138 FOUNDRY WORK I »' liquid pressure, as large copes and the bottoms of deep moulds, should be rammed with stronger mixtures than the sides. But venting must be more thorough, or the casting will be " scabbed ". Green, Dry, and Core Sands.—The feature which these have in common is that they are consolidated with the rammer while in a moistened condition. They are never wet, but sufficiently damped to retain a shape imposed when squeezed in the hand. The retention of the form produced during ramming depends partly on the coherence of the sand, but largely on the means by which it is sustained in flasks, and on grids. Green sands cannot be dried, except slightly on the surface, without losing their coherence. Sand, to be dried, must be of a strong clayey character, and be mixed with horse manure, which, by its carbonization during drying, counteracts the close texture of the mould, favouring venting. But the vent wire must be used freely too. Coal dust is also used. Core sand is mixed with clay wash, peasemeal, or beer grounds; and generally, dry sand mixtures are suitable for cores. Loam Mixtures.—These are made with strong sands, vented with horse manure, with which generally a large proportion of old loam is mixed, the whole being ground in a mill with water, and swept thus while in a pasty, plastic condition, to be dried subsequently. It is used in coarse and fine grades, the first for embedding the bricks in, and for the rough coats, the second, finely sieved, for the final coats. Chemical and Mechanical Analysis.—During recent years, attempts have been made to grade moulding sands by chemical analysis, supplemented with microscopical examination of the grains. These are helpful when new sands are concerned, but their value is discounted when, as is usually the case, large proportions of old sands are mixed with new. It is important that the percentages of silica and of alumina should be known, and also the quantities of iron oxide, lime, magnesia, and alkalies, which tend to lower the fusing point of a sand, and flux it. Silica, the refractory element, must be present in more than 80 per cent, alumina in from 7 to 10 per cent, the proportions varying for weak and strong mixtures. But it is held that the texture of a sand when passed through sieves of different meshes is of more importance when deciding its suitability for a certain class of work than chemical analysis, and that mechanical testing affords an approximate index of the cohesive character of a sand. Weak sands have fine grains, and least alumina. The strong sands possess coarse grains, and a large proportion oi alumina. Castings with smooth skins can be obtained with the use of coarsely grained sands. Fine grains are suitable for dry mixtures, cores, and loam, with a large proportion of alumina.