Crystalline Glazes

About Crystalline Glazes…

You can’t mention the word “crystalline” without saying “porcelain” first.
Porcelain is the preferred clay body for the majority of crystalline potters.
The Chinese are credited with the discovery of  porcelain somewhere around 960A.D., during the Sung Dynasty. The Chinese kept it a closely guarded secret from the rest of the world until the beginning of the 19th century, when a French monk brought the pieces of the puzzle back to Europe. The discovery of crystalline glazes soon followed, but when the effect was first seen they considered it to be a mistake. The Chinese started taking notes and began to appreciate this unique looking glaze.

When the Europeans had the building blocks for crystalline pottery they soon discovered how incredibly difficult it was to produce it. A 50% to 70% rejection rate was quite common. Since the supply was so limited, the art form could only be had by the wealthy.

Fast forward to today. We now have computerized kilns that can maintain the precise temperatures necessary for modern potters to have more success, but the newer equipment does not make crystalline glazing fool proof by any means.

The basic components of crystalline glazes are Zinc Oxide, Silica and a frit, (ground glass) with very little alumina. Because of the limited alumina the glaze is designed  to flow. Crystals can not form unless there is movement in the glaze.  The glazed pot is heated to 2,350 degrees Fahrenheit, which is the point where the glaze will reach its fusion point. At this time a reaction takes place between the zinc and silica, creating a seed crystal. The seed is then nurtured at a temperature that is most desirable for its’ growth. The growth temperature is maintained anywhere from 5 to 10  hours, depending on the desired  crystal size.

Crystals must be grown in an oxidation atmosphere. That means that air is allowed to flow through the kiln during the firing process. There is a fairly recent move to traditional reduction firing after crystal growth has taken place. Reduction is when you reduce the oxygen inside the kiln, thereby reducing the oxygen in the glaze and increasing the potters’ possible color palette.

Crystalline glaze colors are made from various minerals found in the earth. Oxides, dioxides and carbonates of colorants are all mixed together at certain percentages to create the different array of possible colors. I hand mix all of my own glazes and then produce “test pots” to see the final results of the materials I have combined work. The testing process can be very time consuming. Color of background and crystal are considered as well as number, size, and shape of crystal. Adjustments are made. Another test pot bites the dust.  

A collection tray is necessary at the bottom of every pot to dam up the flowing glaze. If you don’t dam the flow of the glaze, you’ll burn a hole right through the bottom of the kiln!

A pedestal is needed to separate the pooling glaze from the pot itself. The trays and pedestals are removed when the firing is done, by heating the base of each pot with a propylene torch. When the pot is ready to be released you will hear a distinct “ping” sound. Then just tap lightly with a small hammer and, voila!, there you have it, a one of a kind, fabulous crystalline pot.