Popular Science Monthly/Volume 52/November 1897/Natural and Artificial Perfumes
|NATURAL AND ARTIFICIAL PERFUMES.|
WE may divide the makers of perfumes into the two classes of those who furnish the raw materials for perfumery and the manufacturers proper. The former provide the essences, the pure or concentrated scents, and the latter mix and extend them, incorporate them into various liquids or pastes, and offer them for consumption. Another division, of recent occasion, may be made between those who extract natural perfumes according to the methods pursued for centuries and those who use chemical processes or make artificial perfumes; for, as dyestuffs are now composed from coal-tar products to the extent that some of the natural materials have been driven out by artificial substitutes, so a number of perfumes have been in like manner synthetized. But there is no probability that the synthesis of perfumes will ever be carried to the extent of which that of dyes seems capable, because, first, some essences are the raw material for the synthesis of artificial products of much greater value; and, secondly, the chemical problem of the synthesis of perfumes is more complicated than that which enters into the artificial production of coloring matters.
The preparation of natural essences is still a genuine agricultural industry. Flowers and leaves are the raw material, and they have to be treated fresh. The original laboratories are therefore generally established very near where the plants can enjoy the most favorable climatic conditions. Hence the crude essences generally come to us from various distant regions—essence of ilang from Manilla, of geranium from Réunion and Algeria, of lemon and citron from Ceylon and China, etc. But as the imported materials are generally scandalously adulterated, European manufacturers have been impelled to bring home such of the crude material as will bear transportation. So sandalwood, cloves, patchouli leaves, and vetivert-grass roots brought dried and with their scents unimpaired are distilled in France and Germany rather than in the countries of their origin.
The most important center of this manufacture is the little city of Grasse, near Nice and Cannes, which, besides being a large center of production for the distillation of plants and woods, is the chief place where these special processes which have been transmitted through ages and are the only ones for the extraction of the perfumes of flowers are in use. The only chemical agents employed in these processes are vapor and fat.
Theof artificial perfumes, on the other hand, are real laboratories of chemical products where the habitual agents of chemical industry are employed, requiring the intervention of chemists and engineers, and are established by preference at the great industrial centers. Hence good reasons exist for these two branches having been kept apart, although it is not certain that this separation will continue permanent.
The simplest process of extraction is by distillation. The flowers or leaves are put into the retorts with water and heated to the proper degree. The perfume passes over to the cooling apparatus with the vapor of the water and is condensed with it, after which it is separated from the water by taking advantage of the difference of density. The heat is applied by means of vapor under pressure. Formerly fire was applied directly, but the amount of production was insignificant compared with what it is now; this method is, however, still in use in small portable apparatus. Some distillations are literally performed on the spot, as those of certain aromatic plants which are not grown very near Grasse, the finer lavenders especially being found wild at considerable heights on the mountains. The communal lands up there are allotted every year, and extractors who make this a specialty establish themselves in their plots with their direct fire apparatus, expecting to dispose of their production to the large houses.
When the quantity produced is regarded, distillation is the most important branch of the perfumery industry. It is simple, inexpensive, requires but little manual labor, and is applicable to large quantities of material. But there are objections to it, and some of them are of so much force as to have led to the substitution for it of seemingly more primitive, and at all events more expensive, methods of extraction. The first objection is the liability of vapor, coming in contact with some of their more unstable constituents, to change the character of some of the essences. A still more important objection is that the vapor may bring over some undesirable constituents existing in the plant, or by its action on some of the constituents may produce new substances in no way allied to the perfume. Impurities produced in this way communicate to the newly distilled essence what may be called the taste of the retort. Such inconveniences, of relatively little account in the coarse extracts, are very important in the more delicate distillations.
A process used at Grasse for conserving the more delicate qualities of the perfumes, and which constitutes a method original to the Maritime Alps, consists in the employment of grease for the composition of perfumed pomades. A grease of suitable consistence which will not become rancid is obtained by preparing a mixture of lard and beef fat and subjecting it to thorough refining processes. The perfume is incorporated with this base by a process of warm maceration, or by absorption.
