Crystal Glazes

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 WW, B-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.

FF3110--- ---------------------52% Calcined Zinc Oxide------------24% Silica 325 mesh-----------------24% To this base glaze add: Rutile ----------------- 2% Bentonite------------- .75% Epsom salts--------- .3% CMC------------------- .5%

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!

Brant Palley

Crystalline Sandia's

By Brant Palley 13"

Making a Pot Pedestal
1. Use same clay body as the pot.
2. Best if outer rim of pedestal is same size as widest diameter of pot (because it doesn't waste kiln space)
3. Center pillar on which pot rests, should be the same height or 1/8" higher than outer rim of pedestal. After glaze firing, it may be necessary to use a hammer and chisel to separate the pot from the base. You want to see the line where they join, and have space enough to place the chisel.
4. Trim pot and pedestal so that outside diameters match exactly. If pot is wider than pedestal, glaze runs under the foot of the pot. The same thing happens if the pedestal is wider than the pot.
5. Pot and pedestal can be bisque fired together, or separately if matching numbers are put on pot and pedestal.
6. Mounting the pot to the pedestal

A. If a closed pot, use the liner glaze first.
B. Apply kiln wash to bottom of pot.
Allow to dry. (Some apply a light kiln wash to the top of pedestal).
C. To glue pot to pedestal Mix alumina hydrate with Elmer's glue to a consistency of thick pancake batter. Apply to top of pedestal (not to the pot). Carefully set pot on pedestal, to align as perfectly as possible. Press gently and allow to dry thoroughly. Do not move the pot after setting until glue is dry. Pot might fall off during glazing.

7. After glaze firing, if pot and pedestal do not separate with a firm tap, you may need a hammer and chisel. Put the chisel on the line between pot and pedestal, tap
gently working around the pot.
8. If glaze has run under the foot of the pot, use a grinder to remove. A small cement drill bit or wire brush will remove kiln wash from the bottom of the pot. (this description was given to me by a customer, i don't know who to attribute it to)