One secret of crystal glazes is firing schedule

The blue line is a crystal glaze firing schedule. While it reaches the same temperature as a typical glaze firing (purple line) it is different in how it does so. Notice key differences (while cone 10 is most common for this type of glaze, we will discuss theoretical differences in a cone 6 version):
-The steep climb: Crystallization needs a clean bubble-free melt, no lingering in temperature zones where they might start prematurely.
-The steep drop to 2000F: Crystals typically grow during a long soak in the 1900–2000°F nucleation zone.
-If the temperature is simply held steady at 1950°F only one type and size of crystal would form, likely smaller and crowding out others. The ups and downs are about manipulating the thermodynamics and kinetics of crystal formation — nudging new crystals to form or existing ones to grow differently.
-Cooling and then raising the temperature in the nucleation zone can re-dissolve smaller crystals or unstable nuclei. Then, cooling again encourages new crystal nucleation, rejuvenates existing ones or even changes the pattern of their growth.
-In the upper range of the nucleation zone, faster diffusion produces larger, more spread-out crystals. In the lower range, slower diffusion produces smaller, tighter crystals or detail-rich growth.
-Crash-cool to finish: Drop melt viscosity quickly to halt all crystal formation - this preserves a clean background and prevents blurring of crystal edges.
Crystalline firings are about precision and timing: Get in fast, melt everything, play within the range where crystals want to grow to get the type, distribution and size you want - and then get out. It is not difficult to see why crystal glazers may do thousands of test firings to discover the curve that produces what they want. The nucleation zone depends on firing temperature and glaze chemistry, testing is likewise required to discover it. Meticulous record keeping is critical to success; not surprisingly, many crystal glazes do it in an account at insight-live.com.

Related Pictures

Growing incredible glaze crystals is also about the chemistry

This picture has its own page with more detail, click here to see it.

Closeup of a crystalline glaze. Crystals of this type can grow very large (centimeters) in size. These grow because the chemistry of the glaze and the firing have been tuned to encourage them. This involves melts that are highly fluid (lots of fluxes) with added metal oxides and a catalyst. Typically, the fluxes are dominated by K₂O and Na₂O (from frits) and the catalyst is zinc oxide (enough to contribute a lot of ZnO). Because Al₂O₃ stiffens glaze melts, preventing crystal growth, it can be almost zero in these glazes (clays and feldspars supply Al₂O₃, so these glazes have almost none or either of them). The firing cycles involve rapid descents, holds and slow cools (sometimes with rises between them). Each discontinuity in the cooling curve creates specific effects in the crystal growth. These kinds of glazes are within the reach of almost anyone today since electronic controller-equipped kilns are now commodity items - one can fiddle with the chemistry and manage the testing of glazes in their insight-live.com account. But be ready for work, others on this path have done hundreds of firings to perfect a recipe and scedule.

Crystalline glazed vase by Rod and Denyse Simair

This picture has its own page with more detail, click here to see it.

This award-winning couple are-all in on crystal glazes. They have learned that success is about data. A lot of data. Thousands of pictures, hundreds of firing schedules, hundreds of recipes, endless notes all come together in the growth of crystals like these! Notice the clear background, no micro-crystals fogging it up. Notice that two fundamentally different types of crystals are being grown. Not to be ignored is the throwing skill it takes to make a porcelain piece like this, these are not small pieces.

Videos

Links

Glossary

Crystalline glazes
A type of ceramic glaze made by potters. Giant multicolored crystals grown on a super gloss low alumina glaze by controlling multiple holds and soaks during cooling

Got a Question?

Buy me a coffee and we can talk