Modern Gasworks Practice

208 MODERN GAS WORKS PRACTICE Whereas with. “ aluminous ” firebricks there is usually an after-contraction the reverse is the case with silica bricks. The Refractory Materials Committee specify the following limits for various Materials so far as contraction or expansion is concerned :— Retort Material.—A test piece heated to a temperature of cone 14 (1,410° C.) for two hours shall not show, when cold, more than. 1| per cent, contraction. or expansion. The test piece is. to be inches long by inches wide, with. the ends ground flat. Readings as to length are to be taken by means of Vernier callipers reading to 0-1 mm. Firebricks, Blocks and Tiles.—A test piece when heated under the above con-ditions shall not show more than. 1 per cent, or 1J per cent, contraction or expansion, according to the grade of material. Silica Articles.—A test-piece when heated for two tours to a temperature of cone 12 (1,350° C.) shall not show, when cold, more than 0-75 per cent, linear contrac-tion or expansion. CRUSHING STRENGTH The crushing strength of a firebrick gives an. indication of the internal strength of the brick, and the manner in which the binding materials are performing their duty. The compressive strength of fireclay articles may be determined by means of one of the forms of apparatus made for the testing of materials. With regard to ordinary Stourbridge articles, these should be capable of withstanding a crushing stress of not less than 1,800 Ib. per square inch. Emery and Mellor 1 have carried out a series of tests with materials of varying composition and porosity. The results show that the crushing strength is reduced by increasing the grain-size of the grog when the proportion of grog is constant ; and also by increasing the proportion of grog when the grain-size is constant. The investigators state that there are not yet sufficient data available to enable any conclusion to be drawn respecting a possible relation between the so-called fusion temperature of the clay and the crushing strength of the cold bricks. There is probably a olose connection between the temperature coefficient of the vitrification. and the crushing strength.; and it is fu.rth.er probable that the crushing strength reaches a maximum during the firing of the brick, and that the strength gradually deteriorates when the bricks are in use. It does not follow that the crushing strength of the newly fired brick is at its maximum. THE RELATION OF REFRACTORY MATERIALS TO THE VARIOUS PORTIONS OF A RETORT SETTING In dealing with. fireclay articles and their relation to the various portions of a retort benet, it is proposed to consider the matter from the purely practical standpoint of the gas engineer, and to avoid, so far as possible, the inclusion. of the more 1 Refractory Materials Committee (Inst. Gas Eng.), 1919.