The quantity of bleaching-powder necessary is subject to considerable variation, depending greatly upon the quality of the grass, and the extent to which it is boiled ; 12 lb. per cwt. of esparto may be considered a fair average quantity. After remaining in the poacher for about 2 hours, the almost perfectly white pulp is run off into large chests or drainers, and allowed to remain there for about 8 hours, or longer if convenient. The chests are supplied with perforated bottoms, through which a large proportion of the liquor drains away. It is said that the action of light assists the bleaching process, and for this reason, the chests axe often placed in an exposed position. The drained pulp is next transferred to hydraulic presses, where most of the liquor left in is got rid of.
The preceding method of treating the grass is the one usually adopted, but, in some cases, other systems are in use. Thus, instead of breaking and bleaching in separate engines, some manu facturers perform the two operations in the breaker, which, in this case, is provided with two drums, one covered with coarse gauze for removing the liquor from the boiling operation, and the other for removing the excess of bleach, when the pulp is considered white enough, which usually happens in about hours. The gauze on the drum for removing the bleach is finer, because at the end of the operation the pulp is much finer, and would therefore pass in considerable quantity through a coarse one. In this modified system, the pulp is not pressed, but, after the excess of bleach has been washed away through the drum, is run off into the "beaters." Beating.—Beating-engines differ but little in appearance from breaking-engines. The revolving roll, however, carries more knives, and is let down much nearer to the bed-plate, in order to complete the process only partially done in the breakers, of reducing the esparto to a sufficiently fine pulp. It is in the beaters that the final preparations of the pulp are made, and much care is neoessary in their management. The bleached pulp from the hydraulic presses, or direct from the breakers, according to the system of bleaching employed, is placed in the beaters together with a quantity of water carefully filtered through a bag of woollen felt.
Immediately after furnishing the engine, a quantity of some loading or filling material is run in. The one usually employed, as being the most economical, is kaolin or China-clay (see pp. 635-9). The addition of China-clay in moderate quantity can hardly be looked upon as an adulteration, since it serves to close up the pores of the paper, and enables it to take a good surface. Of course, if added largely, it tends to weaken the paper. The clay is usually made into a fine milk with water, by being mixed in vessels provided with revolving agitators, placed above the level of the beaters. Before being run into the engine, it is carefully sieved through fine wire-cloth. Occasion ally papers are made without any clay. Other filling materials have been proposed, and several others are in actual use, the chief of these being precipitated sulphate of lime, or " pearl hardening ;" this, however, is used only in the finest class of papers, it being too expensive for ordinary use. No rule can be given for the amount of loading material to be used, as papers are made with quantities varying from 5 to 15 per cent., and even more. When the clay or other sub stance exceeds 15 per cent., it certainly cannot be to the advantage of the user.
The quality of the water is a matter of the utmost importance in the manufacture of fine papers. Above all things, it should be free from suspended matter, and from dissolved iron. While much of the former can be removed by settling in large ponds, and by careful filtration, the latter cannot be economically eliminated, and is liable to be precipitated in the fibre, thus injuriously modifying the colour of the paper.
The subsequent treatment of the pulp depends on the kind of paper that is required. Papers may be roughly divided into two classes, viz., " tub-sized " and " engine-sized." As all papers, oven tub-sized, excepting blotting- or water-leaf-paper, are more or less sized in the engine, this may form the next point for consideration.