For maceration the fat is melted in the marine bath and brought to a temperature of 60° or 70° C, as measured by the skill of the attendants rather than by the thermometer; the rose leaves, or whatever is to be treated, are introduced, thoroughly immersed; after a few hours the mixture is passed through strainers and drained, so as to separate the grease from the flowers as completely as possible. What grease remains on the rose leaves is washed off with warm water. The process is repeated with the same grease and with other flowers, and so goes on for fifteen or twenty days, according to the quality of the pomade that is desired. Some of this pomade is consumed as it stands; but by far the greater part is used for making extracts, or alcoholic infusions of the perfume. For this purpose the pomade is introduced with a suitable quantity of alcohol into a shaker, consisting of a cylinder in which a screw beater is kept going all the time. The mixture is thus actively beaten up, and, as the perfume has more affinity for the alcohol than for the grease, the grease in time becomes nearly inodorous, and the alcohol is charged with the greater part of the perfume. The grease is then sent to the soap maker, for it is not fit to be used again for a pomade.
The question may be asked, and has been asked, Why use the intervention of grease in the process of extraction? Why not apply alcohol or some substance of similar powers directly? Chemists have offered ether, sulphide of carbon, chloride of methyl, chloroform, and other scientifically prepared solvents, to have them rejected, and have blamed the manufacturers of Grasse for adhering to barbarous processes, and for their inability to escape their antiquated routine. A more scientific spirit would lead them to inquire if there were not some reason for preferring grease. Most of the substitutes recommended have more liberal affinities than grease. Alcohol, for instance, unites readily with water, and with water takes substances held in solution by it, which are extremely undesirable in a perfume; and so with the other substances, according to their several properties. Grease has a narrow elective affinity, and takes just what is wanted and nothing else. Various petroleum substances have been proposed recently as extractors, and much may be said prima facie in favor of them; but experiments with them have not yet given satisfactory results.
In the process of absorption or enfleurage wooden frames furnished with glass bottoms are used. These, fitting closely together, are placed one upon another, so that a small inclosed space is left between every two of them, or a sort of greenhouse with glass above and below, inclosed with wood. The surfaces of the glasses are coated with grease, on which a layer of flowers is placed. The flowers are left there to shed their perfume, which is absorbed by the grease, for twenty-four hours, when they are removed and new flowers are put in their places. This is continued for two or three months, at the end of which time the pomade is ready for treatment as in the maceration process. The flowers to which this process is best adapted are the jasmine, tuberose, and mignonette. This enfleurage process has likewise drawn criticism, if not contempt, from men of science; and the question has been asked why a different treatment is given to these from that pursued with other flowers.
If we observe flowers with regard to their odors, we shall find that they may be divided into two categories: those which contain their perfume already formed, or have at least a considerable reserve of it, and those which have no reserve, but develop and emit their fragrance as they grow. When we rub roses or orange blossoms, for example—flowers of the former class—we perceive their fragrance very plainly. Such flowers may be treated by distillation, by the warm pomade process, or by extraction with volatile solvents, with results of a greater or less degree of perfection, but always positive; while if we rub a flower of jasmine, convallaria, or violet—which are of the second class—we perceive nothing but a slight pungent odor in the same plant which an instant before was exhaling a pervading fragrance. In crushing it we have killed it, and it produces no more perfume. Such flowers are not suitable for distillation or maceration or the action of volatile solvents, because there is nothing to be extracted from them.
The production and exhalation of odor go on while the plant continues to live and vegetate, even after it has been cut. When, therefore, it is placed in the limited atmosphere of these absorbing cases, the perfume continues to be given out, and is absorbed by the grease. Contemplating the matter from this point of view, we are astonished at the sagacity of the perfume extractors of past ages, which enabled them to perceive that some flowers could be treated only by this absorption process, and to co-ordinate their operations so logically that the method could not have been better adapted to its purpose if it had been adjusted after a careful study of the physiological principles involved. Everything in the process seems adapted to the prolongation of the life of the flower. The close and consequently moist atmosphere in which it is placed preserves it against dying; the coolness of the apparatus prevents its wilting. The flowers of the tuberose are picked just as the bud is about to open. It blooms within the case, and gives out nearly all its perfume there.
