Modern Gasworks Practice

214 MODERN GASWORKS PRACTICE pressure is not so efEective for most purposes—particularly retort work—as a more plastic clay which is put together by hand. Whenthe clayis plastic, the additional amount of water is afiorded an opportunity of carrying out its function of knitting and binding up the particles. Moreover, when the ram under heavy pressure squeezes. or drives the clay out of a retort machine, there is a tendency to destroy the best setting of the grog and to drive the coarse particle into a more or less regulär formation. On the other hånd, the machine-made retort, prepared with due care, is far cleaner, more regulär, and straigliter tb.an that made in successive lifts by hånd. There is a good deal of controversy as to the relative merits of machine-made and hand-made firebricks, the latter usually being preferred although the reasons for this are not quite clear. Mellor 1 says that the subject is one which requires careful re-examination, and that the prejudice to machine-made bricks is probably the result of an unfair comparison. Evidence shows that the essential difference is not that of machinery versus handwork, but depends rather on the difference in. the time the clay is allowed to stand in contact with. water. Hahn 2 says that the internal cohesion of a brick appears to be better with well-made machine bricks than with hand-made bricks. Mellor points to the faet that there is also a tendency for the machine-made body to be less uniform in texture. Pug-mills are not really good mixers. With. pugs mixing, sav, clay and grog, the eject from the pug has a tendency to be more or less laminated into poor patches and patch.es rich in grog.. The preparation of the clay for hånd and machine working usually difEers in a way which. materially affeets its working properties and texture. If the texture of the brick be not uniform, it is highly probable that strains will set up when the brick is being heated, owing to a difference in the rates of expansion of the different parts. This will lead to the formation of fissures and cracks, and, probably, also to spalling.. A prime desideratum with machine or hand-macle bricks is uniformity of texture. Other things being equal, that process of manufacture which is least liable to yield bricks with an irregulär texture will give the best results. In the dry or semi-dry processes of machine-making, films of air between. the granules prevent their packing togetlier so well as in bricks made by plastic processes, and this again has a tendency to make machine-made bricks less sound than hand-made bricks. Emery and Bradshaw,3 as a result of a series of tests carried out with hand-made and machine-made silica bricks, have come to the following conclusions :— 1. The prevailing objections to machine-made silica bricks appear to be due to prejudice arising from early failures to produce good bricks from irnproperly graded mixtures, and with unsuitable machines. 2. Of recent years the quality of machine-made silica bricks has been greatly improved. 3. Bricks made from the same materials moulded by hånd and by machine were submitted to a series of comparative laboratory tests. In none of these were the machine-made bricks forind to be inferior to the hand-made, while in severaL 1 Proc. Faraday Society, November, 1916. 2 Ber. Ver. deut. Fab. feuerfester Prod., 33 (1912) and 34 (1913). 3 Refractory Materials Committee, Inst. Gas Eng., 1920.