Where starch is used for stiffening purposes, the soap is mixed with a quantity of starch paste in the proportion of 1½ part of starch to 1 part of resin soap. Some manufacturers, Mr. Davis states, mix the starch paste with the kaolin in lieu of mixing it with the resin soap. In either case the materials should be thoroughly strained before being added to the pulp. From 3 to 4 lbs. of the mixture of resin soap and starch paste to each 100 lbs. of dry pulp are about the proportions in which the size is generally used, but the quantity added to the pulp in the beater depends upon whether the paper is to be soft-sized or hard-sized.
Sizing is chiefly applied to papers which are to be written upon with ordinary inks, and also, with a few exceptions, to printing papers, the object being to close the pores of the paper and render it non-absorbent, by which the spreading or running of the ink is effectually prevented. While the finest lines may be written upon a well-sized paper (as ordinary writing paper, for example) without spreading in the least degree, a similar stroke of the pen upon blotting paper, tissue, or unsized printing paper would spread in all directions, owing to the highly absorptive property of the cellulose.
The sizing of the pulp is conducted as follows:—After the loading material has been introduced and well mixed, the resin soap, previously dissolved in water, a little carbonate of soda being sometimes added, is mixed with a paste of starch prepared by dissolving starch in boiling water, and the mixture of soap and starch is then passed through a fine sieve to keep back any particles or lumps that may be present. The proportion of the materials used in sizing vary at the different mills, each manufacturer having formulæ of his own; about 1 part of resin size to 3 of starch paste, and, say, from 9 to 12 lbs. of the mixture, may be used for 300 lbs. of pulp; and, if preferred, the respective ingredients may be put into the engine separately, a method adopted at some mills. Some manufacturers of the finest papers, instead of dissolving the starch in hot water, make it into a thin paste with cold water, in which condition it is introduced into the pulp, the object being to impart to the paper a particular feeling to the touch which is not obtainable by other means.
The mixture of resin size and starch paste, with or without the addition of water, is added to the pulp in the beater, in which the pulp is circulating, and the engine allowed to run until the materials are well incorporated in the pulp. At this stage a solution of alum (about 28 to 30 lbs. for 300 lbs. of pulp), or of sulphate of alumina,[23] is introduced, which causes the resin soap to become "separated," the sulphuric acid of the alum uniting with the alkali of the soap and setting the resin and alumina free in the form of minute particles; the resin in the subsequent drying on the calenders becomes fused, as it were, and thus cements the fibres and alumina together, at the same time rendering them non-absorbent and improved in whiteness by the precipitated alumina. Sometimes ordinary soap is added to the resin soap, which is said to impart a higher finish to the paper in the operation of calendering.
The so-called "concentrated alum," which contains a higher percentage of sulphate of alumina than the crystallised alum, is considered the most economical in use, being proportionately cheaper, and the variety known as "pearl alum" is specially recommended. "Aluminous cake" is another preparation which has found favour in many mills, but since it sometimes contains a large excess of free sulphuric acid it requires to be used with caution, since this acid, although it will brighten the colour of some aniline dyes, will discharge the colour from others, while at the same time it may injuriously affect the brass-wire cloths of the paper machine. The alum solution should be prepared in a lead-lined tank, fitted with a steam pipe for heating the contents when required.
The proportions of the materials used in sizing differ considerably in different mills, but the following may be taken as an average for common writing and printing papers:—
| Per 100 | parts of | dried pulp | 10 to 12 | parts of | resin. |
| " | " | " | 20 " 30 | " | starch. |
| " | " | " | 10 " 12 | " | alum. |
To the sizing solution is generally added from 30 to 50 parts of kaolin. When a colour is present on which alum would have a prejudicial effect this is usually replaced by about one-third of its weight of sulphate of zinc. Many mineral substances have from time to time been added to paper stock, principally to increase its weight, and in 1858 Sholl took out a patent for adding carbonate of lime, a substance which, however, had long been fraudulently used in order to increase the weight, but he found it to have the property of fixing the ink in the pores of the paper, thus rendering it immovable. The only useful addition is kaolin, or some similar aluminous compound, as it attaches itself to the fibre, and, while giving the required opacity and a good surface, takes both printing and writing ink well, and has the advantage, from a manufacturer's point of view, of increasing the weight. It has been proposed that small quantities of glycerine be added to the pulp, in order to give the paper greater flexibility, and especially to give copying-paper the quality of taking up colour readily.[24]
French Method of Preparing Engine Size.—Thirteen pails of water are boiled in a copper-jacketed pan capable of holding about 150 gallons; 90 lbs. of soda crystals are then introduced and allowed to dissolve, when 200 lbs. of finely-powdered resin are gradually introduced, with constant stirring, and the boiling is sustained for about two hours after the last portion of resin has been added. A further addition of water is now made by putting in five pails of cold water, and the water is then boiled for an hour and a half longer. The resin soap is then transferred to stock-chests, in which it is allowed to remain for ten days or longer, fresh batches being prepared in rotation, to meet the requirements of the mill.
To determine whether an excess of resin soap or of alum has been added to the pulp, red and blue litmus papers should be employed, the former turning blue if an excess of resin soap be present, and the latter red when alum or sulphate of alumina is in excess. For uncoloured papers the aluminous material should be added until the pulp becomes faintly acid, which will be indicated by the blue litmus paper turning slightly red when immersed in the pulp.