Fig. 54
As the back is usually hotter than the front the hard glazes should be packed first, and by selecting suitable shapes a good setter will pack a surprising amount into even a small kiln. The clamming should be done with care, as bits are liable to flake off and stick to the glaze. The top plug only need be left out and the lighting up proceeded with as before.
The gradual increase of heat is very essential in firing glaze, for any sudden jump or reduction will play all manner of games in a glost oven. The
slow start is not so important, there being no risk of blowing. Indeed the glaze is best in a sharp quick fire.
Although cones should always be used and are excellent indicators, experience will soon show when a kiln is fired up. It may be advisable to go on a little after the cone is over until all looks fluxed so as to get rid of bubbles. These appear in some glazes until the last. They do not always go down on cooling, but those glazes that bubble through over-firing should be avoided. Possibly the trouble will arise from sulphur in the body which must be corrected. Trials are always useful but they must be small and easily hooked out, as constant poking about at the spy hole lowers the temperature of the muffle at the front, just where it is most difficult to get it up.
When fired, a glost kiln should cool very slowly, for any sudden access of cold air is liable to stunt or crack the pots. When firing over glaze decoration where the glaze does not run, the packing is much simplified. With low-firing lustres the pots may even touch each other.
All this has been set out at great length but we shall find over and above this that each kiln has its own little ways which must be studied and humoured before the best results can be obtained.
The chief thing to avoid is a sudden flush or jump in the heat. The main thing to aim at is a slow start steadily increasing to a sharp finish. The state of
the muffle, a dry or a damp day, will modify each firing a little, but the above generalization will have a fairly wide application to the working of a small kiln.