Kaolin (pure porcelain clay, China clay) is also an excellent clarifying medium. It is a fine white clay, derived from the decomposition of the felspar of granitic rocks, insoluble in water. The potteries and porcelain works are chiefly supplied with this substance. It consists in nearly equal parts of silica and alumina, and is the purest kind of clay. By its weight it also carries down any suspended impurities, but it is not everywhere easily accessible, or cannot be obtained in a pure state, or only for a high price, since pure porcelain clay is not found very frequently. The pure kaolin shares all the preferences of silica and pumice stone, and is also indifferent to the action of fruit-acids therefore applicable to all kinds of syrups.

Alumina (alum-earth) is also a combination of silica and alumina and generally pure.

Pipe clay is a white clay nearly free from iron.

Potters' and brick clay are varieties which contain iron and other minerals, besides alkalies in various proportions, and are therefore unfit for clarifying syrups.

Analysis Of Kaolin

The following process for distinguishing kaolin from ordinary clay (the latter is entirely unfit for our purpose) is recommended by Eisner: agitate the kaolin in a test tube with pure strong sulphuric acid till a uniform mixture is produced, decant the acid after subsidence, dilute it carefully with six volumes of water, and super-satu- rate the cooled solution with ammonia. Kaolin thus treated separates but slowly from the strong acid, and the diluted acid solution gives an immediate white precipitate with ammonia, whereas ordinary clay is but slightly attacked by the acid, separates quickly from it, and the acid after dilution gives but an insignificant precipitate with ammonia.

Kaolin and alum-earth even are frequently found with mechanical admixtures. These can be removed partly by washing with water, partly by a short boiling with a solution of caustic potassa. A common admixture is chalk. If this be present the kaolin or alum-earth is unsuitable for any clarifying purpose, so far as aqueous and acidified liquids are concerned. Chalk is easily detected. Take a sample and pour a few rops of sulphuric or hydrochloric acid on to it; if effervescence takes place, chalk is present.