_“Broke” Paper._—Under this head may be included all the partially formed paper which is always obtained in greater or less degree when a paper-machine is started, or such portions as are occasionally unavoidably damaged in its passage over the drying cylinders, together with the imperfect or rejected portions. It may also include used or waste paper, a large proportion of which, if not too dirty, is re-made into paper. The cleaner portions, especially if they have not been dried, are frequently returned direct to the beaters, and mixed with other pulp. That which has been actually made into paper requires to be softened by boiling in water and {105} gentle breaking in an engine. It may be necessary to heat it for a short time in a weak solution of caustic soda. This may be done either in a breaker or in special tanks provided for the purpose. Paper which has been printed upon requires a rather more drastic treatment, and it must of course be used for an inferior quality of paper, as it is impossible to get it to as good a colour as the original pulp from which it was made—at least not economically. “Broke” paper may be advantageously disintegrated by means of an edge-runner. It consists of a pair of stones arranged in the same way as an ordinary mortar-mill (Fig. 27). [Illustration: FIG. 27.] _Mechanical Wood Pulp._—A very large quantity of pulp is used in the commoner kinds of paper, such as cheap news, &c., which is obtained by disintegrating wood by mechanical means alone, no chemicals being employed. The idea of making paper in this way dates back about 100 years, but owing to the want of suitable machinery it is only lately that a good product has been obtained. The following are, in a general way, the details of the process employed:—