Page:Paper and Its Uses.djvu/68

56 The materials used range from the strongest to the weakest—from hemp rope to mechanical wood and include jute in the form of old gunny bags or sacking, hemp refuse, old rope and string, waste card cuttings, old paper, and wood pulp refuse. The substance ranges from 38 Ib. to 160 Ib. per ream of 480 sheets in double imperial, the colour from "white" to a very dark brown, and the finish from highly glazed both sides to a rough air-dried surface.

Strong materials are boiled under pressure for several hours, lime being employed for hard papers, and soda for softer papers. The fibres receive but little washing, going on to the beaters, where the stronger fibres are first reduced and the softer materials added later. Loading, colouring, and size are added, and the paper made on the Fourdrinier. Air-dried browns are specially tough, very leathery, will stand a great deal of folding, and when packing and unpacking of parcels is required the extra cost is easily recouped. Cylinder-dried browns are dried on the paper machine, and the papers are not so elastic as air-dried papers of the same substance. Glazed browns are usually lighter in colour and cleaner in appearance than the ordinary wrappings, and usually contain a large proportion of wood pulp. Kraft browns may be described as glazed browns, as they are sometimes finished with a glazed surface both sides. A special kind of pulp is used for krafts, wood being digested at a comparatively low pressure with soda solution, the boiling being prolonged. The fibres are loosened, and reduction to pulp takes place in the edge runner (kollergang) instead of the beating engine. By this means the fibres are drawn out, not cut up, and very tough papers can be made, fully entitling the papers to their description as "kraft" (German for "strength").