Crystalline glazes are those special ceramic glazes in which crystals actually grow inside the glaze while it is still very hot (about 2100° F). The crystals start out as microscopic spots in the glaze, called "seeds." These seeds form spontaneously in the glaze, in random numbers and locations. The glaze is first fired to about 2400° (or more) to thoroughly melt all of the ingredients. It is then cooled to the crystal-growing temperature, where the crystals start growing on the seeds. The longer the glaze is held at that temperature (from 6 to 12 hours), the larger the crystals get — until the maximum size for that particular glaze is attained.
In order for the crystals to grow, the glaze must be very fluid. Because of this, much of the glaze runs off the piece during the firing. In order to contain the run-off, a catch basin and specially-shaped pedestal are made for each piece. The piece is placed on the pedestal in the catch basin for the firing. After the firing, the piece is carefully separated (with some difficulty) from the pedestal, and its bottom is ground flat.
There are a few types of crystalline glazes. The most popular one has Willemite crystals. This glaze was developed at Sevrès in France in about 1850 and became very popular from 1890 to 1915, during the Art Nouveau period. About 1915 two things happened: the Art Nouveau movement ended and the production of ceramics became industrialized — but crystalline glaze does not lend itself to mechanized production. Very little Willemite crystalline glaze has been produced since then. As a result most people have never seen an example, except perhaps in a museum.
One of the very interesting features of willemite-glaze crystals is that they are sometimes "optically active." That is, if you change your viewing angle of a crystal, its internal features seem to move. As I find this trait very attractive, I have spent a lot of time developing glazes that enhance this effect.
A very rare feataure of crystalline glazes is the appearance of "secondary" crystals. These are quite small (less than a 1/4") and are usually a very interesting addition to an already beautiful glaze. They come in several different colors (silver, gold, black, etc.) and many different shapes (square, round, needle-like). I have spent a lot of time developing several glazes which can occasionaly show this unusual feature. It is interesting that the chemical stucture of these crystals has never been determined.
This glaze is extremely fickle. One can fire it in exactly the same way two days in a row, and it will come out quite differently each time---frequently with no crystals at all. Consequently, the results are quite unpredictable, both as to numbers and sizes of crystals.
Most potters consider crystalline glazes to be the most difficult and challenging of all glazes to produce. This is because they are unusually difficult and time-consuming to formulate and fire. They require meticulous attention to every detail. Also, the "success rate" is much lower for this glaze than for others. Often less than 50% of the pieces turn out; the others either have poor color, no crystals, or break.
Due to the lengthy time required to master the glaze, the great care that must be taken in its compounding and firing, and the less than optimal success rate, rather few potters today use this glaze.