The glazes I've worked with have a surplus (supersaturation) of Zinc Oxide in them, yielding crystals upon cooling. If you look closely at galvanized steel sheets, you see the same pattern. Hamer's dictionary says the crystals grow from 1508 degrees F to 2300 degrees. Theoretically (and a lot of crystal glazers do it), a slow cooling cycle in this range should promote crystal growth. On a practical level, I don't have a fancy up and down kiln controller, don't really want one, and get good results by shutting the kiln off at cone 10 and letting it coast naturally, even with firing times as short as 6 hours to cone 10. But here are the rules of thumb I use:
Increased beauty, increased pain: To get crystalline pots, expect a higher level of failures: glaze running onto the shelf, pinholes, too hot, too cold. With the glazes I use, probably a quarter still become second quality for some of the reasons just cited.
Wax or clean 1/2 inch up the foot. Wax makes an even line, whereas toothbrushes, sponges etc. are uneven and can cause a little track for glaze to start running. If it can run, it will... Also, a flared foot can help, since glaze has to spread itself thin to run over a wider area.
Make sure the glaze is thick enough. I dip all my pots. I hate double dipping, except in confined areas that I don't have to touch for a day or two, because the glaze takes a long time to dry where double dipped. So make sure the crystalline glaze is thick enough to give a good coating. If too thin, it blends in with the clay, and crystals don't have a good growth environment.
Know your kiln: Depending on which elements you've replaced and other factors, different parts of the kiln can be hotter than others. For crystalline glazes, hotter is better, to a point (the point where it all runs off the pot, for instance.). If you put crystalline pots on every level, you can relate the best results to the best areas. If you find consistent cool spots in the kiln, you might replace the elements in that area, as new elements heat better, or even replace with heavier elements (Duralite and Euclid's can help you with that)...
Interpreting results: If underfired even slightly, the glaze will form microcrystals and appear matte and darker. These pots can be refired usually to good effect. If overfired, the glaze will run off the pot, and appear thin at the top of the glazed area. Often no crystals will be visible on these pots, but a high gloss glaze will be there... Once overfired, don't bother trying to fire it again...
Be ready for drips: For testing, fire in a waste pot or specially made saucer, with kiln wash. I never used to wash my shelves, but these glazes spit a fine spray of glaze, making frequent kilnwashing necessary to avoid losing chunks of the pot to the shelf.
Color development: Rutile is used as a source of titanium, helpful for crystal seeding. Rutile is good for producing variation and opacity in the resulting glaze. Theoretically Titanium Dioxide should work better instead of rutile, but my tests have shown a preference for rutile. In oxidation, Copper Carbonate will produce greens, Cobalt Carbonate-- blue, Iron Oxide-- brown. Click here for my page of color tests.
Integrating with your forms and with other glazes: The
crystals stop as soon as you overlap them with any other glaze, since the
solution is no longer saturated. However then the runniness of crystal
glazes comes into effect. Therefore, if you put the crystalline glaze
on the top of a pot, and dip the bottom in another color, expect the crystalline
to run down fairly far into the lower glaze. I personally don't like the
irregularity that results, but to each, his-her-or-their own...
A recipe for a basic crystalline glaze is found at my Glaze page, accessed from the pottery menu button...