Cone 10 Microcrystalline Vessel

Above is a small crystal glazed vessel, and below, is a macrocrystalline glazed vessel.

Cone 10 Macrocrystalline Vessel

The three general glaze firing temperature ranges are: 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 spreading blobs of color. These are easy and pretty but do not grow real crystals in the glaze.

Matt crystalline glazes give small and subtle crystals and are preferred by some. Some of my tests resemble matt crystalline glazes. They occurred when certain regular commercial glaze were 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 change the lighting angles. Pots with large smooth areas to allow the crystals to grow must be thrown. There are no commercially made glazes for the high fire temperature range, macrocrystalline glazing potters mix their own glazes. Glazing must be extra heavy on top. A catcher must be glued to the bottom of the pot to catch the run-off. After firing, the catcher must be separated from the glazed pot 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.

Most books about crystal glazes give only a few colorant choices. I've used a Peter Ilsley glaze recipe a lot. Fara Shimpo's e-book is more generous with colorant information but her recipes do not always work as stated with my materials in my kilns. However, she is a great place to start and with many trials and much hammered failures you do start to build a stable of glazes that you love. With my retired chemist husband and my artistically minded daughter, I have been varying component amounts and mixing glazes with occasional pleasant new results. PLEASE CHECK OUT MY STUDIO NOTES.

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. The crystals have some relief and could hold bacteria and there is the problem of possible leaching of colorants if acidic food is left for long periods. I have used crystal glazes on the outside of cups and bowls and they have been sturdy enough in the dishwasher. None of my glazes contain lead.