REFRACTORIES FOR GASWORKS PURPOSES 223
the case of those retorts surrounding the combus-tion chamber, the brick-work makes an admirable substitute for special protection. blocks.
COMBUSTION CHAMBER AND
CROSS-WALLS
The greatest in-tensity of heat which is produced in a retort set-ting is in the combustion chamber and its imme-d i a t e neighbourhoocl; hence a material of high, refractory qualities is essential. In this respect
Fio. 122.—Fibbclay Shield Block.
Fig. 123.—4|-inch Division Wall with 9-inch Ewell Ring.
the siliceous or siJica
materials are to be preferred, and those containing upwards of 96 per cent, of silica are now coming into fairly general use for the purpose. In this connection it is always well to bear in mind that while fire-clay of the aluminous or Stourbridge type shows some tendency to con-tract when under working heats, the silica clays invariably expand; and, as a general rule, the higher the silica content the greater will this tendency be. Bricks of the siliceous type, how-ever, being intermediate so far as silica content is concerned (87 to 89 per cent.), remain practically stationary, although at
the more intense heats th.ere may be some slight tendency towards expansion. Owing to this property, also to their high refractoriness, siliceous bricks are largely used for combustion chambers and cross-walJs in modem settings.
The characteristic of certain bricks to diminish or increase in size is rather