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About Kilns: Article or Project

History and types of kilns

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Most historians believe firing of clay dates back thousands of years back to primitive people about 8,000 BC. It is the general consensus that it was discovered by accident. Most likely a chunk of clay fell into a fire and got hard and the light dawned. Once that was discovered, it grew from there. The earliest kilns were nothing more than fire pits. Pottery was loosely stacked in a shallow pit and a fire was built around them. When the fire cooled, the pots were cleaned and used for many purposes. Such firing only reaches about 1000 degrees Fahrenheit and created a very fragile and porous product, but still functional. Early pots were not glazed as that was only discovered in China around 700 AD.

The problem with pit kilns is that they operate backwards. The insulation (earth) should be on top and the fuel should be on the bottom. But over time and with experimentation, it was finally discovered. The next stage of kiln development was the beehive kiln. This kiln was built with clay bricks creating a dome and vent over a fire box pit. This enhanced the firing process in that higher temperatures were able to be achieved. Further experimentation with air flow and design continued to morph to greater success and better ware.

In China, around 500 AD, there was a more sophisticated design of the beehive kiln called a climbing kiln. This was a series of firing chambers like the beehive, linked together and built on the side of a hill where each chamber was higher than the next one. Each chamber had a fire pit, but they vented from one to the other to the next. This enabled higher temperatures for curing the final ware. This design was great for mass production and some communities made a lot for trade goods. It is because of this kind of kiln and the increase and control of heat, that enabled the Chinese to discover the art of porcelain (around 700 AD) and glazing and it was a secret that they maintained for hundreds of years. Porcelain became very popular in Europe around 1400 AD.

The first gas kilns were patterned after the original beehive kiln. The primary difference was that they were fueled by gas rather than wood. By the time they were developed, the basic insulating bricks were improved. Gas flame is at the bottom of the kiln with a vent and dapper at the top to control the air exchange. The most advantage of the gas kiln was the control of the temperature as well as the low levels of pollution. The current advantage is that in most parts of the country, gas is less expensive than electricity. In some parts of the country propane is the fuel of choice. There are safety factors that one must be aware of. Propane has to be vented from the bottom of a kiln after use before relighting or it can explode. Natural gas disperses on its own as it is lighter than air.

Electric kilns are the only new kiln technology - developed in the 20th century. They operate with radiant heat generated by electricity passing through elements or coils wrapped around the chamber. Much the same as a toaster. Most new kilns have shut off devices called kiln sitters and back up timers. They don’t require the same venting as gas or fueled kilns do. Many are also equipped with computer controls which can be set to create multiple staging possibilities.

There are primarily two kinds of firing available for current clay enthusiasts. Oxidation firing and Reduction firing. Oxidation firing has no need for oxygen, ergo the electric kiln is perfect. It does not require the venting control needed in a gas kiln. Any fuel kiln needs oxygen to create the flame. Generally, the fuel uses up all the oxygen from the air flow during the firing process. IF the damper is closed, then the air draft is reduced which prevents the fuel from getting enough oxygen. In that case, the fuel will try to find the oxygen (chemical) it needs from anything else in the kiln, namely clay and glaze materials which contain oxygen in the form of metallic oxides. When this REDUCTION occurs, it changes the chemical balance in the glazes and the clay and changes the resulting colors. So, in summary, Electric kilns are only capable of Oxidation firing and fueled kilns can do both by controlling the air flow with a damper.

 

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