The success and reputation of Cologne water are the natural results of a good product prepared with a thorough knowledge of the subject. However, in order to be enabled to impart to Cologne water its generally known excellent qualities, it is necessary to have a thorough knowledge of the materials used, as well as to understand the method of fabrication, and the proper treatment of all.
For the preparation of actually good Cologne water employment of the best materials is the first condition. The alcohol must be pure, _i. e._, free from fusel oil, and 95 to 96 per cent. strong, so as to effect a ready and complete solution of the volatile oils. The latter also should be of the best quality and proper age, _i. e._, neither too young nor too old. If too young or too recently distilled, the aroma is not thoroughly developed, and if, on the other hand, too old, they have lost the greater portion of their aroma, are thickly fluid, acquire a dark coloration, and are finally converted into a resinous substance in which condition they are entirely unfit for finer products. A cool and dry cellar is required for storing volatile oils, and they must also be protected from air and sunlight.
Regarding the durability of the volatile oils used in the preparation of Cologne water, the following may be said:--
_Bergamot oil_ is quite durable; in well-closed bottles stored in a cool cellar it keeps quite well for 3 to 4 years. _Oil of lemons_ is, however, very sensitive, and has to be especially protected against heat, by which it is readily decomposed and, so to say, turns sour. Freshly-distilled _neroli oil_ should never be used, it acquiring its proper aroma only after being stored for about 2 years; on the other hand, it should not be too old. When 6 to 8 years old, it shows a tendency to rancidity, which can, however, be prevented by mixing it at the proper time with an equal quantity of alcohol of the best quality. _Petit-grain oil_ is apt to spoil when kept for several years, and hence it is not advisable to have too large a supply of it on hand. Besides the French variety of this volatile oil, there has existed for several years past another kind, namely the _Paraguay petit-grain oil_, which is about one-half cheaper than the French. While some consider both oils as equally good, the French product no doubt deserves the preference. Of _lavender oil_ only the best French oil, especially made in France for the preparation of _Eau de Cologne_, should be used. The price of the English Mitcham lavender oil, being six times that of the French oil, is simply excessive. Of _rosemary oil_, an especially fine quality for Cologne water, which can be highly recommended, is also manufactured in France. Of _balm oil_ only the best German product should be taken. The three varieties of volatile oils last mentioned keep for years, especially when kept in hermetically-closed bottles in a cool place protected from the sun.