Fig. 66
Fig. 67
The most recent innovations are gas and oil kilns. They require comparatively little manual labour in the firing, are easily regulated, and the waste heat is frequently used for secondary purposes. Their freedom from smoke and their economy of labour and money must eventually render the other types obsolete.
These large kilns are strongly banded with iron supports designed to prevent too much loosening of the walls when expanded by the great heat. A very important point is the draining of their site,
as large kilns tend to attract moisture, and the presence of steam in a kiln is productive of much damage.
Fig. 68
Enamel kilns are used for the final processes where the glazed pot is painted with over-glaze or enamel colours. They are of no great size and are made of fire-clay slabs or even iron plates when the enamel is soft. The flames play all round the muffle or fire-clay box during the firing, but no flame or fume is allowed access to the inside. (Fig. 68.)