THE MAKING OF SULPHITE There is a new process of separating the wood fibres from other organic substance by chemicals. Everything but the tough cellulose is removed, and it makes a strong paper. I visited one of these mills. The chemicals used produce a pulp called in trade "sulphite." The Wood. This mill, on the bank of the Delaware, soon consumed all the available wood on the hill slopes of the neighbourhood. Now the supply comes in on cars from regions more remote. Hemlock and spruce are the only kinds used here. They are sound and green and cost at the mill about $6 per cord. The sticks average six inches to eight inches in diameter, and are sawed in two-foot lengths. The smaller the sticks, the greater the bulk of clear stock per cord and the less waste in bark.
The blocks of wood are stored in the basement and go in their turn to the peeling machine, whose knives remove the bark in thin slivers, leaving the blocks white and smooth. The bark is carried into the furnace, for it has considerable fuel value, and must be put out of the way. The blocks next pass to a machine where they are chipped into flakes, much like chips on a woodpile. These are carried to a great cauldron called the "Digester," with capacity of four or five tons of chips.
The Acid Solution. In the end of the building farthest away from the stored blocks of wood are the raw materials that combine to convert wood into sulphite. In one bin is dry sulphur, or brimstone, shovelled in by the ton. In another is air-slacked lime. Into a large tank of lime water the fumes of burning sulphur are introduced. The acid solution thus produced is the liquor poured over the chips in the digester. Under a pressure of eighty pounds of steam the chips cook for about twelve hours or longer. The judgment of an experienced tester is needed to decide when the cooking is done.
To scoop out this mass of hot pulp, reeking with strong chemicals, would seem a dangerous as well as difficult operation. On the contrary, it is very simply done. A tube at the bottom
of the digester is now opened. It leads to an empty receiving vat. The steam pressure is increased above, and the mass is driven out by the blast, leaving the walls of the digester as clean as if they had been scrubbed.
The cooking has chemically freed the wood fibre from every thing else. It remains to get the delicate white threads separated from the mass of waste with which they are now associated in the vat. Washing and screening are the means of freeing the fibres. The processes now are purely mechanical. Water is introduced, and by churning and draining alternately the acid solution is washed out. Then the pulp passes, thinly spread, over sets of screens that take out the coarsest of the impurities, brown flakes of pith rays, uncooked knots, and bits of foreign matter.
Water streams over and through the screens, carrying the fine white fibres with it. There is a wide, endless apron of linen, like a gigantic roller towel, that revolves at right angles with the screening tables. As the water pours over this the fibres lodge on the cloth, and as the dripping, tightly stretched sheet of linen passes between the two big steel rollers at the end the filmy layer of fibres has most of the water squeezed out, and adheres as a damp, matted sheet of cotton wool to the upper wheel.
A continual winding of this coating of fibres thickens the roll on the steel cylinder until it is like table felt of heavy quality. The machinery need not stop while it is removed. A pocket knife is run from end to end of the cylinder of steel. The next revolution lays the white sheet of sulphite on the table in front of the machine. The thin film on the steel is the beginning of another.
This is sulphite. It has the colour of unbleached linen or muslin. In fact, it looks much like felt, its fibres being merely pressed together—not woven. It is folded clumsily and stacked for the present. In this form it dries gradually for use or ship ment later, or it may be used at once.