sintered metal-Test Strips for Carbon-Fired Metal Clays

Ever since I´ve sintered metalbeen using carbon-fired clays， I have become a big advocate of test strips。 A lot of work can be wasted without the information that they provide。

Test strips give clues to how completely sintered your piece is， and how good your firing schedule is。 There are so many variables to working in carbon-fired metal clay， it is often difficult to arrive at a proper firing schedule just by following the manufacturer´s suggestions。 Many times you need to customize your own firing plan。

Another use for test strips is as a "witness strip" (a term coined by Mardel Rein， owner of Cool Tools and expert on firing metal clays and troubleshooting)。 A witness strip is fired alongside your piece。 Upon removing your work after firing， you can perform tests on the strip， which will give an indication as to what is going on inside the piece you fired it with。 If it is under-fired you will be able to re-fire the piece。 This isn´t a substitute for testing to arrive at a firing schedule。 This is a technique to do after the firing schedule is already determined， anytime you would like to be informed about a particular piece or group of pieces you are firing。 Gordon Uyehara describes this use for test strips in his excellent book， Metal Clay Fusion。

So， how do we "read" a test strip?

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 about firing bronze metal clay。

There are many tests you can perform on a metal clay strip。 The first one is done after removing the strip from the carbon。 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 topowder metal 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 thispowder metal 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 stripsintered metal should not crack or break。