From Paper-mill to Pressroom

by William Bond Wheelwright

WOOD-PULPS.

Wood-pulps are of two classes, mechanical and chemical. In the lay mind there often appears to be some confusion between the two, leading to an unreasonable prejudice against papers made from either class. The fact is so generally known that news-print, one of the cheapest grades of paper, is made from wood, that the partially informed person is prone to think that all wood papers are of low quality, whereas paper of permanence and excellent quality may be made from the high grades of wood cellulose chemically prepared.

Ground Wood.—The mechanical, or ground wood, as its name implies, is made by grinding logs from which the bark has been removed. The logs are shipped, or floated from the lumber camps to the mills, where they are cut to convenient length and the bark is removed. Next they are taken to the grinders. One type of grinder consists of a vertical grindstone encased in an iron jacket. There are three pockets over its circumference into which the logs are placed. They are held by hydraulic pressure against the revolving stone, over which flows a stream of water, and are rapidly reduced to fibers. These fibers are carried by the flowing water into a chamber below the grinders, passing through a screen which catches the coarser bits, the fibers of suitable size thus being separated from the rest. This pulp is still not sufficiently fine or uniform, so it is pumped into screens and forced through the finely perforated plates. The fibers are carried through with a large quantity of water, and are formed into thick sheets by means of a so-called “wet machine.”

Wet Machine.—The wet machine consists of a vat, in which a partially submerged hollow drum rotates. The surface is covered by a wire cloth, and the hollow axle of the drum acts as a drain for the fiber-laden water, which, in passing through the drum, deposits a film of fibers upon the revolving surface. This soft pulp film, continuously forming, is removed from the top of the drum by an endless felt running tangent to it, and held in close contact with it by a couch roll, the pressure of which causes the web of pulp to adhere to the felt.

The felt passes between two squeeze rolls, and the pulp adhering to the upper roll is wound up until a certain number of layers have accumulated, when it is cut across by a knife and removed as a thick sheet.

WOOD GRINDER

The sheets, folded to a convenient size, separated by alternate pieces of sacking, are put in a hydraulic press and squeezed to remove the water. The pulp is taken from the press about fifty per cent moist; the sheets are separated from the sacking and are now ready for use or for shipment. It is also quite customary to ship the pulp without having pressed it. In this case it contains about 70% water, due allowance for which is made in billing.

This pulp contains practically all the constituents of the original wood, has little strength, inferior felting properties, and is not of permanent character. Its utility results largely from its cheapness. When made into paper with a suitable admixture of sulphite pulp, for strength’s sake, it proves to be admirably adapted for the fast-running newspaper presses, as ink dries upon it almost instantly.

It is also used in the making of boxboards, cheap cardboards, pie plates, wall papers, etc. It should, however, be strictly excluded from all papers of more than ephemeral purposes, because of its lack of permanence. The appearance of a paper containing much ground wood is inferior, as the color is poor and small shives of wood may be discerned on the surface. An easy and reliable way to ascertain the presence of ground wood is to moisten the paper with a drop of strong nitric acid, which develops a dark-brown stain if ground wood is present. Another good test is phloroglucine, which turns ground wood to a bright carmine shade. The quantity of ground wood is roughly indicated by the intensity of the stain.