I have been asked by PMC Connection to test the compatibility of the new sterling clay with copper clay. Here is my report, which includes instructions for applying projects from my book Patterns of Color in Metal Clay to a combination of sterling and copper, as well as some suggestions for avoiding disasters and saving material. Please note that I have not tested the new sterling clay on its own, just the way it works with copper clay.
When firing silver clay with copper (and this applies to both fine silver and sterling, and copper clay in general), the firing temperature should be lower then the manufacturer’s suggested temperature. Fire according to the White Bronze schedule (Schedule C in my Instruction Manual, p. 27). I fired sterling and copper at 1320°F/715°C in a brick top loader, which would be equivalent to 1400°F/760°C in a front loader, and my pieces melted. This temperature is way below the recommended one for the new sterling clay.
The piece on the top right was not fired at this temperature.
The inlay ring in the photo above was made with copper clay and the new sterling clay. Both clays were fired together at the low temperature for White Bronze. The ring shrank 3 sizes. Because of possible reactions with the skin, I chose to make the shank from sterling. I noticed 2 problems:
1. As you can see in the photo, some copper made its way into the inner shank, probably while I was smoothing the inside with a wet finger. This could not be detected prior to firing.
2. Although I was able to hammer the ring into shape on a mandrel without breaking it, I am not sure how strong it is because it was fired below the recommended temperature. Theoretically, the shank could have been fired first at the high temperature for best strength. In practice, since the shank was not going to shrink again in the second firing, the surrounding layer would probably crack while shrinking around it. The same thing would have happened if the shank was made out of copper.
Copper was fired first. Silver was wrapped around and fired with a torch. It worked just fine.
Silver was fired first, copper strips were wrapped around it and fired in carbon.
The copper strips sintered, but always split and came off. At first I thought it was a shrinkage issue, but eventually I started to suspect that some rejection reaction was taking place. This hunch was supported by the recent experiments that I have been doing with combining the new sterling with copper.
Mokume Gane Patterns
In these patterns silver and copper have to be fired together, without firing one of them first. I have tried it before with home-made sterling clay, and it worked just fine. You can see photos on p. 112 of my above-mentioned book. Here is my first try with the new sterling clay:
I noticed the cracking and separation while I was still burning the binder on a stove top. It took 4 more rounds of patching and firing to bring the earrings to a decent state:
This has never happened to me before when combining either fine silver or home-made sterling sterling with copper. However, before rushing to conclusions about the new sterling clay, I remembered what happened when wrapping copper around silver, as shown above. As you can see in the photo above, the mokume gane pattern is wrapped with copper. Perhaps it was the rejection reaction that I suspected four years ago.
So, I tried pieces without framing them in copper. I watched them on the stove top and no cracking occurred.
Because of the high shrinkage rate of the new sterling clay, I fired the above rectangular earrings with my Traditional (Flex) copper clay, which shrinks more than the Quick-fire version. The circular earrings were fired with Quick-fire copper. The shrinkage rate of both pairs of earrings was the same. However, while the surface of both was not flush after firing and required some grinding, the Traditional copper was easier and quicker to grind.
By the way, with home-made sterling clay the shrinkage rate of mixed pieces is very low.
How do we make sure that the outer layer is always silver and not copper?
Most of the mokume gane patterns introduced in my book are created with a clay extruder. The instructions are always to stack the circles with the copper closer to the opening to the extruder. That way the extruded cane always has copper on the outside. When working with copper and silver, make sure the silver circle is closer to the opening, so the extruded cane is silver on the outside.
Making sure that the outside layer is always silver sure helps a lot. However, cracking did occur in all my experiments, not in the first phase but in the second. All pieces required repair and re-firing, although not as may times as when the copper was the outside layer. Cracking did not occur when I used home-made sterling clay.
How to Save on Silver
Naturally, when sanding and filing pieces from copper and silver before firing, the silver powder gets mixed in with the copper powder. I have been saving the powder, as well as mixed clay that was leftover in the extruder. I have used this clay for making backing layers for the pieces and for patching in hidden spots. The earrings in the following photo were backed with mixed clay:
The backing layer has a yellow tint. Also notice the pits. I was not able to remove them, even with heavy-duty sanding.
The circle on the left is what was left over from the mix after I’d finished the experimentation. It has a pinkish color.
Just as a point of interest, I have noticed that you can mix up to 35% copper in fine silver before any color change is detected in the fired piece.