Two-Stage Metal Clay Firing and Issues with Firing Containers

When I make fine silver pins, or pendants with hidden bails, I tend to fire the metal clay in two steps.

During the first step, I fire the main design on the pin. For cloisonné the main design is usually just a simple frame. With champlevé pieces the main design consists of raised areas of metal, and recesses that will eventually be filled with enamel.

Two freshly fired pieces intended for champlevé enamel

During the second firing, I place the hidden functional pieces like the pin catch or the tube bail. I use art clay oil paste to cement the components down (even though it smells terrible!!), because it seems to create a strong bond without bubbles or issues with porosity.

The back sides showing hidden pin and bail components

I settled on doing a two-stage firing for a number of reasons:

1) By firing the main design before adding components I am able to concentrate on getting a clean design impression. This way I won’t accidentally deform the front face of greenware clay when I am working on the back.

2) After the first firing, I can clean up the back side to a nice polish. I find that even when I polish and sand greenware clay, the pieces always show scratches after they are fired. Polishing the back to a consistent shine without ridges or marks is much more difficult after the component pieces are added.

3) I can easily experiment with the placement of the pin backs or bails without making a mess of the back side of the piece. I can easily wipe away smudges of oil paste with a cotton swab dipped in the thinning oil. This gives me the freedom to change my mind if I don’t like the spot where I originally placed a bail or pin component. This also makes it very easy to clean up messy oil paste seams before firing.

During the second firing, I usually support the pieces on a bed of vermiculite. This helps prevent the pieces from deforming or sagging oddly during the firing. I usually put the vermiculite in a ceramic fiber firing container like the ‘square head’, but in this case since I was wanting to fire two pieces at once I used a larger unglazed terracotta saucer.

I have had good luck in the past using terracotta pots and saucers from the dollar store. They do eventually crack, probably due to the stress of being exposed to high temperatures, but I can usually get quite a few firings out of them before that happens.

The particular terracotta saucer I used here was from a garden center, and it cracked spectacularly on the first firing.

My pieces fired nicely, despite the vermiculite spilling from the saucer. The mess in my kiln is easily cleaned, and is only a minor irritation. I suspect that what happened was that the saucer from the garden center had been exposed to a humid environment and had not completely dried out before I put it in the kiln.

Since I focus on making metal clay pieces for enamelling I avoid using stainless steel firing containers in my kiln. Stainless steel bowls tends to shed flakes of oxide that could cause impurities in enamel. I have also heard that the shed oxides collect on exposed kiln elements and burn them out more quickly, though this is not something I have had any personal experience with. For those of you interested in working with copper, bronze or steel clay, it is totally possible to keep your kiln clean enough for enamelling and still work with base metal. I simply use unglazed ceramic or ceramic fiber firing containers for the embedded coconut carbon firing.

Using ceramic fiber firing containers such as the ‘square head’ or ‘round head’ firing pots is the most reliable in my opinion. If you decide to use unglazed terracotta, make sure the pots or saucers are completely dry. Also, be prepared to do a bit of clean up once in a while.

I’m off to clean up my kiln…