The sizing materials used, above all others, are rosin alum and gelatine-alum. Papers of the filter paper or blotting paper type depend for their characteristic quality on an absence of sizing, though a small quantity of starch, as binder added to the beater, is sometimes used in making them. A great many other papers, certainly all writing and printing papers, are sized. Rosin alum sizing is always added to the beater, i.e., before the pulp goes on to the wire of the paper making machine, and hence it is called "engine sizing": gelatine-alum sizing is always employed for hand made papers, and these papers are dipped in a tub (originally by hand, now more frequently by machine) containing the sizing solution and hence gelatine sizing, sometimes called "animal sizing" is also commonly known as "tub sizing." The method is also employed for sizing machine-made papers, which is sometimes achieved by intruding a sizing tub between two of the stacks of drying cylinders ; more frequently by employing a separate unit. Many attempts have been made to gelatine size paper by adding the size direct to the pulp in the beater, as is done with rosin sizing. This would be the most convenient method, if it were feasible; but as yet no successful method has been evolved capable of giving favourable results proportionate to cost.
Rosin, the residue from turpentine distillation, is an acid anhydride or mixture of anhydrides, insoluble in water. It is rapidly dissolved by heating with alkaline solutions, e.g., caustic soda or soda ash, to give sodium salts which are the so called rosin soaps, so frequently used as ingredients of the yellow soaps of commerce. Ordinarily, neutral rosin soaps are thus obtained containing ioo of rosin to 17 of soda: but, by prolonged heating and stirring, a further large extra quantity of rosin may be brought into solution or suspension to give very "acid" sizes, con taining as much as 35 per cent of combined rosin, 24 per cent of free rosin and 3.5 per cent of soda. They give milky emulsions, "size milk," containing suspended globules or droplets of precipi tated rosin, on dilution with water. Much care is therefore re quired in diluting these acid sizes, for addition to the beater, because there is always the danger present of these precipitated droplets agglomerating together, to form particles of such dimen sions as would cause resin specks in the finished paper. The danger is diminished by adding starch to the diluting water. Still more recently, acid sizes containing as much as 45% of free rosin have come into use, and their use requires still more care and skill. They are used with atomiser injectors by means of which the strong size is mixed first with a small proportion of hot water and then with a larger quantity of cold water. The manufacture of these sizes is now a separate industry and the paper maker can purchase concentrated acid sizes, ready made for dilution.
The quantity of acid size usually required is that representing two to three pounds of original rosin per i oo lb. paper : but it depends greatly upon the nature of the fibre and the degree of beating. The sizing operation consists, firstly, in adding the
diluted size to the beater after passage through a fine screen or coarse cloth filter bag, and it is usually added when the pulp is one-half to two-thirds beaten: secondly, a solution of alum, or, more commonly, aluminium sulphate, is added to the beater which fixes the rosin in the pulp, by double decomposition between the rosin soap and the aluminium salt, with formation, perhaps, of aluminium rosinate, but more likely of a coagulated colloid absorp tion complex of rosin-alumina. The optimum quantity of alum is determined by experiente and is a good deal higher than the equivalent quantity calculated from the chemical equation. To economize alum it is sometimes desirable to add a proportion of dilute sulphuric acid to the beater with the object of neutralizing the basic ingredients present : e.g., esparto and soda pulps are fre quently alkaline in reaction. Experience has shown that the best sizing results obtained with rosin are those given by a finished paper slightly acid to litmus.
Gelatine size is prepared from paper makers' gelatine of good intermediate grade, i.e., of not too high viscosity and proportionately lesser penetrating power, and alum. The gelatine is usually purchased by the paper maker in thin sheets which swell and disperse easily in water and give a clear pale solution, after filtration; the alum solution is then added to it and the viscosity increases up to a point and thereafter diminishes on addition of more alum solution : it is then ready for use as sizing. The proportion of alum is io to 5% of the weight of the gelatine : the alum prevents the bacterial decomposition of the gelatine, but its principal purpose is to make the gelatine much more resistant to ink, and the paper therefore less penetrable. A certain proportion of soap is sometimes added, for hand-made papers, which renders the paper capable of taking a high finish by calendering and supercalendering. The soap should be firm and very white, and therefore free from rosin: in presence of the alum it forms insoluble aluminium soap. An acetylated starch named "feculose," made in several grades varying in penetrating power, has been employed as a substitute for gelatine, in tub sizing: it is used without alum and its chief sizing characteristic is that its effect is pronounced on the surface and with little pene tration: it is, therefore, economical in use. The use of casein for sizing is limited, because of its relatively high cost : it imparts a certain degree of toughness and a good "handle" to the finished paper. Many attempts have also been made to use viscose as a sizing, which by reversion to cellulose would realize the ideal of sizing cellulose with cellulose : but its technique is difficult, and the action of the sulphur compounds on the metal of the machinery, with the parallel action on minute specks of metal in the paper itself, derivable from beating, is an adverse factor.