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About Crystals...

There are three general glaze firing temperatures: low fire (cone 06 around 1800 F), mid fire (cone 5/6 around 2200 F) and high fire ( cone 9/10 around 2300 F).

There are low fire glazes called crystal glazes with chunks of glassy material which melt to give blobs of color. These are easy and fun but not true crystals.

Matt crystalline glazes are smaller and subtler and preferred by some. One of my samples resembles matt crystalline glaze but is not matt. This occurred by accident when a regular commercial glaze was fired in a crystal firing run.

Macro crystalline glazes have big crystals with wonderfully varied shapes and a three dimensional quality of shifting highlights as you move around them. Pots with large smooth areas to allow the crystals to grow must be thrown. Glazing must be extra heavy on top. A catcher must be glued to the bottom of the pot to catch the run-off. Then the catcher must be broken off and the edges smoothed down. Firing is more complex than regular glaze pots- there is a quick heating, sudden partial cool down and a temperature hold for the crystals to grow. The results can be surprisingly good or just surprising.

Crazing is a cracking of the glaze due to tension while cooling. Commercial glazes generaly don't craze. All of my vases have a commercial glaze inside and so, do not have crazing inside so they are quite ready for water and flowers. (Exception-when the neck is very small, the inside is unglazed. Glaze will not pour in, neither will water. These pots are only suitable for a few dry stalks.) The general consensus is that crystal glazes are not food safe because the crystals have some relief and could hold bacteria. I have used crystal glazes on the outside of cups for family use and they have been sturdy enough in the dishwasher. None of my glazes have lead.