Oil-cake

Oil-cake is used mainly for feeding sheep and cattle. It is made from the solid residue of oleaginous seeds (linseed, rape-seed, cotton-seed), after a large proportion of their oil has been extracted. The following is the usual process of manufacture in Britain. The seed is crushed between iron rollers, then damped and ground upon a mill of the following construction. Two large circular blocks of hard granite are set edgewise on a bed-stone of the same material, which is slightly hollowed out; these two upright stones are connected by a horizontal shaft which passes through the centre of both, and is fixed at its middle to a revolving upright shaft. The stones are thus made to revolve about their vertical axes, while at the same time they are left free to be turned round the horizontal shaft by the friction of grinding. The meal thus obtained is heated in kettles formed of two compartments, in the inner of which the meal is placed, while the outer is filled with steam. The meal is then filled into small woollen bags of the shape it is wished to make the cakes—usually oblong, about 30 inches by 12 inches, and $\backslash frac\{1\}\{2\}$ to $\backslash frac\{3\}\{4\}$ inch thick. These bags are then placed in wooden 'wrappers,' which consist of two pieces of hard wood, of the same size as the cakes, hinged together at the end; the wood is usually corrugated and furnished with a stamp to mark the cakes. The wrappers containing the bags full of crushed seed are then placed in the compartments of a press worked on the same principle as a Hydraulic Press (q.v.), except that the oil from the seed is used instead of water. In this way about 90 per cent. of the oil the seed contains is squeezed out of it, leaving sufficient to bind the residue of ground husks into a solid firm cake. Sometimes the process is varied, in that the seed, instead of being ground under stones, is repeatedly crushed on iron rollers; in this case the crushed seed is steamed in the kettles to give the necessary moisture, not merely heated as described above. Sometimes oil-seeds are subjected to a chemical instead of a mechanical process—viz. solution of the oil in bisulphide of carbon. By this means the oil may be almost completely extracted. Mustard, rape, castor-oil, undecorticated cotton-seed cake, and some others are also used as fertilisers.