Pulp from linen rags ·25 ash per cent. 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 CaSO_{4} + 2 H_{2}O, and therefore that every part of calcium sulphate obtained represents 1·26 parts of “pearl-hardening” actually added. _Sizing._—Papers that are intended to be written upon with ink, and also most printing papers, require to have the interstices of the fibres filled up with some material which will to a greater or less extent resist the action of water, and thus prevent the ink or other liquid applied to them from “running.” This may be accomplished in two ways, viz., by “engine-sizing” and “tub-sizing”; the former is so called because it is performed in the beating engine itself, the latter being done on the finished paper, as will be subsequently described. Engine-sizing consists in precipitating a compound called resinate of alumina in the pulp. This compound when dried has the property of being with difficulty wetted with water. In order to obtain a thorough mixture of this compound with the fibre it is 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. 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 (Na_{2}CO_{3} 10 H_{2}O) 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.