Second of a Two Part Series: Sintering, Test Strips and Carbon-Fired Clays
In my last post I talked about why test strips are a good idea. For this article, I talked with Bill Struve, inventor of BRONZClay and COPPRClay, and owner of Metal Adventures, manufacturer of those brands. Bill provided me with a wealth of information, and also directed to me Mardel Rein’s very informative Learning Center page at her Cool Tools web site, about firing bronze metal clay.
So, how do we “read” a test strip?
There are many tests you can perform on a metal clay strip. The first one I usually do after removing the strip from the carbon is to drop it onto a steel bench block. It should make a particular type of ringing sound, very metallic. If it has a dull sound or a “clunk”, something is wrong.
The next test is to bend the strip. This test is designed to help determine if the piece is sintered enough to be malleable. You bend the strip into a tight “u” shape, approximately .2 inches (5 mm) inside the “u”. During this test, the outside surface of the strip is stretched, while the inside is compressed. The strip should not crack, tear or break. Thicker pieces will crack or break more easily than thin, so that is why test strips are done at a thickness of 4 or 5 cards—sort of a happy medium thickness.
The last test is to mix a tiny bit of detergent into a small amount of water, then place a drop of this water on the strip. The water should not be absorbed*, but sit on top of the strip. This is because with the progression of sintering, the piece becomes less porous. The water test indicates a “closed” porosity.
*This test will indicate a relatively full sintering. It is possible for the strip to pass the bend test but not this test. Not passing this test does not always indicate a problem with your piece. But if you are looking for full sintering, this is a good test.
Another test that can be performed on a strip is to hammer it really hard. The strip should not crack or break.
Unsuccessful strips can provide clues to what went wrong with your firing. And there are a lot of them! Mardel Rein describes them best in her articles that I mentioned before. I highly recommend looking at her page, there are a series of troubleshooting photos that are invaluable.