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.
A clarifying powder or compound is employed for separating and precipitating the resinous matter of essential oils or essences, etc. The best known remedies for these purposes are, calcined alum and magnesia, also to some extent powdered chalk.
Alum and magnesia appear frequently combined as " Double Clarifying Powder or Compound". Finely powdered calcined alum has an extensive application for clarifying purposes, especially in clarifying liquors, besides isinglass and gelatine.
Calcined alum is prepared by melting the commercial alum; it loses thereby its water of crystallization, expands, and then crystallizes as a white, light and porous mass, which is called calcined alum, and is but slowly and with difficulty soluble in water. By this calcination process the alum loses more or less of the combined sulphuric acid, as the higher the temperature the greater will be the loss; the consequence is a, greatly diminished solubility, and this makes it particularly adapted for mechanical clarification of alcoholic liquids and essential oils.
For the clarification of aqueous or aqueous-alcoholic solutions it cannot be used, as it would impair the flavor and taste of the liquids to be clarified, imparting an alkaline reaction.
Commercial alum is never employed for the purpose in view here, only for the purification of water, as described in Part First.