Creation of this blog and how I came to cone 6 crystal glazes

I'm creating this blog to provide a site for folks interested in exploring zinc silicate, or 

macro-crystalline glazes, and for folks already working with these glazes who prefer to fire their kiln to cone 6 rather than cone 10.

Most independent studio potters who own a kiln, have a top loading kiln capable of firing to cone 10 (2350°F), but this is the manufacturer's recommended top temperature limit for this type of kiln. This is like owning a car capable of reaching a speed of 110 mph, but if one were to drive at that speed all the time, one would expect the car to wear out faster. Top loading kilns are built by manufacturers, using cost effective materials, to provide a product most potters feel they can afford. Most kiln makers also produce front loading kilns, that are better insulated, structurally stronger and more capable of reaching higher temperatures.

I became interested in crystalline glazes while in graduate school. At the time, there was very little published information that I could access about crystal glazes and this was before the advent of the computer and internet. During my graduate studies, Herbert Sanders published his book, Glazes For Special Effects, 1974, Watson Guptill Pub, which contained recipes for several types of crystalline glazes. The next year, David Snair wrote an article, Making and Firing Crystalline Glazes, December 1975, Ceramics Monthly. All of the crystal glaze recipes in these publications were listed for firing to cone 9/10.

Information in hand, I began making pots, mixing test batches of recipes, preparing catch plates and pedestals for the pots. If there was a mistake to be made, I soon discovered I was quite adept at finding them. From glaze peeling off the pots, to pots falling over, to catch plates breaking allowing glaze to leak out onto the shelf, I discovered them all. One of the most frustrating discoveries was finding the electric kilns I had available for firing were either incapable of reaching cone 10 or could not reach that temperature in a reasonable period of time. After several hundred pots and many failed firings, I gave up and moved on to explore other areas in ceramics.

It wasn't until the 1990's that I would revisit crystalline glazes. Some of my students saw images of crystalline glazed pottery and asked me how it was done. I scheduled an advanced independent study class for the next semester and we went about the process of making pots and glazes for testing. It soon became apparent that the electric kilns I had in the school studio could be fired to cone 10, but the firings would take far too long to complete in the time I would be on campus - a 16 to 18 hour firing was not unusual.

This lead me to begin another period of research, seeking information about firing crystalline glazes at a lower temperature. I found several sources that mentioned working at lower temperatures, but very little about glaze formulation or firing schedules. Since we were already conducting firing schedules at cone 6 in the studio, I decided to target my efforts at that temperature range. I purchased a small electric kiln to begin testing glazes in my own studio. Though programmable kilns were on the market, I bought a manually controlled kiln. The kiln has 3 sections and infinite control switches. This configuration allowed me to have fairly good control over heating ramps and holding temperatures over long periods of time. I still use this kiln, but now it is primarily used for bisque firings.

Crystalline glaze firing schedules place greater demands and stresses on kiln than most any other type of firing. Heating ramps are generally faster than in other types of glaze firings. The most critical part of the schedule are the holding temperatures and times. Instead of firing glazed ware up to a certain temperature, holding a short period of time, slowing the cooling a bit, crystal glazes are brought to temperature rather quickly, held at top temperature, cooled a couple hundred degrees and held at that slightly lower temperature for several hours before the kiln is shut down. This firing schedule places an extra strain on the heating elements and other parts of the kiln. Potters who fire cone 10 crystalline glazes find they will have to replace elements more often to be able to maintain their desired firing schedule. Firing at a lower temperature does save time and energy, but more than that, it saves money because elements do not have to be replaced as often.

I wrote an article about my discoveries entitled: Crystalline Glazing at Cone 6, published in Clay Times, Volume 6, No. 5, Sept/Oct, 2000

I finally arrived in the 21st century and purchased a programmable kiln in 2005. This kiln has allowed me the flexibility to program very specific firing schedules and repeat the schedule again and again. My previous experience with a variety of kilns led me to have this kiln equipped with a couple optional features. Instead of heavy duty Kanthal A1 elements, I had APM elements installed. APM' s cost at least twice the amount than the A1's. But by firing only to cone 6, I expect the APM's to last twice as long as a set of the heavy duty elements. I currently have more than 50 firings on this set of elements and have yet to see an increase in times to achieve desired heating ramps. I also had "S" type thermocouples installed instead of the typical "K" type. At temperatures above 2000°F, the "S" type is much more accurate.

Here's an image of my kilns, showing the vent system arrangement. I highly recommend using a direct vent downdraft vent system on all electric kilns