Crystalline is a special effects glaze found in pottery which contains large (or macro crystals) of zinc orthosilicate. The naturally occurring mineral form of zinc silicate is called Willemite. Many pottery glazes produce crystals but most of them are too small to display their structure. Iron, barium, and titanium all produce crystals in glazes, but only zinc makes the spectacular crystals of up to three inches or more in diameter.
Most people who observe crystalline for the first time are amazed and usually ask "What is it and how did it get on the pot". Some think it's painted or explodes on there or the crystals are applied to the pot in some mysterious manner. I've even been accused of blowing the crystals onto the pot through a hole in the kiln during the firing.
How is it really done? Well, here's the secret. The glaze contains three basic ingredients necessary for crystal growth: zinc oxide, silica (or ground quartz) and a frit which is fused glass that is ground to a fine powder. These three powdered ingredients are mixed with water to form a thick glaze which is then applied to the pot. The pot itself is turned in porcelain clay for best results. Stoneware clay can also be used. Various metallic oxides such as copper or cobalt oxide, iron, manganese or nickel oxide are added alone or in combination to give colors ranging from white (absence of color) to blue, green, gold, purple, orange, etc.
Crystalline glazes are very runny, that is when the pot reaches peak temperature the glaze is actually slowly flowing off the pot. Here's where things get a little tricky. Each pot must rest in a small dish mounted on a pedestal. The dish collects the glaze so it won't run off onto the kiln shelf and ruin it. The pedestal keeps the bottom of the pot from sticking to the dish. The pedestal is carefully removed after the pot cools to room temperature with a mallet and chisel and the bottom of the pot is ground smooth on a silicon carbide grinding wheel.
When the pots in the kiln reach about 2350 degrees Fahrenheit (1304 degrees Celsius) the kiln is turned off and the temperature allowed to drop about 300 degrees Fahrenheit. At this point the glaze is no longer flowing off the pot but is still molten. This is the point where the crystals form. Since the glaze is saturated with zinc silicate, which has formed when the zinc oxide and silica are disssolved in the molten glaze, the crystals form as the pot is held at this cooler temperature. This is roughly the same thing that would happen if you dissolved a large quantity of salt in boiling water and as it cools, sodium chloride crystals would form. The kiln must be maintained at this lower temperature for several hours to allow the crystals to grow. The longer you hold it there the larger they get provided enough zinc silicate remains in the glaze to form the crystals. I use a small kiln controller which contains a microprocessor to automatically hold the kiln at the proper temperature. The kiln is shut off after the crystals have grown to the desired size.
The crystalline process contains so many variables that consistent or predictable results are almost impossible to obtain. This is what makes it most interesting to me. The challenge lies in gaining some small measure of control over the process. There is also much room for experimentation to obtain different results. Lowering or raising the temperature slightly as the crystals are growing, produced by change of temperature or by adding various chemicals yields anything from fan, bowtie, flower-like or needle shaped crystals.
One interesting technique I use is the production of copper red crystals. This is done by adding copper to the crystalline glaze. The resulting pot comes out of the kiln green. It is then placed in the kiln again and heated up to 1500 degrees Fahrenheit. At that point vegetable oil is slowly dripped into the kiln, and as the burning oil uses up oxygen from the surrounding atmosphere of the kiln and from the glaze, a red colloidal form of copper or more often a copper luster, which is a thin layer of metallic copper, forms on the crystals.
Needless to say I can't wait to get into the kiln to see the always different results. As soon as the kiln has cooled sufficiently to look inside, i'm out there with a flashlight, shining it in the peephole of the kiln, burning off eyebrows to see what kind of crystals I got. At around 550 degrees Fahrenheit I remove the pots from the kiln. This results in pine resin being drawn out of the pine floor of my shop, creating a pleasant aroma of pine and many little round scorched places all over the floor. Glazing and preparing the pots for firing sometimes gets a little tedious, but I never fail to get excited about opening the kiln.
Historically, crystalline has probably been around a long time. Some say the Chinese produced crystalline hundreds of years ago, but I have never seen an example in a museum or pictured in any books dealing with Chinese pottery. I imagine crystals did occur by accident throughout history on pottery and the Chinese with their rich and ancient pottery tradition more than likely encountered them.
It was actually around 1890 in France at the Sevres porcelain factory that a chemist developed the macro-crystalline process that we know today. I'm sure they were difficult to produce since they didn't have the readily available frits and kilns that can easily maintain a constant temperature that we have today. But some very spectacular and beautiful crystalline pottery was produced by the art potteries of the late 1800's and early 1900's such as Rookwood Pottery. In the early 1900's in the North Carolina Mountains, Pisgah Forest Pottery produced some crystalline pottery achieving some lovely results in a wood fired kiln. These are very rare and expensive as is all of the early crystalline pottery. More recently, extensive work was done by the late Dr. Herbert Sanders of California State University, and much information appears in his book published in 1974 entitled "Glazes for Special Effects".
I first encountered a crystalline pot in 1978 made by Mike Feree, instructor of pottery at Montgomery Community College, where my wife Milly was a pottery student. His class had tried crystalline glazing based on an article by David Snair in the December 1975 issue of "Ceramics Monthly". I began doing crystalline in 1990. I was helped getting started quite a bit by Beven Norkin, a creative genius, and excellent crystalline potter from Norfork, Va. He freely shared information he had collected from twelve years of crystalline experimentation. Beven continues to work alone in relative obscurity trying to master the control of crystal growth and is currently producing some spectacular results I've never seen before.
After having produced well over 1000 crystalline pots over the last few years, I'm just now starting to get a feel for the process. I'm not able to devote all my energies to crystalline and would not want to do so, but I keep trying out new ideas all the time with this challenging glaze process.
So if you want to see some crystalline pottery come to Dover Pottery, Turn off of highway 705 onto Dover Church Rd. accross from Westmoore School, go 2 1/2 miles and look for signs.