Page:EB1911 - Volume 20.djvu/790

Rh to be driven. The power consumed by the two engines will average from 40 to 100 h.p. The drying cylinders of the paper-machine form a convenient and economical condenser for the two steam engines, and it is customary to exhaust the driving engine into the drying cylinders and utilize the latent heat in the steam for drying the paper, supplementing the supply when necessary with live steam. The speed of the machine has frequently to be altered while in motion. An alteration of a few feet per minute can be effected by changing the driving-speed of the steam-engine governor; for a greater change the machine must be stopped and other driving wheels substituted. Arrangements are made in the driving-gear by which the various parts of the machine can be slightly altered in speed relatively to one another, to allow for the varying contraction or expansion of the paper web for different kinds and thicknesses of paper. The average speed of a paper-machine on fine writing-papers of medium weight is from 60 to 90 ft. per minute, but for printing-papers, newspapers, &c., the machine is driven from 120 up to as much as 300 and 400 ft. per minute. The width of machines varies greatly in different mills, from about 60 in. to as much as 150 in. wide. Mills running on higher classes of papers as a rule use narrow machines, as these make a closer and more even sheet of paper than wider ones. On fine writing-papers an average machine will make from 20 to 40 tons per week, while for common printing and newspapers the weekly output will amount to 50 to 70 tons.

J. Milne & Son, Ltd. . 14.—Super-calender.

The bottom roll and the 3rd, 6th, 8th and 10th rolls, all reckoned from the bottom, are made of highly polished chilled cast-iron; the others of highly compressed paper.

All hand-made papers, and many of the best classes of machine made papers, instead of being sized in the beater with a preparation of resin are what is called "tub-sized," that is, coated with a solution of gelatin. Such papers, when machine made, are reeled off the machine straight from the drying cylinders in the rough state. The web is then led slowly through a tub or vat containing a heated solution of animal glue or gelatin mixed with a certain amount of alum; after passing through a pair of brass rolls to squeeze out the superfluous size, the web is reeled up again and allowed to remain for some time for the size to set. The paper is then led by means of continuous travelling tapes over a long series of open skeleton drums, about 4 ft. in diameter, inside which revolve fans for creating a circulation of hot air; rows of steam-pipes underneath the line of drums furnish the heat for drying. Slow and gradual drying is essential to this process to get the full benefit of the sizing properties of the gelatin. In and-made papers, the sheets are passed by handfuls of three or five on an endless felt through the gelatin solution and between a pair of rolls, and then slowly dried on rope lines or "tribbles" in a steam-heated and well-ventilated loft.

The cheaper kinds of paper are glazed on the paper-machine in the calenders as before described. For the better class or very highly-glazed papers and those that are tub-sized, a subsequent glaring process is required; this is effected by sheet or plate-glazing and by super-calendering or web-glazing. The plate-glazing process is adopted mainly for the best grades of writing-papers, as it gives a smoother, higher and more permanent gloss than has yet been imitated by the roll-calender. In this method each sheet is placed by hand between two zinc or copper plates until a pile of sheets and plates has been formed sufficient to make a handful for passing through the glazing-rolls; this handful of about two quires or 48 sheets of paper, is then passed backwards and forwards between two chilled-iron rolls gearing together. A considerable pressure can be brought to bear upon the top roll by levers and weights, or by a pair of screws; the pressure on the rolls, and the number of times the handful is passed through, are varied according to the amount of gloss required on the paper. The super-calender (see fig. 14) is used to imitate the plate-glazed surface, partly as a matter of economy in cost, but principally for the high surfaces required on papers for books and periodicals to show up wood-cuts and photographic illustrations. It usually consists of a stack of chilled cast-iron rolls, alternating with rolls of compressed cotton or paper so that the web at each nip is between cotton and iron; it will be seen from the illustration that there are two cotton rolls together in the stack for the purpose of reversing the action on the paper and so making both sides alike; pressure is applied to the rolls at the top by compound levers and weights or screws. A very high surface can be quickly given to paper by friction with the assistance of heat; the process is known as "burnishing," and is used mostly for envelope papers and wrappings where one surface only of the web is required to be glazed. It is produced by the friction of a chilled-iron roll on one of cotton or paper, the ratio of the revolutions being as 4 to 5; steam is admitted to the burnishing iron roll.

At the end of the 19th century a large and increasing demand sprang up for papers embossed with a special pattern, such as linen-finish, &c.; these are used principally for fancy writing papers, programmes, menu-cards, &c. This embossing is effected usually on the plate-glazing machine, in the case of linen and similar finishes by enclosing each sheet of paper between two pieces of linen or other suitable material to give the desired texture or pattern on the surface of the sheet. Each sheet of paper with its two pieces of cloth is placed between zinc plates and passed backwards and forwards between the rolls of the machine as in plate-glazing.

Except for special purposes, such for example as for use in a continuous printing-machine, paper is usually sent from the mill in the form of sheets. A number of reels of paper is hung on spindles between two upright frames to feed the cutting-machine (see fig. 15); the various webs of paper are drawn forward together through two small rollers, and ripped into widths of the required size by means of a number of pairs of circular knives or "slitters"; they then pass between another pair of rollers, and over a long dead-knife fixed across the cutting-machine, on which they are cut into sheets by another transverse knife fastened to a revolving drum and acting with the dead-knife like a large pair of shears. The cut sheets then fall upon an endless travelling felt, from which they are stacked in piles by boys. It is often necessary, as in the case of water-marked papers, that the sheets should be cut with great exactness so that the designs shall appear in the centre of the sheet; the ordinary cutter cannot be relied upon for this purpose and in its place a machine called a "single sheet cutter" is used. In this cutter only one web of paper is cut at a time; between the circular slitters and the transverse knives is placed a measuring-drum, which receives an oscillating motion and can be adjusted by suitable mechanism to draw the exact amount of paper forward for the length of sheet required.

James Bertram & Son, Ltd. . 15.—Reel Paper Cutter.

All that now remains to be done before the paper is ready for the market is overhauling or sheeting. This operation consists in sorting out all speckled, spotted or damaged sheets, or sheets of