True Cone 10 Zinc Crystal glazes have fascinated potters for the last hundred years. One major advance, digital controllers, put Crystal glazes in reach of any potter. In this short article, I will outline the basics and give some recipes.
Clay: use a smooth, white clay for cone 10; such as New Mexico Clay' s Domestic, CK-mix, or any cone 10 porcelain. Try making plates or tiles at first, as crystal glazes are VERY runny*. If you wish to make a vase you must throw a little dish to fire your pot on; after bisquing, glue the dish and the vase together with alumina hydrate.
The glaze: After trying many different glazes, I recommend one based on Ferro Frit 3110. Herbert Sanders book on Glazes for Special Effects (sadly, out of print) lists this as Glaze #3.
Notice that this glaze is very high in Zinc and contains no Alumina. The excessive amount of Zinc crystallizes into Zinc Orthosilicate crystals in the glaze, the same way sugar when super saturated in water will turn into rock candy. Low alumina allows the necessary fluidity* that the crystals need to be able to grow.
For color add these oxides to base glaze:
Cobalt Carbonate .75% to 1% for dark blue crystals with a lighter background.
Iron Oxide 4% for brown crystals.
Nickel Oxide Green 4% for blue green crystals on brown background.
Manganese Dioxide 1% to 4% for lavender on tan.
Uranium Oxide 6% to 10% for yellows and golds.
Praseodymium Oxide for white with yellow edges.
Apply the glaze by spraying onto bisque, as it contains no clay and is horrible to brush on. Try layering the glazes, as opposed to mixing the oxides, for interesting varied colors.
Firing: In general fire to cone 10, cool the kiln to 2000 degrees and hold for 3 to 5 hours in-between 2000 and 1800 degrees.
With a digital kiln a firing would go like this:
Ramp 1: rate 250°, to 1000° no hold.
Ramp 2: rate 500°, to 2320° hold 15 minutes. (00.15)
Ramp 3: rate 500° (to 9999 to cool faster), to 2000° hold for 03.00 hours.
Ramp 4: rate 500°, to 1800° hold for 01.00 hours.
Let cool, no peeking!
*These glazes are very runny. A catch plate is a base you throw that matches the bottom of the pot, it is then glued onto the bottom of the pot with a mixture of alumina hydrate and elmers glue. After firing the catch plate is removed and the bottom is ground to a nice finish. That is why I just made plates and bowls, not vases.
The Sandia Mountains - Plate by Brant Palley Cone 10 crystal glazes 1988
Domestic Porcelain is a blend of three American kaolins, NO ball clays, and two plasticisers to create a porcelain clay less expensive than the Grolleg bodies, a little less translucent, whiter, and a lot easier to throw.
Cone 10 shrinkage 15.00%
Zinc Oxide Calcined
In general, zinc increases the maturing range of a glaze, promotes higher gloss, brightens colors and reduces expansion. It is used extensively in once fired Bristol Glazes and crystalline glazes.
Great for old fashion bristol glazes.