Any pattern or name required on the paper is obtained by means of a raised pattern on the wire-cloth mould: consequently, less pulp lodges there, and the paper is proportionally thinner, thus showing the exact counterpart of the pattern. Such devices are known as “water-marks.” (See also p. [156].)
The Paper Machine.
A modern Fourdrinier paper machine is shown in plan and side elevation in Plates I. and II.
The pulp, after leaving the beaters, passes into a large vessel called the stuff-chest, of which there are one or more to each machine. As soon as the beater is empty, water is run in to thoroughly rinse out the remaining pulp, the washings also going into the stuff-chests. These may be {146} made either of wood or iron, and should be provided with arms fixed on a vertical shaft, made to revolve by suitable gearing. The arms are for the purpose of keeping the pulp thoroughly mixed, and should only work at a moderate speed, otherwise they are liable to cause the fibres to form into small knots or lumps. The pulp is drawn from the stuff-chests by means of the pump A, and is discharged into a regulating-box (not shown). The object of this box is to keep a regular and constant supply of pulp on the machine. It consists of a cylindrical vessel, having two overflow-pipes near the top, and a discharge-pipe near the bottom. The pulp is pumped in through a ball-valve in the bottom, in larger quantity than is actually needed, the excess flowing away back into the stuff-chests, through the two overflow-pipes. By this means, the box is always kept full, and therefore the stream of pulp issuing out of the bottom pipe is always under the same pressure. It flows from this pipe, the quantity being regulated by means of a cock, according to the thickness of paper required, directly on to the sand-tables. These may be of various sizes and shapes, but should be so large that the pulp takes some little time to travel over them. They consist of long shallow troughs, generally of a sinuous form. The bottoms are sometimes covered with woollen felt, or with thin strips of wood placed across the direction of the flow of the pulp, and at a slight angle. These serve to retain any particles, such as sand and dirt, that may have escaped removal in the previous treatment of the pulp, and that are heavy enough to have sunk down during the passage of the pulp along the sand-tables. In some mills, where great care is exercised, the pulp is caused to flow over sand-tables 200 yards in length. As the pulp, when it leaves the stuff-chests, does not contain sufficient water for the purpose of making paper, it is mixed, where it enters the sand-tables, with a quantity of water from the “save-all” (see p. [154]), flowing from the box B placed at a higher level.
In some mills, instead of being pumped into the {147} regulating-box, the pulp flows into a small vessel below the stuff-chest, and is lifted on to the sand-tables by means of buckets fastened on the circumference of a wheel.
The pulp, after leaving the sand-tables, passes on to the strainers. These consist of strong brass or bronze plates, having a large number of very fine
-shaped slits cut in them, the narrowest end being on the outside.
Strainers.
