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.
If all the ingredients of a compound syrup, such as extracts, essences, tinctures, colors, etc., are prepared as directed in this work, they will be perfectly miscible with aqueous liquids, and no clarification or filtration of the compound syrup whatever will be required. If commercial preparations are employed, buy none that are not entirely soluble, or make them so by the manipulations as directed in a former Chapter. If essential oils are not thoroughly cut, the essences, when intermingled with the syrup, will, after a short time, shed from itself particles of oil, that by close observation will be found swimming on the surface the next day. Extracts, if not water soluble, will separate extractive matter in the syrup, and cause turbidity. If syrups are not given a rest of at least one day, that this oily substance receives an opportunity to come to the surface, or the separated extractive matter has precipitated; and the syrup has been decanted or filtered and clarified, it will of course be bottled with the water, and, as is commonly known, will turn it blind, milky, and even cause it to precipitate after a while. Numerous bottlers are daily breaking their heads why it is that one day they obtain a clear and bright soda, while on another they cannot. By giving the above special attention, much time, labor, and loss of material may be saved.
Test the condition of syrup in regard to its turning out a bright beverage, as directed under "General Directions," by pouring an ounce into a bottle, charging and comparing the beverage against the light. Where it can be avoided, syrup should never be filtered or exposed in any way after it has once been mixed with any volatile flavoring matter, as a considerable percentage of flavoring matter is lost by this fallacious process. Should, however, on account of the impure or improper materials employed, a clarification and filtration be unavoidable, proceed as follows: Mix with the syrup some glass sand, powdered artificial pumice stone, powdered asbestos or paper pulp, the same as directed for plair syrups (see page 619 and following). These clarifying mediums are indifferent to the acids mixed with the syrup, and are therefore adapted for all kinds, acidified or not. Never use carbonate or calcined magnesia for clarifying plain or compound syrups (page 620); it is entirely unfit for acidified syrups. Filter through a felt or flannel bag, enclosed in a well-enameled, silver or tin-lined metal case (no wooden case should be used, as wood absorbs flavor and syrup) to protect the syrup during and after the process of filtration, and prevent the evaporation of the aqueous portions and volatile flavors. Cold syrups filter slowly, and they should be kept in a dark, cool place, where they are not likely to be disturbed. For each kind of compound syrup a separate filter is required, Figs 373, 374, 375, 425 and 426 representing practical arrangements for protected filtration, but if after each kind has been filtered, the filter is scalded, and separate bags used, one arrangement will answer for all kinds of syrup, but it is advisable to keep such quantity of filtered syrup as will be necessary to meet the requirements of manufacturing.