196
MODERN GASWORKS PRACTICE
foot, and a finished retort measuring 24 inches by 16 inches would be moulded before burning to 25| inches by 17 inches.
The selection and. preparation of “ grog ” has to be carried out with great care, since a superior clay may be completely ruined by the careless iutroduction of an inferior “ grog.” “ Grog ” is usually prepared from a clay which originated in the same seam as the fireclay with which. it is to be mixed. This clay, in the form of lumps, is burnt in a kiln out of contact with the fuel; and the firing temperature sh.ou.ld be greater, or at least as great, as that to which the finished article will be subjected. The lumps are then ground to the desired size. The quantity of “ grog ” added varies, of course, with the material required and the kind of clay made use of. For firebricks and smaller articles, however, it is usually added in. a proportion of from 15 to 20 per cent., while for larger articles, such as retorts, the amount is increased. to from 30 to 40 per cent.
On the Continent the preparation of chamotte is an industry in itself, and lumps of selected burnt clay are dispatched long distances for the use of manufacturers. In some Continental fireclays ordinary “ grog ” is dispensed with, and in its stead a special china clay is employed, the shrinkage of which is so small that it may be used without previous burning.
THE CLASSIFICATION OF REFRACTORY MATERIALS
Refractory materials are, in general, classified in accordance with the percentage of silica tliey contain. “ Aluminous ” clays are those of the Stourbridge type, con-taining about 65 per cent, of silica, “ siliceous ” those containing from 80 to 92 per cent, of silica, whilst those containing upwards of 92 per cent, are known as “ silica ” materials. It is the two last-named materials which are assuming great importance in present-day gasworks practice. Typical highly siliceous clays are the Dinas, Ewell, Ganister, and some Scotch varieties. Analyses of the various materials as now used on gasworks are given below.
1. Aluminous Clays. (See also under “Scotch Clays.”) Stourbridge Type.
Silica. ........ 64-62 per cent.
Alumina ........ . 21-65 „
Ferric oxide ...... ■ 1'48 „
Lime ....... 1-88 „
Magnesia ....... . 0-62 „
Moisture ..... . 9-62 „
2. Siliceous Clays. (By water.)
(a) Ganister. Ougbtibridge (Type A).
Silica. ....... 90-2 per cent.
Alumina ....... . 7-06 „
Titanic oxide .... . 0-36 „
Ferric oxide .... . 1-03 „
Magnesia ........ . 0-05 „
Lime ...... . 0-40 „
Potash ........ . 0-56 „
Soda ........ . 0-30 „