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.
Gum arabic is the gummy exudation of the gum-yielding species of Acacia, a shrub, or tree indigenous to the East Indies, Arabia, Egypt and Abyssinia. Various sorts and grades of gum arabic are commercial articles, of which only a fine grade should be selected for the purpose of making gum foam. It should always be bought in tears, never powdered, as in the latter state it undergoes frequently adulterations with flour and dextrine.
Prepare as follows: take four pounds white gum arabic in tears, selected quality; pour four pints of boiling water upon it, and stir briskly until dissolved, or heat the water with the gum arabic over a water-bath (never over a free fire, as part of the gum would become charred, making the solution useless as gum foam), and stir until dissolved. Strain through coarse flannel if necessary. One or two pints of simple syrup may be added to this solution for better keeping. Previously we have mentioned the unstable character of this "gum foam," it is therefore necessary to add some preservative. The same means of preservation, and in the same proportion as directed for the "aqueous extract of soap bark," should be applied. The addition of from one ounce to two ounces of this solution to the gallon of syrup will produce the desirable head on the beverage. This kind of gum foam should only be used with draught beverages, and mixed with the dispensing syrup. In bottled beverages its presence is very undesirable and usually causes harm.