.
© 2010 Mary Ellin D’Agostino