Ruling on papers with hard surface is rendered less difficult by the use of a small amount of gall in the ink. For hand-made papers the ink always requires such manipulation, while for other tub-sized papers a little gum arabic in addition to the gall will render even ruling more easily attainable. In ruling engine-sized papers a small amount of gum arabic and carbonate of soda (ordinary washing soda) will make the colours lie better. While all work can be done on the pen machine, papers with soft surfaces, blottings, duplicating, metallic, and coated papers generally, will give the disc machine opportunity to prove its superiority for this class of work. Cockled papers and very thin papers can be dealt with successfully at the ruling machine by a little manipulation of the pens and feed.
Although rolling, hot or cold, may be effectively used for giving finish to the printed work, the paper is subjected to such great pressure that it is liable to stretch. As pointed out earlier in the chapter, stretching of paper is not equal in both directions of the sheet, and it is advisable, in order to preserve the strength of the paper, to roll in the same direction as the paper was made and rolled in the papermaking machine. Discover the machine direction by the method described on page 86, and feed the paper to the rolling machine in the same way as it left the papermaking machine.
Tub-sized papers may contain or develop a fault which will not occur in engine-sized papers, that of unpleasant smell. A preservative of some kind is frequently added to the sizing solution, but if the gelatine has commenced to decompose the smell will be at least unpleasant. Coated papers contain glue in the coating mixture, and are liable to the same fault. Printers should be careful when buying job lots of tub-
