The sulphate process is really a derivative of the soda process, sodium sulphate being used instead of the caustic soda. The action is much gentler, the yield greater, and the fibre stronger, but the pulp is not capable of giving so high a white colour as soda pulp: it is much used for strength and when colour is not vital. Kraft wrapping papers are made by this process; imitation Krafts often by the soda process, and imitation manila by the bi sulphite process. The bisulphite process is based on the use of a solution of sulphur dioxide (sulphurous acid gas), to which lime or magnesia is added in quantity insufficient for complete neutral ization. The process has been developed greatly, with a number of modifications, since its first successful industrial introduction by Tilghman, Ekman and Mitscherlich. In the early days the liquor thus used contained 2.5-4.0 per cent of total sulphur dioxide of which rather less than half was neutralized by lime or magnesia : indirect heating by steam coils at 8o-11 o lb. steam pressure was used, and the time of cook was 16-48 hours. Ritter and Kellner raised the sulphur dioxide concentration and the cooking temperature and introduced the use of live steam for heating : this procedure has been developed still further and the modern "quick cook" at still higher concentrations is now some times completed in 8-16 hours, and temperatures of 15o°-16o° C are reached. In general terms the gentlest treatment gives the
largest yield and the strongest pulp which may be, however, relatively difficult to bleach. The boilers for this process are always stationary, vertical and brick lined with litharge cement mortar. They are commonly of enormous size and capable of taking 3o tons or more of wood chips in one charge.
BIBLIOGRAPHY.—Annual reports of Empire Forestry Association. Annual reports of Forestry Commissioners (H.M. Stationery Office). Census of Industry reports on the Pulp and Paper Industry (Dominion Bureau of Statistics, Ottawa). The Committee on Vocational Educa tion (pulp and paper), the manufacture of pulp and paper, Vol. HI. Preparation and Treatment of Wood Pulp (1927) gives an authorita tive account of the present methods of the pulping industry.