REFRACTORIES FOR GASWORKS PURPOSES 201
tridymite does not change at this temperature, so that so far as immunity from trouble is concerned, the condition to be aimed. at is that of obtaining a brick in wliich the fireclay, by reacting, has converted all the silica present in the mass into tridymite.
Experience with silica retorts tends to show that when new material is employed some appreciable outward. diffusion of gas takes place, although the formation of “ scurf,” a fairly speedy operation, will soon act as a check. When demolishing old retort bench.es, it is customary to find a black vein running completely round the section of the retort. This vein (Fig. 105), starting about a quarter of an inch from the inner surface of the material, will
usually run to about an inch in width, and on analysis the portion of the refractory at this point shows a free carbon content of 10 per cent.
THE TEXTURE OF FIREBRICKS
It has been pointed out previously that the service which a refractory article is likely to give cannot finally be judged from the composition and refractoriness of the material from which it has been made. Texture is a property of equiva-lent importance, and largely determines the suitability of the article for any special purpose. In general, it may be said that vitrification of a clay is favoured the doser the particles are to one another. Accordingly, the coarser the gram, the more highly refractory will the article be. A limit as to coarseness is, however, imposed by considerations of physica] strength. Mellor1 has stated that when clays are heated on a gradually rising tem
perature the surfaces of the more fusible granules melt first. Hence again, quite mdependently of the action of the fluxes, a fine-grained clay with its augmented internal surface will vitrify faster and at a lower temperature than a ‘coarser grained clay of the same composition. There is therefore a cumulative action in favour of coarse-grainedness for highly refractory bricks.
On the other liand, Mellor states that a fine-grained texture gives the property of resisting abrasion and attack from fluxes, slags, flue dust, and salt vapours. Thus, bricks which have to resist the corrosive action of slag should have a close texture and be of uniform size, with. joints as small as is practicable. The jointing clay should also be chosen so that it quickly vitrifies without cracking, and welds the
Fig. 105.—Section of Used Retort showing Carbon Vein.
1 Proc. Faraday Society, November, 1916.