The paper, after passing the 2nd press rolls, travels over the drying-cylinders M (Figs. 1055 6-7), the number of which varies somewhat. In the machine shown, there are in all eight cylinders. Between the 2nd press rolls and the cylinders, a passage S allows easy access to the other side of the machine. The paper generally passes over the first two, which are only slightly heated, alone ; after wards it is led over the others by means of felts, as shown. The arrangements shown at it (Figs. 1055-6) are for the pur pose of stretching the felts. The cylinders are all heated by means of steam, and are generally di vided into two sets, between which, is a pair of chilled-iron, highly-polished rolls N, called "smoothers," the function of which is sufficiently explained by their name. They are also heated by means of steam. The cylinders are usually made of slightly decreasing diameter, in order to allow for the shrinking of the paper on drying. After leaving the cylinders, the paper should be quite dry ; it is then led through the calenders, of whioh there are in some ma chines as many as three sets, though only one is shown. These are similar to the smooth ing-rolls, just described. Pres sure is applied by the screws 0' (Fig. 1056), or by levers and weights. The friction of the hot calenders on the dry paper de velops a large quantity of elec trioity, which discharges itself in bright sparks.
The finished paper, after passing through the calenders, is wound on the reels P. The gearing by whioh the whole machine is driven is shown in Figs. 1053,1057.
Tub-sizing.----The foregoing description is of a machine for the manufacture of engine-sized papers ; some slight modifications are necessary in the case of tub-sized papers. One method, usually applied to the cheaper qualities of tub-sized papers, is to pass the paper, after being partially dried over a few cylinders, through a vessel containing a solution of gelatine or glue (see farther on). It then goes between a pair of rollers, which press out the ex cess of size, and then again over drying-cylin ders. In the other, and perhaps most general, way, at least for the better qualities, the paper is wound off immediately after leaving the last drying-cylinder A (Fig. 1058), and sized at some future time, or it may be passed directly into the vat B containing the size. After passing between the squeezing-rolls C, it is generally wound off as at D ; and after having stood some time to allow the size to be evenly absorbed by the paper, it is wound off from F, and passed over the cylinders H, of which there may be a very great number, some machines having over 300 of them. These cylinders are made of light spars of wood ; inside' them, and re volving rapidly in an opposite direction, are the fans G. The paper, iu travelling over these drums, is only slowly dried, and is supposed by this means to be more perfectly sized, and increased in strength. It is wound on to reels again at I. Only the first and last two drums are
shown. This method was devised to Imitate, as far as possible, the sizing process of hand-made paper. Even now, paper that has been made on the machine is sized by hand, after having been cut into sheets, much in the same way as hand-made. The sheets are sometimes passed between two endless felts dipping into a bath containing the solution of gelatine, the excess of size being squeezed out by the pressure of rolls on the felts. Such paper is dried on sticks in a large room kept at a temperature of about 21° (70° F.), and is called " loft dried " on this account.
The gelatine used for tub-sizing is made up somewhat after the following manner. As a general rule, size in the form of manufactured glue would be far too ex pensive, therefore paper-makers almost invariahly make their own. A great many animal substances, such as clip pings of hides, horns, bones, &c., yield gelatine when heated for some time with water. Any of these substances are suitable, the first being perhaps most generally used. They are first softened by soaking in cold water for a day or two ; after that, they should be well cleaned by washing in running water. The next operation is to boil or rather beat them with water. The temperature should never be allowed to rise much above 85° (185° F.), as if it does, the size will he injured, as gelatine strongly heated for any length of time undergoes a slight change, and loses its power of gelatinizing. The operation should be conducted in an iron or copper vessel, provided with a false bottom, or a casing outside, where steam may be intro duced, and it should extend over about 15 hours. The solution should then be drawn off, and filtered into some convenient receptacle. The residue can be again heated with water, and a fresh quantity obtained, which may be added to the bulk. A quantity of alum (about 20 per cent. of the clippings) dissolved in water, is added. The alum is necessary to prevent the decomposition of the gelatine, and to assist in the sizing process, as it helps to give hardness to the paper. After the addition of the alum, the size should be well filtered through woollen felt, after which, it requires no further treatment.
The paper-making ma chine before described is known as the " Fourdinier," from the names of the ori ginal inventors. Modified forms of this machine have been introduced to meet various requirements. One, suitable for the manufacture of very thin papers, resem bles the ordinary machine as far as the couch-rolls. The paper is taken off the wire cloth on to a long endless felt, running round the upper couch-roll, and extending in a slanting direction over the wire-cloth, It is taken off from this felt on to a large cylinder, about 10 ft. in dia meter, heated by steam, and placed above the wire-cloth. After passing round nearly the whole circumference of this cylinder, the paper is sufficiently dry, and is then wound on to reels.