BBBBBBBBBBBB the passages for the hot air.
CCCCC the outlets for the hot air.
The kiln is circular, built on a foundation of brick or stone, and strengthened with bands of wrought iron. At all four sides of the central chamber, where the pottery is placed, are fire-boxes where the coal is fed on to a grate below the level of the floor of the chamber. The hot air from the fire-boxes finds two outlets, one through a direct escape, close to the box, the other through a passage and out at the centre of the floor. Within the central chamber are piled the saggers—great circular boxes or drums made of fire-clay—all of which, if possible, contain pottery, but, whether full or empty, they must fill up the chamber, as their clay holds and radiates heat which would otherwise be lost. The office of the saggers is to protect the pieces of pottery from direct contact with the flame, smoke, and ashes of the fire.
The heat from the fire-boxes, if it rushed directly into the kiln, would fire the pieces near the bottom of the chamber too hard, so slabs of fire-brick, called firebacks, are arranged so as to send the heat up (see [Fig. 44]). It then, passing up between the saggers (which, stacked one above the other, form flues to improve the draught), passes off at the top opening. This aperture, as will be seen in Fig. 44, has a slab supported by bricks above it to control the draught. The heat escapes through the cone, and thus through the chimney. The cone is made of ordinary bricks, and its office is to help the draught. There is another kind of kiln much like this model, except that it has a down draught. The heated air, as it escapes, passes down through an opening in the floor, along a horizontal passage, up and out. This keeps the heat in the kiln for a longer time, but it also confines the gases, which affect the colour of the ware so that it is not practicable for art pottery.
Seen from the outside on a day when the kiln is being filled, its aspect is quite different. Inside the great brick chamber, workmen are stacking the saggers filled with pieces of pottery, some of which, not yet packed, are standing on tables near at hand. They look wan and pale, faint grayish-green, or leaden gray. Not until they have felt the fierce heat of the kiln will they glow with living green or will the beauty of their blues appear. Yellow that is almost orange seems to be only a deep cream colour before it is fired. Then there are pieces of green, or unbaked clay, to be fired in the biscuit—that is, without the glaze. These are put in another part of the kiln.
Just a word about the saggers. These huge, rough boxes of burnt fire-clay, whose office has already been spoken of, are sometimes bottomless, simply rings. Should a sagger be too low to contain a piece that is placed in it (as is often the case), one can build up the clay-box to the required height with the bottomless saggers. Between every two saggers rolls of clay are laid (coiled around the edge) to hold them together. When the kiln is filled at last with its stacks of saggers, the doorway is bricked up and the fire started. The open-fire kiln requires the best part of three days to complete it. The first day it is filled with pottery and the fire started. In firing a kiln, the heat should be raised very slowly. All that night the fire is watched and fed, as it must be kept at the requisite temperature, for if it gets low, or, as the potters say, “slips,” the glaze is spoiled and the pottery must all be reglazed and fired again. The weather affects the kiln, the draught not being so good on a damp or rainy day as on a fair one; and the coal also must be specially chosen for the kiln. It takes several tons to fire one kiln, so that the success or failure of a firing is no light matter. The second day much attention must be given to keeping the fire up, and at about nightfall the guides are drawn as follows:
At equal distances around the kiln are four peep-holes—small, round apertures on a level with the eye, which are closed with a cylindrical tube having a transparent asbestos end. In the glare of the kiln, directly on a line with these peep-holes, one can see a row of clay rings, which were daubed with glaze before the fire was started. Having withdrawn the cylinder which closes the hole, the potter runs in a long metal rod, with a hook on the end, catches a ring, and draws it out. He can tell from the appearance of this ring, or guide, how the kiln is firing and about when the pieces will be done. Usually they are finished that night, but the kiln is not opened until it has cooled for many hours.
THE MUFFLE KILN