This month I wanted to revisit my previous test of enameling on sterling silver. Last fall, while testing for Mitsubishi I had a bad outcome with enameling on the new PMC Sterling silver clay. The test piece ended up with lots of large bubbles.
These bubbles showed up during the last firing. I had already fired two layers onto the metal with successful outcomes.
All pieces in today's test are made from the same PMC Sterling silver clay package; have the same size, thickness, and texture; and were fired together using the firing schedule recommended by Mitsubishi. Each piece had three layers of transparent Thompson Enamel applied to recessed areas. I used these colors: 2335, 2350, 2330, and 2190. All enamels were washed and wet packed into the recesses. No clear flux was applied. Each firing was performed at 1450˚ F - 1500˚F for one minute.
After firing the third layer, each piece was stoned under running water, brushed with a fiber brush under running water, rinsed in distilled water, and then fired again allowing the enamel to become shiny. I did not sand or remove tarnish as it developed on any pieces between firings. I did not pickle between firings. As in my test last fall and now, I made sure the enamel and metal was completely dry before firing.
My enameling tests are as follows:
Test 1 – Can air from the porosity of the metal cause bubbles in the enamel? Does burnishing the metal keep this from happening? Does the fired PMC Sterling silver require burnishing before firing enamel on top of it?
1A – the sterling silver piece is depleted* and then tumbled and burnished.
1B – the sterling silver piece is depleted and not tumbled nor burnished.
1A – It has very fine bubbles.
1B – It has very fine bubbles.
Theory- My best guess as to why I had large bubbles is that I may have piled too much enamel on my piece, thus trapping air in the enamel when it fired. This time I applied thin layers. I still have tiny bubbles, which may be inescapable when enameling on metal clay. (My enameled pieces on sheet don't have these.) All in all, the enameling came out looking nice the tiny bubbles are only visible by using magnification.
Another reason why I am asking this question is because typically when applying keum-boo to sterling silver, it must be depleted first. Celie Fargo has had much success with keum-boo without depleting the sterling silver metal clay.
2A – deplete the sterling silver, tumble and burnish metal before applying enamel.
2B – tumble and burnish the sterling silver metal before applying enamel but do not deplete.
2A – The enamel attached and it has very fine bubbles just as in test 1.
2B – The enamel attached and it has very fine bubbles just as in test 1.
Summing It Up
For enameling on metal clay, sterling and fine silver are both very nice. If I wanted to do something like this in sheet I would have to etch the design into the metal which would take much more time to make.
In the meantime, have fun claying around. I hope to see you all at the PMC Conference!
*Depletion - Removing the copper alloy in the outside layer of the sterling silver. This keeps the metal from tarnishing when heated. See my previous post where I explain how to deplete sterling silver.