Page:Paper and Its Uses.djvu/58

46 printings, lithographic papers must be firm and free from permanent stretch. In letterpress printing, only a portion of the paper is pressed by the printing surface, but in lithography the whole of the paper is brought into contact with the stone or other surface. If the printing surface is full or solid, as in the case of printing a ground tint, the pull on the surface of the paper is heavy, and unless the paper is well made the surface will pluck or pull up in patches, or even all over the sheet. The pressure exerts a stretching influence on the paper, and the moisture from damping induces expansion of the sheet. Lithographic papers require special care in selection of material and manufacture, so as to introduce and preserve all the necessary qualities of easy printing, perfect register, and quick drying.

Esparto fibre is short and soft, prints easily, and experience has proved that esparto papers stretch less than most other papers, and therefore litho papers usually contain a large proportion of this useful material. An all-rag litho. paper is the first quality; then rag and esparto, all esparto, chemical wood and esparto, mark the various grades of paper for lithographic printing. While the papers should not be hard-sized, they should not err on the other side, or absorption of moisture may cause trouble when registering. Soft materials, beaten quickly, dried gradually, not drawn too fast by the drying cylinders, are necessary to produce a satisfactory paper. The surface must be perfectly smooth, and this is obtained by super-calendering or plate-glazing, both of which tend to reduce the liability of the papers to stretch. The latter method is the better but more expensive method of producing the desired surface, and by turning the piles of paper and rolling in each direction of the sheet, subsequent stretch in working is reduced to a minimum.