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Aluminum Contamination and Silver Clay

by Mary Ellin D’Agostino

Some time ago, I ran some experiments to help one of my former students track down a problem.   She and some of her students experienced curling, warping, surface deformation, spots, and what they thought was incomplete sintering of the silver clay. Was it a problem with the PMC she was using? I asked her to come over and bring the problem clay so we could try to figure out what was going on. On her way over, she had a light-bulb moment and realized that the work surfaces she was using might be made from aluminum. That gave us an excellent place to start.

Most of us know that silver clays (PMC or ACS) can be contaminated by contact with aluminum, but many of us ask just what is the problem and how long must the contact be to contaminate the silver?

The essence of the problem arises from an electrolytic reaction between the silver and the aluminum (or aluminum oxide). A full technical description is beyond me, but my understanding is that the reaction between the two is essentially the same as what happens in an alkaline battery. What we are getting is similar to the corrosion/crud that forms on the battery contacts.

What I can give you is a description of what happens to the silver clay from a practical viewpoint. The symptoms of aluminum contamination of PMC or ACS are:

  • Discoloration: Dark spots or areas of discoloration may appear on the unfired silver clay. Discoloration might not appear until after firing your PMC/ACS. After firing, discoloration can sometimes be polished away, but "ghost spots" will reappear later.

  • Warping: During firing, contaminated pieces may warp, curl, and deform dramatically. This far exceeds the minor kinds of warping we sometimes see when firing normal silver clay jewelry.

  • Brittleness & Flaking: Pieces may be brittle and surface areas may flake or spaul off of the piece. This can be confused with incomplete sintering of normal metal clay. It is, in fact, due to an incomplete sintering because the contaminated metal clay is chemically changed and cannot sinter properly.

The Experiment
Going back to the original problem to test whether the problem was due to "bad clay" or had been contaminated by working it on aluminum surfaces, we tested the aluminum contamination theory by exposing silver clay from a single package to varying lengths of time in contact with aluminum. One package of PMC+ clay was broken into four pieces. Each piece was rolled out into a slab 1mm thick and marked with numbers 1-4 and treated as follows:

  1. The clay was worked on the aluminum and dried elsewhere,
  2. Worked and dried on the aluminum,
  3. Worked on plastic and dried on aluminum, and
  4. Worked and dried on non-aluminum surfaces.

    All the pieces were dried on a heating tray for about 15 minutes and then fired in a kiln at 1650F for 10 minutes

The longer the silver clay is left on the aluminum, the worse the reaction. The contamination caused dark spots and a brittle clay with a flaking surface. The most affected was the one worked and dried on aluminum, the next was the one dried on aluminum, and the third was only worked on aluminum. The piece made without contact on aluminum was fine.

Generally, if you are just using a cookie cutter made of aluminum and it is not left in contact with the clay you will be fine. If you really want to be safe, you can coat aluminum or suspected aluminum cutters with a coat of plastic or varnish. Make sure all drying and work surfaces are coated or made from non-aluminum metals.

Last updated on Thu, September 17, 2009 by Metal Clay Guru