First of a Two Part Series: Sintering, Test Strips and Carbon-Fired Clays
Ever since I’ve been 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.
It’s tempting to just dive right in with new clay and create a beautiful piece of art. But I always hold myself back until I’ve successfully fired a test strip. Then I can fire my work knowing that I have a pretty good firing schedule. There is no bigger disappointment than putting a day’s worth of work in the kiln, only to have it come out ruined—either over-fired or breaking easily from under-firing.
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 your piece. 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.
I really like this template from Metal Clay Supply for cutting out test strips. Since I make them often, it is a time-saver and also provides uniformity. I roll the clay to five cards thick. I also measure the strips before and after firing to help get a feel for the shrinkage rate of the clay.
Next time I’ll talk about tests to perform on the strips and what they can tell you about sintering.