required the pulp would be let down to the stuff-chest; but usually other things are added before the pulp is ready for the machine. Filling or loading, colouring matter and sizing material, are mixed with the pulp, thoroughly incorporated, and then the engine is emptied. Paper can be made without filling or loading; in fact all-rag papers seldom contain mineral matter, and many excellent papers are made from other fibres without loading. The purposes of loading are to fill the spaces between the fibres, to give opacity to papers, such as those made of sulphite wood pulp, which would otherwise be very transparent, and to enable the paper to take a higher finish than would be possible in a paper without loading: a smoother and more absorbent, even if a little weaker, sheet, resulting. In a blotting paper mineral matter is an adulteration; in writing papers 5 per cent, is sufficient for improvement of surface; in printings 10 to 16 per cent, is as much as is permissible. In an imitation art paper as much as 25 per cent, may be added, and yet a serviceable paper result; but of course the tenderness of imitation art paper will be present.
China clay is the usual material used for filling or loading. It is mixed with water, and strained before filling into the beating engine, and the colour is added, either to produce a coloured paper, or to correct the tendency to greyness in the finished paper. In the latter case, a little blue and perhaps a little red is added, while in the former case the colour may be added, or formed in situ by the mixture of different chemicals in the beating engine. Dry colours, whether pigment as ultramarine or aniline colours, are mixed with water (dry patches being difficult to deal with in the pulp), and then added to the pulp in the engine; when the colours are thoroughly mixed, alum is put