Sizing.—The principle upon which engine-sizing depends is briefly this, viz. the precipitation and intimate mixture with the pulp of a substance which, when dry, will to some extent fill up the interstices between the fibres of the paper, and which possesses the property of being with difficulty wetted with water. Such a substance is the compound of common rosin with alumina. In order to obtain a thorough mixture of this body with the fibre, it is always formed in the engine by adding an aqueous solution of rosin-soap to the pulp, and when this is intimately mixed with it, running in a solution of alum. The rosin-soap is made in the following manner. Ordinary rosin,
the quality depending on the quality of paper required, is boiled, preferably in a steam-jacketed boiler, with a solution of carbonate of soda (ordinary washing soda) until a sample of the soap formed is completely soluble in water. This takes place in about 2} hours. The amount of carbonate of soda used differs widely, though why, it is difficult to see ; usually about 1 part to 3 of rosin is oonsidered necessary. Any excess above the quantity required to thoroughly dissolve the rosin is useless and wasteful, because, if left in the soap, it consumes an equivalent quantity of alum, and cannot possibly serve any good purpose. The boiling being completed, the charge is run off into iron tanks, and allowed to settle. The soap soon cools to a semi-solid mass, and a quantity of liquor, containing a considerable portion of the impurities, such as colouring matter, of the rosin, rises to the top, and can thus be removed. The soap so purified is next dissolved in water, with the addition of a small quantity of carbonate of soda, if necessitated by imperfect boiling in the previous operation, and is mixed with a quantity of starch-paste, made up in separate vessel by dissolving farina in hot water. The mixture is then carefully sieved, and is ready for use. Some manufacturers prefer caustic soda for forming the soap, and others use soda ash ; all are equally suitable. The proportion of starch to rosin differs in nearly every mill, and the quantity of size to be added to the beater varies according as the paper is required to be soft- or bard-sized. Abont parts of starch to 1 part of rosin is an average quantity ; and about 3i lb. of the mixture to 100 lb. of dry pulp is a fair proportion. In some mills, it is the practice to make up the starch with the China-clay, instead of mixing it with the rosin. (For Rosin, see Resinoua Substances.) It is better, but not absolutely necessary, to dissolve the size in water before putting into the engine. After allowing the rosin-soap and starch to get thoroughly mixed with the pulp, the alum may be run in. It is made up in a large leaden tank, furnished with steam-pipes for heating; it is necessary to use lead, as the alum in strong solution rapidly attacks iron vessels. The choice of a suitable alum is a matter of very great importance; care should be taken that it be free from excess of sulphuric acid, and from soluble iron. The sulphuric acid is deleterious on account of its action upon the colouring matter used subsequently, some colours being completely discharged by it, and because of its effect upon metal-work in contact with it, especially upon the brass wire-cloth on which the paper is made. As the sulphate of alumina is the only active agent in the alum, the sulphates of potash and ammonia being without any action on the rosin-soap, some paper-makers use a preparation called " aluminous cake," which consists entirely of sulphate of alumina. The only objection to this substance is that it not unfrequently contains an excess of free sulphuric acid and soluble iron. If it could be guaranteed free from these impurities, there is no reason why it should not supersede the more expensive alum. A considerable excess of alum over the quantity necessary for precipitating the rosin is employed, as it has the effect of brighten ing the colours added subsequently. Other materials have been suggested as substitutes for rosin in the sizing process, but none can compete with it in point of economy. Wax, dissolved in soda, and precipitated with alum, will answer the purpose, but it does not appear to have been used on a manufacturing scale. The addition to the rosin of a small proportion (about 2i per cent.) of gum tragacanth is said to be very advantageous, giving the finished paper some of the characters of a tub sized paper. (For Alumina, see Mordants, pp. 1295-6.) It is necessary at some stage of the treatment of the pulp in the heaters, and before the addition of the colouring materials, to add some substance that will entirely get rid of any bleaching-liquor unavoidably remaining in the pulp. The substance usually employed is sulphite of soda, techni cally known as " antichlor." Its action depends upon the fact that it is converted into sulphate of soda by the active chlorine in the bleaching-liquor, the latter being at the same time changed into a chloride, in which state it is quite inert. If any free chlorine were allowed to remain, it would be very hurtful, as it would bleach the pink employed, and would at the same time act injuriously on the wire-cloth. Other substances have been used instead of the sulphite of soda, almost any reducing agent being suitable. A very cheap substitute is often prepared by boiling sulphur with milk of lime. It is the practice in some mills to wash out the excess of bleach with water, the beaters being supplied with drums for this purpose.