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I have found that having the instruments measure and control the temperature in the kiln when firing glass is essential for successful results. When I first started cooking glass, some 30 years ago, all I had were on/off switches and an observation chair with a seatbelt, which I called the “electric chair”. At least I had a switch for each of the four circuits in my kiln. Pyrometers were available, but I did not know that. So were controllers, but they were confined to the industrial sector. And controllers were not compact and computerized as they are now. As your projects increase in scale and mass, the necessity for control of temperature becomes increasingly important.
Visual firing techniques have been used since 2500 B.C. with wood, oil, and gas burning kilns; that is convection heating. Now most of us use electric kilns; that is radiant heating (direct drive). Electric kilns are fast and efficient (as long as no more utility companies go bankrupt). However, you cannot fire any faster than the glass will let you. Glass is the boss.
In this article, I will be directing all attention toward electric kilns. The temperature measuring instruments can be used with gas-fired kilns, however the controllers will need some modification.
Forward into the Past
You might notice that I am not calling this part “Back to Basics”, because understanding how glass acts under heat is the most important part. It may seem trivial, but it is in firing procedure where most failures occur. This is why these instruments are important. In fishing, you depart from the harbor, you cruise and do your thing, and you return safely. It is the same way with firing glass, except that you don’t go off shore. Glass is also a three-part journey except that it is the glass that makes the trip.
There are three phases in firing glass. They are heating, process, and cooling. Each phase has its own goal. In the heating phase, it is to prevent thermal shock. In the process phase, it is to get the glass what you want it to do. In the cooling phase, it is to give the glass a good thermal memory so it won’t be grumpy after it returns. When firing small pieces such as jewelry, temperature measurement and kiln control is not that critical. As the size and thickness of the work increases, so does the need for control.
The control system includes: the temperature measuring instruments, and devices that control the rate of heating and cooling of the glass during firing. Temperature reading is usually done with a pyrometer, although some glass artists use pyrometric cones. Controlling devices can be something as simple as an on/off switch and as sophisticated as a computer. Most new kilns come with these systems already built in so all you have to do is plug in the kiln, read the directions (actually you should read the directions first), turn it on and “let her rip”. It’s not quite that simple, but it isn’t really difficult either. The following details about instruments and devices will provide you with a better understanding of what options are available when buying a new kiln, as well as considerations involved in buying a used kiln or upgrading your existing equipment.
Pyrometers
A pyrometer is an instrument that basically reads the temperature in the kiln. It does not control the kiln; it just lets you know what’s happening. Don’t get the pyrometer confused with a controller. All controllers, both set-point and programmable, have pyrometers built into the system. Here in this section, we stick with temperature measurement. Then we will move on to getting in control of your kiln (if I don’t get out of control first).
There are two types of pyrometers: analog and digital. The analog meter is the kind that has a window card with numbers on it (for degrees F and C) and a needle that moves with temperature changes inside the kiln. A digital meter shows the temperature on screen, usually via
L.E.D. (light emitting diode) or L.C.D. (liquid crystal display). The latter is more expensive, but much more accurate. All of the programmable controllers use digital with
L.E.D. display.
Analog Pyrometers
The analog pyrometer is still most commonly used and until recently, was standard equipment on kilns. That is changing because the price of analog meters is creeping up while the price of digital meters is going down. The face of the analog meter has a needle that rotates position to two scales of numbers that denote temperatures in both Fahrenheit and Celsius. Analog meters are portable, and you don’t have to change batteries. They are also easily available at almost any ceramic supply store, and, of course, stained glass suppliers who deal in fusing supplies. These meters are electro-mechanical. The thermocouple (probe) in the kiln sends a micro-voltage to the pyrometer. The wires at thermocouple tip, lead wires, and inside the meter itself are of two dissimilar metals. The heat creates that micro-voltage. There is a bi-metal spring inside at the base of the needle. As the heat inside the kiln increases, so does the micro-voltage. As the micro-voltage increases, it expands the spring and moves the needle to the right (uphill). This points to the numbers on the scale to indicate the temperature. If the needle goes to the left (downhill) that means your lead wires are reversed. (More about this in the section on thermocouple.)
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