The Potter's Craft: A Practical Guide for the Studio and Workshop

by Charles Fergus Binns

CHAPTER XVII
The Fire

Kilns and burning form the pivot upon which the art of the potter turns. M. Doat has said, "A potter can no more express himself without his kiln than can a violinist without his violin," and yet there are some who try to make out by sending their work to some nearby pottery to be burned. Let it be at once understood that he who finds it impossible to procure and manage a kiln had best take to some other craft.

Kilns are of two types, open and muffle. In the open kiln the flames pass through the firing chamber and the ware may be exposed to their action, as in stoneware and brick; or it may be enclosed in the fire-clay cases, called saggers, as in the many forms of pottery, dishes or faience. The muffle kiln is a closed chamber which is surrounded by flames but which is not entered by them. These kilns are used in the manufacture of terra cotta and heavy enamel wares, and the portable kilns made for studio use are of this type.

There are certain advantages to be gained in the use of either type of kiln but inasmuch as the open kiln involves the use of saggers and as, moreover, it must be properly constructed of fire-brick by a skilled mason, it will be best to consider only the portable studio kiln.[M]

It must not be expected that any kiln will give perfect satisfaction. Neither built kiln nor portable kiln will do this, but either may be relied upon to do excellent work in the hands of those who will take trouble. A kiln of the proper size having been purchased, it must be carefully installed. A good chimney is an absolute necessity and if one can be built on purpose it will be best. It should be at least twenty-five feet high with the bottom lined with fire-brick to a height of six or eight feet. The portable kiln is set on iron legs which raise it about one foot from the floor. This is not enough for easy work and a platform of brick or stone, ten inches high, should be prepared. This will greatly simplify the observation and management of the burners which are beneath the kiln, and if it should make the inside of the muffle hard to reach, it is easier to stand on a box to attend to the kiln than it is to go on one's knees to the burner.

The kiln room should have a cement floor and should be both well drained and well ventilated. At the window there should be a stout bench where the work of preparation may be done and at a convenient spot there should be shelves for stilts, cones, wash, stopping and all the minor accessories of burning. If there is room for a barrel of oil it will be a convenience, and if the room be fire-proof the insurance company will not object.

The kiln having arrived it is mounted on the platform and the asbestos-lined pipe is securely connected with the chimney. The inside of the muffle is examined with care to see that no part has been jarred in transit. The reservoir cans are filled with oil and a slow fire is started. This should be allowed to burn very gently for an hour or two in order to thoroughly dry out and season the kiln. It is a good plan to make up a wash of equal parts of kaolin and flint and to brush this all over the inside of the kiln. It should not be put on so thick as to shell off from the walls but at the bottom a good coating may be laid. This protects the walls of the kiln from the attacks of glaze and will make them last longer.

In order to fill the kiln economically a number of props and bats must be provided. Some of these are sent out with the kiln but one is always needing odd sizes and extra pieces. The props are simply legs of burned clay; they are of any height desired and should be thick enough to stand alone. The bats are slabs of burned clay and they rest on the props to form shelves. The bats must be thick enough to bear the weight of any pieces which they may be called upon to support, but they need not be very large as two or more may be used to bridge the width and length of the kiln. Bats and props are best made of sagger clay to which has been added about one-third of crushed fire-brick. Broken bats serve well for this after the first supply has been secured. This crushed burned clay, called grog, has a very important influence upon wares which have to be heated again and again. The size used should be about what will pass through a 16-mesh sieve, and if the dust be sifted out through a 48-mesh sieve, the resulting ware will be stronger. That is, only the grog which passes a 16 sieve but lies upon a 48 sieve should be used.

The relative proportions of clay and grog in the mix will depend somewhat upon the nature of the clay. Three parts of clay to two of grog by measure will be about right.

The first charging of the kiln should be with pieces of no great importance. The temperature in different parts must be carefully ascertained. In order to do this a number of pyrometric cones[N] are prepared in groups of three.