Copyright © Annie Cap, Crystalline Potter
Crystals in glazes have been around since pottery first began, however they were thought to be flaws and undesirable blemishes! It was only relatively recently, during the Art Nouveau period, that a crystalline glaze was developed on purpose at The Sevres Porcelain Factory in France (around 1850). Yet, by about 1915, as Art Nouveau was ending, ceramic production became industrialised and crystalline glazing was abandoned as too expensive and unsuitable for commercial production. For many of you, this may be the first time you have ever seen crystalline glazed pottery.
Crystalline pottery is highly unpredictable, time-consuming and costly to make. Coupled with an 'accepted' exceptionally high failure rate, it's not surprising there are so few crystalline potters in the world, and why it's usually only found in galleries or museums. Developing and working with crystalline glazes is simply difficult!
If you are lucky, your crystals will 'seed' forming nuclei randomly within the molten, fluid glaze matrix and your stars are born! The zinc-silicate molecules, able to float about in the glaze, start to cling together, first forming rods, and then, given enough heat and time, they expand and blossom, much like flowers.
The high and prolonged heat required to grow dramatic macro-crystals doesn't just increase firing costs, however, it dramatically increases problems as your pots and glazes are stretched to their limits and often-times beyond. With crystalline glazing you get more of every difficulty there is in pottery. There's more warping, bloating (when gas gets trapped between the clay body and the glaze), pitting, pin-holing, cracking, slumping, and generally A LOT more breakage, along with other snags less common in typical firings, including dunting and crawling. And frequently, the crystalline glaze process is too much for your pots and/or your kiln. I've had entire loads of pottery simply fall to pieces on cooling or pots where the glaze popped off and adhered instead to the kiln shelves! It's not uncommon for crystalline potters, or crystallieres as we are called, to have only a 30% success rate.
Additionally, crystalline glazes are extremely finicky to apply and fire. They are applied very thickly and once melted in the kiln are extremely runny. To avoid ruining the kiln with each firing, like most crystalline potters, I throw individual ceramic pedestals (rings to elevate the pots) and separate or integrated catchers (shallow bowls or saucers) to collect the glaze which runs off the vertical pots. These pedestals and catchers must then be broken from the fired pots (another chance to ruin your pieces ) and the bottoms are smoothed with a diamond lap grinder.
Crystalline pottery is something rather special. It's not just difficult to make--but each piece is unique. Like each of us, no two will ever be the same. It's impossible. Maybe that's the real allure of crystalline ceramics for me; each piece is unpredictably individual and has its own special unnreplicate-able beauty. That makes the aggravation and time all worthwhile.
Even when you think you've developed a 'dependable' crystalline glaze, sometimes you will get crystals, and sometimes you won't. And other times, there will be so many crystals the glaze looks horrible and rough. Although with years of experience, you learn to control (or encourage) particular crystal colours and shapes, even growth rings and halos, you can never predict where or if crystals will actually form on a piece. They or nature decides this.
Macro crystalline glazing requires firing your kiln to very high temperatures, too (Cone 10 or 11, upwards to 1300 degrees C. ). Once the glaze materials are thoroughly melted, the kiln is allowed to cool, but only just slightly, down to within the crystal-growing temperature range (which varies by glaze)--and this is when the magic begins!