Page:Paper and Its Uses.djvu/80

68 fill some of the air spaces between the fibres, and the expansion can never extend to the 20 per cent, mentioned. Experiments carried out on a litho. paper, 36 lb. royal, showed the maximum expansion from absorption of moisture to be 2½ per cent, but papers do not expand as much as this in working, or register work would be extremely difficult.

Expansion, or stretching as it is usually termed, is caused by absorption of moisture by the finished paper from the atmosphere. The atmosphere always contains some moisture, the amount varying not only from day to day, but from hour to hour. When there is an excess of moisture in the air, as on wet days or when fogs occur, paper will readily absorb the extra moisture, and the absorption will be accompanied by expansion of the sheet, principally across the web, or as it is generally termed, in the cross direction. This propensity of paper really points to the remedy. Paper should be matured and kept in that state, or to put it in other words, it should contain an amount of moisture which is neither increased nor diminished.

Few printers treat the machine-room, letterpress or lithographic, or the ruling-room as places where scientific conditions should be maintained. The use of the wet and dry bulb thermometers in other factories is for a definite purpose, to indicate the state of the atmosphere, and to guide in regulation of temperature and humidity, in order that the manufacturing processes may be carried out under scientific conditions. But the machine-room of the printer, closed for more than half its time, heated perhaps by hot water or steam pipes, sometimes hot, sometimes cold, in wet weather damp, in summer alternately very dry and damp, what wonder if paper expands, contracts, and causes trouble at machine.

The establishments where scientific conditions are