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 is done by mixing calcined or carbonate of magnesia with part of the liquid or essence to be clarified in a mortar, rubbing with the pestle until it becomes a paste or jelly, and then adding and mixing gradually the balance of the fluid; or add some magnesia to the liquid in bottle, shake the whole contents together and allow time to subside. In both cases filter through filtering paper or filtering bags. The magnesia adheres to the resinous and turbid matter of the fluid and thus clarifies it.
Some oils, such as lemon, orange, or others that are with difficulty soluble, are combined directly with magnesia in a mortar and mixed to a paste; then the alcohol is gradually added to effect their solution, and finally the mixture is filtered through filtering paper. This is called the "cutting of essential oils".
The use of either calcined or carbonate of magnesia refers only to strictly alcoholic solutions or liquids. For clarifying aqueous or aqueous-alcoholic liquids (such as for instance "water-soluble extracts or essences") it should never be employed, as both the calcined as well as the carbonated magnesia impart a distinct alkaline reaction to the aqueous portion of the liquid, which will produce noticeable effects upon the delicate flavors.
We are aware that the majority of carbonators employ principally magnesia for all sorts of clarifying purposes, and especially for cutting essential oils in order to prepare water-soluble essences, and we urge them to discard with its use, substituting the materials we shall recommend for the special purpose of preparing water-soluble extracts and essences later on.