Page:Paper and Its Uses.djvu/79

Rh the majority of fibres remain in a position parallel to the machine direction of the web of paper. Most machine-made papers are dried on the heated cylinders of the paper machine, the diameters of the cylinders being arranged to allow for the consequent contraction of the web, but the fibres are not given the opportunity to adjust themselves as in the case of air-dried papers.

When it is remembered that the papermaking fibres may expand in diameter to the extent of 20 per cent., but only one per cent, in length, it will be seen that the expansion will show itself chiefly in one direction, for the majority of fibres lie side by side. Fortunately the full expansion does not take place. Paper which is properly matured contains water equal to 7 per cent. of its weight. Without this moisture, paper would be brittle, and when this amount is exceeded the paper expands. But paper, as it leaves the calender rolls of the paper machine, contains less than 7 per cent. of water. It is essential that all the water should be dried out of the paper, and the paper is sometimes reeled almost bone-dry, but if the paper is to be super-calendered it is damped before reeling, and left until the paper mellows before calendering. Many papers are cut and packed without much opportunity for maturing, that is, as regards paper, attaining a degree of stability which should be maintained during its manipulation by the printer, and, it is hoped, during the remainder of its career.

All papers have some spaces between the fibres, sometimes partly filled with sizing and loading, but always containing some air space, the amount depending upon the density of the paper. Heavy or dense papers and light or bulky papers are the extremes, 30 to 70 per cent. of air space being examples of the two ends of the scale. The fibres, when expanding,