Fig. 45
The kinds of kilns usually found in schools are the gas and the oil kiln. The English gas kiln has an arrangement of nine or twelve burners beneath the muffle. (Fig. 45.) This is a fire-clay box, open at the front, set on fire bricks and cased round with fire tiles within an iron frame (see cut). There is an air space all round, except at the open end, leading to the flues on top which have dampers to regulate the draught. The open end may be closed by a hinged door or bricked up with fire tiles cut to fit. The defects of this kind of kiln are too sudden access of flame to the bottom of the muffle, causing it to split, and the impossibility of getting the front, where trials are usually placed, fired up equally
with the back. An ideal muffle of this kind would be one with flues all round, gradual access of flame on all sides, spy holes each end, and the top to lift off, for placing. American kilns have flues in the door, and the chimney at the top is placed slightly forward, thus making it easy to fire the front up hard. They are usually fitted with two large burners, with air mixers, and a handy mica spy hole.
The oil kiln differs considerably from the gas kiln. The oil is fed through tubes into fire boxes some distance below the bottom of the muffle. It burns on asbestos fibre in an iron pan to which the draught can be admitted. The flames strike the bottom of the muffle and pass up through fire-clay pipes, which project into the muffle, then pass off through a twin flue regulated by dampers. With this kiln a long flue is necessary and any excess of carbon is liable to choke the pipes, and further it cannot be “sweated” up at the finish so easily as a gas kiln having a large number of burners.
It is a good plan when a coarse fire-clay muffle is used for glaze and biscuit to give the sides and top a sagger wash of lead and stone. This renders the muffle less liable to absorb glaze from the pots in glost firing. It also lessens the danger of small bits scaling off and sticking to the finished ware. The bottom should always be kept dusted with finely powdered flint. When cracks appear or
joints open, they should be stopped with a pugging of fire clay and grog. A mixture of egg silica or water glass with fine grog and quartz sand will stop small cracks. Siluma, a fire-proof cement, with equal parts of sand, answers admirably for patching.
In biscuit firing the green shapes may be packed close together, with the lighter shapes on top of the stronger, but all must be 1⁄2” to 3⁄4” away from the sides of the muffle. Triangular pieces of biscuit, called saddles, are used to raise the shapes off the bottom, but often a fired tile, sanded and placed on a spur or saddles, gives the best foundation. Where two layers are required, small props and fire bats, perforated to let the heat through, will be necessary. These form shelves as the exigencies of the packing dictate. (Fig. 47.)
For light shapes, thimbles and fired tiles will serve the purpose. (Fig. 48.) Where a shelf or prop rocks insecurely, a small wad of pugging (grog and clay) will give a steady bearing. Thrown bowls, if dried together and well fitting, may be fired together, and large thrown pots may be filled with little ones. Cast shapes can be placed on top of thrown ones, but no liberty is to be taken with them. Flint should be used liberally to prevent sticking, which may happen if the biscuit be over-fired. Tiles can be fired two together in tile boxes or
stacked as dried. Flat ware fired in a small muffle requires very careful handling. Whenever possible, it should be placed in the centre, on a flat flint-covered fire tile or bat.