The exchange succeeds the operation last described; it is conducted in a hall contiguous to the vat, supplied with several presses and a long table. The workman arranges on this table the paper newly fabricated, into heaps, each heap containing eight or ten of those last under the press, kept separate by a woollen felt: the press is large enough to receive two of them at once, placed the one at the other's side, and must have a power from 70 to 100 tons. When the compression is judged to be sufficient, the heaps of paper are carried back to the table, and the whole turned, sheet by sheet, in such a manner that the surface of every sheet is exposed to a new one; and in this situation they are again brought under the press. If the stuff be fine, or the paper slender, the exchange is less frequently repeated: in this operation it is necessary to alter the situation of the heaps, with regard to one another, every time they are put under the press; and, also, as the heaps are highest toward the middle, to place small pieces of felt at the extremities, in order to bring every part of them under equal pressure. A single man, with four or five presses, may exchange all the paper produced by two vats, provided the previous pressing at the vats has been well performed.
The work of the exchange generally lasts two days on a given quantity of paper. The sheds for drying the paper are contiguous to the mill; they are furnished with a vast number of cords, upon which they hang the sheets both before and after the sizing. The sheds are surrounded with movable lattices, to admit a quantity of air sufficient for drying the paper. The cords of the sheds are stretched as much as possible; and the paper, four or five sheets together, is placed on them by means of a wooden instrument in the form of a tall T. The principal difficulty in drying the paper consists in gradually admitting the external air, and in preventing the cords from imbibing moisture.
The inconvenience of the expansion and contraction of the cords from alterations in their humidity, might, we conceive, be remedied by saturating them in a solution of caoutchouc, which would not destroy their flexibility, but would enable them to resist moisture, and render their durability almost everlasting. In some mills the paper is hung upon smooth, rounded laths, and the drying is effected by steam or hot water, circulated in pipes through the room.
The size for the paper-makers is made of the shreds and parings procured from the tanners and parchment-makers. All the putrefied parts, and the lime, being separated from them, they are enclosed in a kind of basket, and let down by a rope and pulley into a cauldron. When the solution of the gelatin is found to be complete (which is ascertained by drawing up the basket), it is allowed to settle for a while, and then twice filtered, before it is put into the vessel into which the paper is dipped. After this a certain quantity of alum, also of smalts, or other pigments calculated to improve the tint, or bestow a peculiar hue upon the paper, is added. The workman then takes a handful of the sheets, smoothed and rendered as supple as possible, in his left hand, dips them into the vessel, and holds them separate with his right, that they may equally imbibe the size. After holding them above the vessel for a short time, he seizes on the other side with his right hand, and again dips them into the vessel. When he has ten or a dozen of these handfuls, they are submitted to the action of the press. The superfluous size is carried back to the vessel by means of a small pipe.
The vessel in which the paper is sized is made of copper, and furnished with a grate, to give the size, when necessary, the requisite temperature; and a piece of thin board, or felt, is placed between every handful as they are laid on the table of the press. After the paper is sized it is carried to the drying house, where a gradual drying of the sized paper is considered to be very important; the exchange, likewise, at this stage, requires great attention, as the grain of the paper, which may then receive impressions, can never be restored. When the sized paper is also exchanged, it is possible to hang more sheets together on the cords of the drying house: the paper dries better in this condition, and the size is preserved without any sensible waste, because the sheets of paper mutually preserve the rapid operation of the external air; and as the size has already penetrated into the paper,:and is fixed on the surface, the insensible progress of a well-conducted drying house renders all the good effects more perfect in proportion as it is slowly dried. When the drying is completed, it is carried to the finishing room, where it is pressed, selected, and examined; folded, made up into quires, and, finally, into reams.
It is here put twice under the press; first, when it is at its full size, and, secondly, after it is folded. The principal labour of this place consists in assorting the paper into different lots, according to its quality or defects; after which it is made up into quires. The person who does this must possess great skill, and be capable of great attention, because he acts as a check on those who separated the paper into different lots: he takes the sheets with his right hand, folds them, examines them, lays them over his left arm till he has the number requisite for a quire, brings the sides parallel to each other, and places them in heaps under the table. An expert workman, if proper care has been taken in assorting the lots, will finish, in this manner, about 600 quires in a day. The paper is afterwards collected into reams of 20 quires each, and, for the last time, put under the press, where it is continued for ten or twelve hours, or as long as the requirements of the paper-mill will permit.
The art of making paper in one continuous sheet of any required length, originated from an ingenious Frenchman of the name of Didot, who, in conjunction with the Messrs. Fourdrinier, succeeded, after the expenditure of enormous sums of money, in perfecting this important improvement, which has now, in a great measure, superseded the desultory mode of operating we have just described. The action and arrangement of the improved mechanism may be thus briefly explained. A horizontal frame, of any required length or breadth, is furnished with a roller or cylinder at each end, over which is stretched an endless web of brass wire, of the requisite texture or fineness, for the paper to be manufactured by it. At one end of the frame, parallel with, and immediately over one of the cylinders, is a long angular trough, or sluice, into which the pulp is received from a vat above, wherein it is continually agitated, whence it issues through a long slit or opening, regulated by a screw, falling in an uniformly thin stratum upon the whole breadth of the endless web beneath, at which time the cylinders are in motion, carrying forward the stratum of pulp, and a joggling motion is communicated to it laterally by the alternating motion of a rod, produced by a revolving crank; this agitation of the pulp, as the water drains from it through the wire-work, produces the felting, or interweaving of the fibre, as perfectly as it is done by hand; and the pulp is prevented from flowing over the sides by means of two leather straps, one on each side, which move round with the web; and by the shifting of which straps nearer to, or farther from the centre, the width of the paper may be regulated.