Sunday, June 1, 2008

How to Identify Types of Kiln

When working with pottery you will always have need of a kiln to fire and harden your wares. Without a kiln you've got little more than soggy clay. over the years many different types of kilns have been invented, each with their own particular use. Here is a guide on most of the kilns a potter is likely to encounter and their uses.

Kiln Types

Portable Kilns-The term portable does not mean that a kiln can be picked up and carried around, but merely that it was manufactured in a factory and shipped in one piece. The manufacturers of such kilns have illustrated catalogues with complete technical information about burners, blowers, and other features. This is the most diverse of kilns given the huge numbers of companies that build and ship their own.

Up Draft Kilns- An updraft kiln is made up of essentially two pieces; a firebox in which the heat and flame is concentrated and contained, and a flue damper system by which the heat and flame is controlled. The placement of the burner can be from the bottom or on the sides, but the flame and heat is designed to flow and rise upward from the bottom of the firebox. This is a very simple system where the burner produces heat at a steady rate and the flue in the top can be widened, narrowed, or cut off completely. Because of its simplicity, relying on air flow to distribute heat, this model can rarely produce heats much higher than 1,940 degrees Fahrenheit, which is barely enough to mature earthenware and nowhere close to maturing stoneware or porcelain.

Muffle Kilns- A muffle is a chamber made of refractory material into which ware is placed for firing. Between the muffle and the outer wall of the kiln is a space through which flames circulate, entering at the bottom, passing all around the muffle, and leaving through a flue at the top. In some muffle kilns, the wall of the muffle contains passage for the flames; in others, the flames travel in tubes just inside the muffle itself. Muffle kilns protect ware from contact with the flame, an important factor in most glazed work. They do not used fuel economically; however, as too much heat escapes up the chimney

Down Draft Kilns- A down draft kiln has no muffle. The flames enter from the front or sides near the bottom corners of the kiln, pass over a baffle or bag wall, and reach the ware at the top. They are then drawn down through openings in the floor into a flue connected to a chimney. In their passage through the kiln the flames come in direct contact with everything in it. This is alright for most stoneware, but for certain types of porcelain the ware must be stacked in saggers, or boxes made of refractory materials which act as separate muffles. Down draft kilns use fuel more economically than muffle kilns and they are essential for work requiring an extremely high temperature. Some manufacturers produce portable models, but the majority of down draft kilns must be built in the place where you intend to use them.

Car Kilns- As the name implies, car kilns are ones in which ware is loaded onto a car or shelf and rolled into the kiln. This makes loading the kiln much easier, especially for large pieces. They work on the same principle as the down draft kiln, and thus are extremely versatile and sell well for small business and commercial use.

Tunnel Kilns- All the previous kilns described are periodic. Meaning they must be fired, allowed to cool, unloaded, reloaded, and fired again. The tunnel kiln is meant for industrial use, often spanning more than one hundred feet in length. The ware is loaded onto a moving platform at one end and passes through the tunnel, encountering successively increasing temperatures until, at the very center of the tunnel, full maturing temperature is reached. Continuing on its way, the ware cools slowly until at the end of the tunnel it can be safely lifted off the platform and shipped to whoever ordered it. The firing cycles typically take between seventy and ninety hours.

Tips & Warnings
Often local craft or hobby groups will have kilns you can use if you can't afford to buy one or don't have the space. Check your local newspaper listings first.
Though the principle behind a kiln is simple, don't try to construct your own without professional aid and supervision. When dealing with combustible fuel, heat, and heavy building materials to possibilities for disaster are endless. Never use a kiln indoors because of the hazard for fire damage and smoke inhalation.

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