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Small Kilns and Kiln temperature control.
Small kilns are now in common use for a number of interesting and intricate crafts. Fused glass, Glass bead annealing, Precious Metal Clay, Enameling, Cloisonné and Ceramics are popular.
Artists just starting out often think all they need is a small kiln and the simplest controls to be able to successfully pursue their craft. In fact, most of the artists who persevere eventually realize that they need more than the minimum means of control.
If you talk to an artist who has a real kiln controller they will usually credit much of their success to the fine results they can achieve because of the much greater accuracy and repeatability they are provided by their control system.
Beginners often think that the kiln is similar to their stove in the kitchen and to some degree this is true, but crafts have materials that are much fussier about temperature and time control than is true for cooking. On a stove, you set a temperature and the oven heats to and holds that temperature using the built in thermostat. Actually most stoves have a long cycle time and will vary from 30 to 60 degrees both over and under the temperature they are set for as they heat full on for a while and then are full off for a while. Also they tend to often be intrinsically inaccurate within a 50 or 60 degree range. You learn to compensate for your particular oven based on how the food comes out and by manual tests (toothpick for instance).
In a kiln it’s not quite so simple, manual intervention means you have to open the lid to look at it and even if you can judge your article’s condition, the mere fact of opening the lid drastically changes the temperature and you sure aren’t going to be sticking a toothpick in it
The “infinite” switch a small kiln is normally supplied with does not allow you to “set a temperature” It is merely an electromechanical switch that switches on and off at regular intervals usually between ten and 100 percent. Many small kilns will eventually climb to temperatures that will destroy the kiln on all but the lowest settings and the infinite switch will definitely not accurately maintain “any” temperature. In fact the kiln requires continuous monitoring, adjustment and periods of being turned manually off and on to even roughly control the temperatures.
On a small kiln the small dial type pyrometer is supposed to provide temperature information. These miserable items shouldn’t be allowed to be sold. The basic concept of a dial pyrometer is good, but these minimal small ones suffer two debilitating deficits. They are extremely inaccurate often over a hundred degrees off or more and their scales are so small they are almost impossible to read closer than a hundred degrees anyway. (You really need ten degree or so accuracy for really good, consistent results.)
As you can see, some means to more accurately control the temperature is a really important asset, if fact, as many artists will confirm, accurate temperature control is just as important as the kiln itself.
This is where electronic kiln temperature controllers come in. They are inherently much more accurate than a dial pyrometer and as their name says, at a minimum they actually control and accurately hold at least the final temperature.
There are two primary types of kiln controllers.
The “set-point controller” allows the kilns temperature to rise at full power or the rate established by the infinite switch to the temperature that has been set on the controller and then will accurately maintain that temperature until it is turned off.
The other kind of controller controls both temperature and time to a fine and often complex degree. Some “Ramp and Hold Kiln Temperature Controllers” can even allow complex “firing profiles” to be programmed into them.
The Ramp and Hold Controller allows the user to enter a target or set temperature, a time to hold at that temperature and a ramp time to get to that temperature from the current temperature. More capable controllers will allow you to string multiples of these ramp, hold and set temperature steps together to form a “firing profile” and most of these will also allow you to save and retrieve several “firing profiles”.
For some kinds of operations, the “set point kiln temperature controller” can be quite adequate. Small glass object annealing (beads for instance) works quite well if you simply set the kiln to an “annealing” temperature and deposit torch worked glass items in it as you complete them. After the last bead is inserted you allow the kiln to run for an additional 30 or 40 minutes before you turn it off. PMC also requires quite accurate temperatures but still responds quite well to the single process set point controller.
Generally the more expensive “Ramp and Hold Multi-step Kiln Temperature Controller” is more desirable for all kiln operations and can virtually guarantee a successful outcome, time after time. For many kinds of kiln craft uses, this kind of controller will truly spell the difference between success and failure.
Glass in particular is a fussy and particular material that requires very careful control of both temperature and time. Glass has a sensitive heating phase where the primary goal is the prevention of thermal stress, cracking and even shattering. Glass then has the working phase where it fire polishes, tacks, melts, slumps or flows into its desired configuration. And then glass has a very touchy cooling phase where it is extremely important to cool it slowly and evenly through the “annealing” temperature range so that it doesn’t crack or shatter and so that the finished piece of glass will not have built in stress that will cause or allow it to break easily later.
Although small glass items such as jewelry are somewhat less sensitive than large glass objects to temperature control inadequacies, small kilns respond much more quickly to both heating and cooling than larger kilns and thus require considerably greater “control” to achieve the same results.
Bubbles are one of the easiest problems to fix using a controller. Bubbles are caused by uneven stacking of glass and the edges of the glass fusing before the middle.
So if you hold your glass at around 1250 degrees for 25 -30 minutes to equalize the temperature and allow the middle to heat up as much as the edges, you can virtually prevent bubbles in the middle of your pendants.
In fact, really good temperature control is the single most overlooked requirement of small kilns and is easily the most common cause of failed, damaged or lack luster projects.
A good perspective on the importance of temperature control for a small kiln is from my wife Barbara who has been a fused glass artist for the past 15 years.
When I started firing glass we got a big kiln with an infinite switch and a pyrometer. I produced quite a few glass art objects, plates, bowls and sculptures, but I had a lot of breakage and cracking (frustrating and often unsatisfactory). We got a controller and overnight success went up to over 90 percent and with a little practice approached 100 percent.
For the past few years I have concentrated on small dichroic glass jewelry using small kilns. When I started firing pendants every day without a controller, success was spotty at best. (So I started “suggesting” to Gary that he make me a controller) Although catastrophic failure wasn’t too much of a problem it was very difficult to get the results that I wanted. I spent a lot of time checking my kiln and if I was distracted for a few minutes (not that difficult in my studio), the whole batch could end up over fused. I never even knew that my kiln pyrometer was off by 100 degrees till I got the first controller.
Then my husband built me a simple set point controller and every thing changed (again). Even though I had to turn it off manually at the end of its firing time, I could now achieve excellent, repeatable results every time (at least as long as I heard my kitchen timer go off).
Now I have his newest programmable ramp and hold controller, and as much as I dearly loved the old set point controller( I kept one in the studio as backup for a month in case I couldn’t figure out how to use the new controller), you couldn’t get the new programmable controller away from me for any price. I can now get the glass to do exactly what I want every time without having to pay any attention at all to it. My only contribution is to refine the process a little based on each firings outcome. Now if I want to fire polish my pendants I don’t end up completely fusing them again.
I should also say that although set point controllers are generally pretty easy to use, the more complex ramp and hold controllers often are not. Many of them are extremely difficult to program, and can cause more frustration than they solve.
When my husband, the engineer, was designing the controller I use now (the EZ2Set) I made it really clear to him that ease of use was the most important goal. He and his best friend, also a programmer worked over a year developing and programming it and they succeeded. The EZ2Set really is easy to set. It’s also the most accurate controller I have ever used and has virtually no over or undershoot, a problem that plagues other controllers.
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