The quality of the pulp may be varied by the conditions under which it is made. By limiting the proportion of water so that the wood remains in contact with the stone for a longer time the temperature of the mass in the pockets rises. Such _hot ground pulp_, as it is termed, is tough and strong. When the fibres are washed away from the stone as fast as they are produced the temperature does not rise, and _cold ground_ pulp is made, which is not characterised by the somewhat leathery feel of the pulp made at the higher temperature. The surface of the stone plays an important part also. If the stone is smooth the wood is rubbed away slowly, but if the surface has been roughened and grooved by means of a special tool the fibres are torn away quickly. In the first case the pulp comes from the stone in a finely-ground state and in a uniform condition, while in the second the pulp is coarse and chippy. The output of the machine is, however, much increased by the use of sharp stones and by the application of considerable pressure to the blocks of wood. [Illustration: FIG. 30.--A Vertical Grinder for making Hot Ground Mechanical Wood Pulp.] _Screening._--The mixture of water and pulp leaving the grinder falls into a tank below the stone, all large chips being retained by means of a perforated plate. The finer pulp, still too coarse for use, is then pumped to the screens, which serve to remove all chippy and coarse fibres and produce a uniform material. The _shaking sieve_ consists of a shallow tray, the bottom of which is a brass plate or series of plates perforated with small holes or slits. The pulp flows on to the tray, which is kept in a state of violent agitation, the fine pulp passing through the holes and the coarser pieces working down to the lower edge of the tray into a trough which carries them away. The _flat screen_ is somewhat different in construction, but the principle of separation is the same. It consists of brass perforated plates forming the bottom of a shallow cast-iron tray, continually agitated by means of cams fixed to the under surface of the trays. [Illustration: FIG. 31.--Centrifugal Screen for Wood Pulp.] The _centrifugal screen_ is a cage made of finely perforated brass sheeting which revolves at a very high rate of speed inside a circular cast-iron vessel. The pulp flows into the interior of the cage, the fine fibres being forced through the screen by the centrifugal action of the machine, and the coarse material is retained. [Illustration: FIG. 32.--Section of Centrifugal Screen for Wood Pulp.]