The speed of the machine has to be altered frequently, while in motion, and arrangements are provided whereby the various parts of the machine can be slightly altered in speed relatively to one another, to allow for varying contractions and expansions of the web, for different thicknesses and different pulp materials. Adjust ments are also possible by changing the driving speed of the steam engine governor, and, if necessary, also by changing the driving wheel. Skilful control is obviously required and, in practice, it is found advantageous to have two separate steam engines: the first, at constant speed, drives the strainers, pumps, shake motion, etc., and the second, working the machine, varies in speed according to the rate at which the machine requires to be driven : their combined H.P. will be 40.-100 or more. The drying cylinders form convenient and economical condensers for the two engines and it is customary to exhaust one driving engine into the drying cylinders and to utilize the latent heat of the steam for drying the paper, supplementing it, if necessary, by further live steam.
The mills making better class papers commonly use machines up to 90 in. wide, and they make closer and more even sheets than do wider machines : but newsprint sheets are made up to 24o in. wide running up to 1,200 ft. per minute. The former machines will make 20 to 4o tons a week, the latter up to 750 tons. Tinting and Colouring.----The colouring of paper, including the colour correcting of unfavourable tinges of the pulp, is always effected by adding suitable substances to the pulp in the beater, i.e., before the pulp goes on to the wire. Originally, pigments were practically the only substances available, and many of the pigments used then are still used to-day. Smalts (very finely powdered glass, coloured blue with cobalt) are extremely perma nent and are also expensive : they are therefore used for hand made papers only. Ultramarines, of various shades from reddish blues, through pure blue to greenish and yellowish blues are made and they are also much used. They are sensitive to acids and even to solutions of alum, which is slightly acid in reaction, as well as to oxidizing substances such as bleaching agents : hence the colour is apt to be evanescent and care must therefore be taken, espe cially by effective washing, to ensure that the pulp is free from unfavourable substances. Red pigments in use are rouge, Indian red, Venetian red, etc.: they are natural earths, refined by leviga tion, and rich in iron oxide to which their redness is due : an artificial red of similar chemical character is also prepared by calcining ordinary green vitriol (iron sulphate, copperas). Chrome yellow (lead chromate) is extensively used for the production of brilliant yellows. It is decolourized by strong acids, dissolved by caustic soda, and blackened by sulphides: it is therefore apt to be discoloured by exposure in cities in the open air, on, e.g., posters. Ochres, which are natural earths containing iron oxide, are used and they give various shades of dull yellow or buff : golden shades are commonly made by using mixtures of ochre and chrome yellow : solutions of iron salts are also used for the same purpose. Green is usually provided by using chromium oxide and brown by using umbers, which are natural earths containing manganese oxide in addition to iron oxide : the latter are much used as com bined colouring and loading agents for brown wrapping papers. There are also some organic pigments and lakes in use, the most important of which is Prussian blue, made by precipitating a soluble ferrocyanide solution with a solution of an iron salt and supplied to the paper maker in paste form. It is sensitive to
alkali, which precipitates a brown stain of iron oxide, on paper, when it is present. A number of organic synthetic pigments are valuable by reason of their qualities of permanence : indanthrene blue is very permanent and much used for high grade writing papers : soluble dyes are also largely used.
Briefly the uses of available colouring agents for tinting pur poses are as follows. Smalts are the only blueing agents used for the highest grades of papers. For other high grade papers, such as high class all rag machine made papers and high grade' sulphite pulp papers, ultramarine is the usual tinting agent : it is practically as permanent as smalt, provided that it is not exposed to the action of acid fumes : indanthrene blue is, however, coming into increasing favour in competition with it. Of lower grade blues there are many water soluble blues, acid coal tar dyestuffs, which are completely precipitated by alum and they are therefore used with sized papers only, usually esparto and chemical wood papers: Prussian blue is also used. For the tinting of the cheapest grades of paper, e.g., newsprint, the very powerful, cheap and light-fugitive, methyl and ethyl violets, blues, greens and reds are used.
Many of the colouring matters now so commonly used are therefore rather sensitive to chemical action : and the commonest active chemical thus to be dealt with, is of course bleaching powder, a solution of which is used in developing the high white colour required in writing and printing papers. Washing will remove the superfluous bleach remaining in the pulp, in time ; but time may be saved, frequently with advantage, by using a dilute solution of an "antichlor". Sodium thiosulphate, the "hypo" of photography—has been much used for this purpose : but sodium sulphite is preferred by many paper makers because it has less action on the "wire" of the paper machine, even though about three times as much of it is required for the same effectiveness. These substances react at once with any surplus bleach residues present in the pulp, to form harmless substances. Hydrogen peroxide would be the ideal substance, for this purpose, and it has been used: but its high cost makes its use prohibitive. What ever antichlor is used, an excess of it must be avoided because they all of them have some slight unfavourable effect on the sizing and to a small extent on some colouring substances. The beater man, in practice, has to add to the pulp the minimum quantity of antichlor that will just prevent the blueing of starch iodide test paper by the pulp: he then knows that there is no free chlorine (or hypochlorite) residue left in the pulp. It is worthy of mention that an excess of antichlor is most objection able in any paper, such as tissue paper, used for wrapping brass or silver, and in printing papers used for bronze or so-called gold lettering : these antichlors, being compounds containing sulphur, are likely to blacken silver and copper by formation of sulphides, and it is therefore best that such papers should be made without antichlors, and that they should be absolutely condemned for these uses if any antichlor is present, even in the minutest traces. (Ultramarine also contains sulphur, and its presence is similarly objectionable.) The present tendency is to avoid the use of anti chlors and to rely on very perfect washing.