Zinc Crystal Pieces
Zinc crystal pieces are a relatively new addition to my repertoire; they have a presence and verve that I really like. A beautiful zinc crystal piece can dominate a room.
The bottom line on zinc crystals is that the glaze is incredibly runny. In fact, the glaze IS going to run, and it will run a lot--probably a third of the glaze will flow off of the pot during the firing. Knowing this, the wise zinc crystal potter takes steps to ensure that the pot does not become permanently wedded to the shelf upon which it stands in the kiln. My method for this is to make cylindrical stands that are exactly the same diameter as the foot of the pot, which raise the pot about three inches off the shelf. I glue the pot to the stand, then place the glued pot-stand combination into a saucer on the kiln shelf. During the firing, the glaze runs off the pot, down the stand, and into the saucer. After firing, I break the stand and saucer from the pot and proceed to grind and polish through eight different grades of sanding and polishing. The final product is very smooth, and the innocent bystander would never suspect that that the foot of the piece was once razor sharp and jagged.
Another interested aspect of zinc crystal pots is the firing process, which can be quite complicated. These pieces are oxidation fired in an electric kiln, and it is important for the kiln to reach 2350 degrees Fahrenheit in a short period of time. Because the kiln heats more slowly with a greater number of pots in it, I stack only three or four pieces in each firing. After quickly firing to temperature, I cool as fast as possible to around 2000 degrees Fahrenheit and hold the kiln temperature for 40 minutes. It is during this period that the crystals form and begin to grow on the surface of the glaze. After holding, I drop the temperature to 1950 degrees Fahrenheit and hold for another 30 minutes. This new temperature also allows the crystals to grow, but they grow differently at the lower temperature, adding interest to the final glaze. I repeat this cooling and holding process two more times, and then slowly fire the kiln down to 800 degrees Fahrenheit. After that, I allow the kiln to cool naturally.
Each different glaze, each different colorant, requires a different cooling profile as some materials, such as copper and manganese, help crystals to form, and others such as cobalt hinder crystalline growth. As a corollary to the previous sentence, I always fire the kiln with the same glaze on every piece.
For me, the key to really nice zinc crystals is to balance the crystals with the background. The best pieces are ones in which the crystals appear to float in the glaze.
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