With circular furnaces, some arrange for the expansion of the roof, with a series of plates and sets-crews, which are loosened as the furnace heats up.
Arches over doors and openings may have an occasional shingle built into the joints to provide for expansion.
With straight walls a slip joint can be arranged that will close up when the furnace heats. Care should be taken, how ever, not to allow for more expansion than will actually take place, as otherwise there will be an open joint that may prove a weak point.
All users of silica brick should strictly observe the following rules : 1st. That the brick are kept perfectly dry.
2d. That in building, the brick are not laid too tight.
3d. That ample provision is made for the expansion of the brick as the heat is raised, without weakening the support of the furnace.
4th. That great care be taken in heating up the furnace to do it slowly.
5th. That the same material be used in laying the silica brick as that from which the brick are made.
6th. That when, for any reason, it becomes necessary to cool down the furnace, it be done as gradually and slowly as pos sible.
The appearance of the " Dinas " brick is peculiar in its color and in the roughness of its surface. Silica brick should be
regular in size, thoroughly burned, and the chemical analysis of brick made by the same manufacturer at different times should show perfect uniformity in quality, and these require ments only can be secured by the most intelligent and unceas ing watchfulness in all the details which pertain to the produc tion of this high grade of refractory material.
The way of manufacturing " Dinas " or silica brick for many years was a closely guarded secret, and it is only of recent date that there has been any certain knowledge of the mode of pro ducing such brick.
The material, or, as it is locally named in Wales, " clay," of which silica brick is made, is found at several places in the Vale of Neath. It occurs in the state of rock and disintegrated like sand. Its color, when dry, is pale-gray. The rock, when not too hard, is crushed to coarse powder between iron rolls. By exposure to the air the hard rock becomes somewhat softer, but some of it is so hard that it cannot be profitably employed with out the use of a Blake or other crusher. The composition of Dinas " clay," from two localities in the Vale of Neath, has been found to be as follows :