Sulphate of Lime.—This compound is valued chiefly for its brilliancy of colour, being used in high-class papers. It is slightly soluble in water, to the extent of about 23 lbs. in 1,000 gallons, and this fact must be taken into account when the material is added to the pulp in the beating engine.
It occurs naturally in a variety of forms, such as gypsum, alabaster, selenite, the first of which when finely powdered is sold to the paper-maker as gypsum, powdered plaster, and under other fancy names.
It can be prepared artificially by adding sulphuric acid to solutions of calcium salts; and the precipitated product so obtained is sold as terra alba, pearl hardening, satinite, mineral white, etc.
The tests for sulphate of lime in paper ash are based upon the following reactions:—
Calcium sulphate is soluble in dilute hydrochloric acid. The addition of a few drops of barium chloride to the solution produces a dense heavy precipitate, indicating the sulphate. A small quantity of ammonium oxalate solution added to another portion of the dissolved calcium salt previously neutralised with ammonia produces a precipitate and indicates calcium.
A microscopic test of paper for the presence of sulphate of lime is based upon the slight solubility of the salt in water. The paper is boiled with some distilled water. The water is evaporated to a small bulk and transferred to a glass slip, and the gradual formation of characteristic sulphate of lime crystals can be seen by means of the microscope as the water cools down.
French Chalk.—This material is prepared by grinding talc into a fine powder, and possesses a good colour and a somewhat soapy feel. It is a silicate of magnesia, having the approximate composition—
| Silica (SiO2) | 62·00 |
| Magnesia (MgO) | 33·00 |
| Water | 4·30 |
| Traces of oxides, etc. | 0·70 |
| 100·00 | |
Other silicates of magnesia used for paper-making are agalite and asbestine, the latter being a finely ground asbestos.
The composition of asbestos is approximately—