| ash per cent. | |
|---|---|
| Pulp from linen rags | ·25 |
| Pulp from cotton rags | ·10 |
| Pulp from esparto | ·50 |
| Pulp from straw (soda process) | 4·80 |
| Pulp from straw (Leunig process) | 1·36 |
| Pulp from wood (soda process) | ·50 |
| Pulp from wood (bisulphite process) | ·06 |
| Pulp from wood (mechanical) | ·40 |
If the paper contains calcium sulphate the ash obtained may consist partly of calcium sulphide, due to the reducing action of the carbon found on ignition, and the amount will {137} therefore not represent the true amount added. The ash should be moistened with a few drops of sulphuric acid, and again ignited, in order to reconvert it into calcium sulphate. It should also be borne in mind that the sulphate of lime as present in the paper is combined with two atoms of water CaSO4 + 2 H2O, and therefore that every part of calcium sulphate obtained represents 1·26 parts of “pearl-hardening” actually added.
Sizing.
Rosin (colophony) consists of a mixture of pinic and sylvic acids, which, when heated with a solution of carbonate of soda, combine with the alkali to form a soap, carbonic acid being evolved. The soap is prepared in the following manner:—
Ordinary rosin, the quality depending upon the quality of paper, is boiled in a jacketed pan for from two to three hours with a solution of carbonate of soda, until a sample of the soap formed is completely soluble in water. The quantity of carbonate of soda (Na2CO3 10 H2O) required is about 30 per cent. of the weight of the rosin. It is usual to employ crystallised carbonate of soda (soda-crystals), but {138} soda-ash of good quality, or even caustic soda, might very well be employed, in which case of course the proportion would be different.
A very convenient form of carbonate of soda has lately been introduced by Messrs. Gaskell, Deacon & Co., which goes by the name of “Crystal Carbonate.” It is obtained in the form of minute crystals of the mono-hydrate, Na2CO3 + H2O. It contains 50 per cent. of alkali (Na2O), and possesses this advantage over soda-ash, that is dissolves readily in water without forming a hard cake. It is much cheaper in proportion than soda crystals, as the expense of crystallisation and carriage is saved. The latter contain only 21·68 per cent. Na2O, 62·93 per cent. of their weight consisting of water of crystallisation. One hundred parts of soda crystals are equivalent to 43·36 parts of the “Crystal Carbonate.”
An excess of soda should be carefully avoided, as it consumes an equivalent quantity of alum (see below): on the other hand, it is very essential that the rosin should be completely dissolved, otherwise small particles are sure to find their way into the pulp, and would form clear transparent specks in the finished paper.
The boiling being completed, the charge is run off into iron tanks and allowed to settle; the soap forms a semi-solid mass, while a dark-coloured liquor, containing the impurities of the rosin, rises to the surface, and can thus be removed. The soap so purified is next dissolved in hot water containing a small quantity of carbonate of soda, in case complete solubility has not been attained, and is then mixed with a quantity of starch paste prepared in a separate vessel by dissolving starch in boiling water. The mixture is then carefully sieved and is ready for use.
The proportion of starch to rosin differs in nearly every mill, and also the quantity of size to be added to the beater varies according as the paper is required to be soft or hard-sized. About 3 parts of starch to 1 of rosin, and between 3 or 4 lbs. of the mixture to 100 lbs. of pulp may be considered an average quantity. Some manufacturers prefer to add the {139} starch and rosin size to the engine separately; others again do not dissolve the starch in water, but merely make it up to a thin paste, in which state it is added to the pulp. This method is employed by makers of very fine papers, as it is said to give a certain feel to the paper which cannot be obtained in any other way. The method is costly, as only a small proportion of the starch added is actually found in the paper.
Some papers, which are not intended to be sized, such as blotting and filter-papers, are made with the addition of starch only, this being used to bind the fibres together to some extent. The presence of the starch does not prevent the paper from being absorbent. Papers which are intended to be tub-sized only are usually made up with a small quantity of starch, and are sometimes sized to a small extent with rosin also. The addition of a certain amount of ordinary soap to the rosin soap is said to give good results; it enables the paper to take a higher finish when calendered. Sizing with animal size (gelatine), or Tub-sizing as it is called, will be described in Chapter X.