Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/239

 PAPER 221 mixed with the starch. This mixture is put into the beating engine in which the pulp is circulating, and when it is thoroughly incorporated with the pulp the solution of alum or sulphate of alumina is added. This forms a finely divided precipitate of resinate of alumina on the fibres. The pulp, after the sizing material is thoroughly incorpor ated with it, is now ready for colouring. Even to produce a pure white, colour must be added to the pulp. In general, for white papers, either cochineal and ultramarine blue are employed, or magenta and aniline blue. In all cases where permanence of colour is of importance, the former are to be preferred. For blue papers, ultramarine is generally used. Tinted papers are, as a rule, pro duced by the use of aniline colours. Coloured papers are produced by the use of various pigments. The operation of beating the pulp is of the greatest importance, and too much care cannot be devoted to it. In America, where the mills are generally driven by water-power, the pulp is kept for a much longer time in the engine than in Great Britain, and this accounts to a considerable extent for the superiority of the American papers. 1 After the pulp is prepared in the beating engine it is run into the chests of the paper machine (figs. 4 and 5). These chests A are fitted with agitators, and from them the pulp is pumped into the supply-box B, which com municates with the sand-traps C by means of a regulating cock. Along with the pulp a certain amount of water is allowed to flow into the sand-trap, so as to thin it down sufficiently; in most cases the save-all water (see below) is employed for this purpose. The pulp flows backward and forward here in a shallow stream, so as to deposit any heavy impurities which it may contain. After issuing from the sand-traps it is delivered on to the strainers, which are made in many varieties, the most common being the revolving strainer D, shown on the plan. This is a rectangular trough into which the pulp flows. In the centre of this the strainer, rectangular in form, composed of four sets of brass plates bolted to a frame in which very fine slits are cut, revolves slowly. The size of this is about 7 feet by 2 feet. The pulp is made to flow from the FIG. 4. Paper Machine Vertical Section. outside through the slits to the inside of the strainer by means of suction produced by bellows or disks in the interior of the plates, and is discharged by the pipe E into a box from which it flows on to the apron F, which is placed on the top of the breast roll. The apron is made of a piece of moleskin or india-rubber cloth the fall width of the wire, and prevents the pulp from running away down the back of the wire. It covers the wire for 12 to 18 inches at the beginning. The wire consists of an end less sheet of fine wirecloth (about 66 wires per square inch) which stretches from the breast roll G to the couch roll H, returning underneath by the leading rolls I. Underneath the first portion of the wire are the tube rolls K, and farther along are the vacuum boxes L, L. These communicate by pipes with the vacuum pumps M. As the wire revolves in the direction shown in fig. 4 the pulp is allowed to flow from the strainer and spreads itself out in a thin film, covering the surface of the wirecloth. It is prevented from flowing over the sides of the wire by the deckle straps, endless india-rubber straps N. Part of the water runs off through the meshes of the wire by gravitation, and the rest is removed through the suction boxes L by the vacuum pumps M. Stretching along under the wire from the breast roll to the first suction box is the save-all, a shallow trough into which the water which passes through the wire falls. The contents of this box flow into a cistern at the back of the machine into which the vacuum pumps also discharge their water ; and from this cistern the water is pumped into a service box and used instead of fresh water for mixing with the pulp as it flows on to the sand-trap. There is a considerable saving in this, as the fine fibre, size, &c., contained by the water passing through the wire is all in this way 1 Another form of beating engine which is finding great favour i is the Umpherston engine, which differs little from the ordinary
 * beater, except in having, instead of a midfeather, a passage under

the roll by which the pulp circulates. It is claimed for it that one area than an ordinary beater for 3 cwts. The pulp is also said to travel more freely, and does not lodge about the corners as in the ordinary engine.
 * capable of preparing 10 cwts. of paper does not occupy more floor