This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
This in powdered form is sometimes recommended as a clarifying agent, but it is also objectionable, being a siliceous magnesian mineral, and we reject it for the same reasons as we did magnesia. A hard species is the so-called "French chalk or soapstone," employed in various industries. Talcum contains iron also, which, when treated with tartaric or citric acids, passes into solution, unless the talcum has previously been likewise freed from it. When this has been done, it may be used in preference to magnesia, and has then the advantage of being nuch cheaper.
To remove the iron from talcum more completely than by simple washing, take 100 parts of it and boil for some time with a mixture of 100 parts of diluted hydrochloric acid, and 400 parts of distilled water. The turbid mixture is allowed to settle, the liquid decanted, and the residue again boiled for a few minutes with a like quantity of hydrochloric acid, of about one-half the strength of the foregoing. After standing and decanting the acid, water is added to the residue and thoroughly shaken up with it, and the decantations repeated several times, until a specimen of the water poured off no longer gives the Prussian blue reaction for iron with potassium ferrocyanide (see page 25). The moist powder is then thrown upon a filter and washed out with distilled water, until the removal of all the hydro-chloric acid is indicated by a failure of the filtrate to produce a cloudiness with silver nitrate (page 35).
Talcum thus prepared forms a better and cheaper material for clarifying purposes than magnesia; however, it remains a siliceous magnesian mineral, and we have still other, almost unobjectionable, means of clarifying, which are pure enough to be employed without previous purification, such as paper pulp, glass-sand, pumice and asbestos.