already described. Broad, ribbon-like fibres, some pitted, and others perforated, all are very thin, lying closely together, so that a paper made entirely from sulphite wood pulp is rather harsh and fairly transparent. Papers produced from soda pulp are softer and more opaque than those made from sulphite pulp. Mechanical wood pulp is made up of little pieces of wood with all the resins and other impurities of the original wood, and when examined carefully, the fragments of wood can be seen, and the splinters appear to be held together by the plant cells. Mechanical wood pulp possesses very little felting quality, and requires the addition of larger fibres, such as chemical wood, in order to make paper successfully.
From the foregoing list and descriptions it can be seen that papermakers have a variety of fibres at their command, and it is by selecting, sometimes by blending, fibres of different characteristics, that the manufacture of the large variety of papers is possible. Some of the demands made by the world of paper users are for papers which are very strong or very soft, absorbent, resistant to grease or water, very light, very dense, and the selection of the fibres and their treatment call for special knowledge and skill on the part of the papermaker.
In addition to fibre, most papers contain sizing, sometimes loading or filling. Sizing may be animal or vegetable, the animal size being gelatine or glue obtained from various animal substances, and the vegetable size being made of a combination of resin with soda. Alum is used to assist in fixing the size in the paper. Mineral matters are employed for filling or loading; china clay is the principal, others being barium sulphate (barytes, blanc fixe), calcium sulphate (gypsum, terra alba, annaline, pearl hardening, crown
