During a controlled cooling period of the firing, heavier elements separate out from the rest of the glaze to form the nuclei around which the crystals develop. The number and position of the crystals is entirely random, and on any piece one can usually see crystals in all stages of development; from a tiny dot of colour through an intermediate spiky stage to the fully-formed flower-like crystals. Sometimes they are the same colour as the background glaze within which they grow, but just as often the crystal colour contrasts vividly with its background, e.g. when nickel is the added colourant the resulting crystals are deep turquoise floating in an amber magma.

In order for the crystals to grow, the glaze must be fairly fluid; therefore every crystal glazed piece requires a catch basin to catch any glaze which flows off the piece during the firing. If the catch basin fits the base of the piece exactly there is no space for glaze to flow between the two, and it is straightforward to separate the two after firing with a few taps of a chisel. Then the base can be ground smooth. I formulate the glaze so that the crystals have all they need to grow and, if I fire it correctly, they do. I have a fair amount of control over the colour of the glaze, (except in the oil-drip reduction,) but until I unpack the kiln I have no idea where the crystals will be. They are all zinc crystals with metal oxides added to the glaze to affect the colour. The rate of crystallization of glazes or glasses at ordinary temperatures is extremely slow: what transpires in my kiln during the 22-24 hour 1280 degrees C firing would take centuries in nature.

For ten years I have been experimenting with a short period of reduction atmosphere in the kiln after the crystals have formed. This transforms glazes, which have any amount of copper from greenish hues to iridescent reds, purples, and all sorts of metallic surfaces reminiscent of real copper and brass.

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