We draw from these observations the two lessons that the extraction of natural perfumes is not, as is often believed, a question of pure chemistry, but is primarily one of vegetable physiology; and that in this domain, as in many others, practice is often in advance of theory. For my own part, I have never met with well-established processes sanctioned by long use which do not rest upon correct though frequently unconscious observations over which theory has nothing to boast.
As when people wanted to travel faster they devised the railroad instead of improving the stagecoach, so the achievements of chemistry in the domain of perfumes have not been made in building up the old industry, but through creating another. To enumerate and describe the artificial products now used in perfumery would be like composing a manual of chemistry. It will be of more interest to indicate a few categories of syntheses or chemical fabrications, and to point out the various chemical or financial questions they raise, illustrating them severally by typical examples described in detail.
There are several kinds of synthesis. One kind consists in isolating a natural principle, studying it, and trying to reproduce it. Such a synthesis may be called a methodical one, as following out a line traced from the beginning. Take, for example, the synthesis of ionone, or artificial violet. The authors of this synthesis, MM. Tiemann and Krüger, started with the natural perfume of iris root, which they found to be very characteristic and fixed. It was consequently considered to be a single very definite and stable substance. All these conditions were favorable. The preliminary researches of the authors showed that this substance existed in extremely weak proportions in iris powder. The proposed synthesis, therefore, promised to be lucrative; for, while all chemical syntheses are interesting, the probable financial bearing of the discovery is of considerable importance; and it is not the absolute cost but the probable profit that most deserves consideration. A primary characteristic of perfumes is their immense strength; a very minute quantity of one is sufficient to produce a large effect. Consequently, they may bear a large price without limiting consumption. But, to isolate this as yet unknown perfume, it was necessary to treat enormous quantities of iris powder, and this required industrial resources which the inventors could not command. These were secured by making suitable arrangements with two large houses which became parties in interest. The experiments lasted ten years. First, irone, the principle to which the odor of the violet is due, was isolated. A complete chemical study of it was made. Having remarked that citral, an aldehyde abundantly diffused in Nature, gave, on condensation with acetone, an acetone of the same crude formula as irone, these authors effected this condensation under the influence of hydrate of baryta. They thus obtained, as they had expected, an acetone of the same crude formula as irone—false irone; this, it is true, had neither the odor nor the properties of irone, but by the action of dilute sulphuric acid it was transformed into its cyclic isomer, ionone. From the chemical point of view, ionone not being identical with irone, but only an isomer, the problem could not be said to be solved; but it was fully solved from the industrial point of view. Ionone possesses, like irone, the perfume of the violet, with a slight shade perceptible to practiced noses, but lending itself admirably to all the uses of perfumery. We have in this a complete example of methodical synthesis, although it must be admitted that the authors were aided by happy combinations of circumstances that might not always occur.
There are also chance syntheses. Thus, a substance, the odor of which may be utilized, is sometimes fallen upon in pursuing researches undertaken for another purpose. This is what happened, for example, in the case of Baur's artificial musk. M. Baur had undertaken the study of two carbides of hydrogen—two butyltoluenes contained in the essence of resin. He isolated and separated these substances, and then in experiments connected with the performance of the synthesis he perceived an extremely pronounced odor of musk appertaining to the trinitrile derivative of isobutyltoluene. M. Baur was not led to his researches by chance, but the discovery of his musk, the most precious result of them, was not anticipated by him.
There are other cases in which a series of bodies are prepared with full expectation of what the chemical results will be, but without knowing what odor they will have, or whether they will be odoriferous, but with the expectation that they will be, and that some may be found among them the odors of which can be utilized.
Syntheses of this kind only rarely lead to natural principles; more frequently what perfumers call chemical products are obtained, or perfumes which betray their origin to a greater or less extent, and can not be used in the preparation of the finest products, but have cheapness and great strength in their favor. We should observe that chemical synthesis takes to the odorous principle itself, while essences contain only a very slight proportion of the active substance.
Finally, it sometimes happens that no new material is discovered, but some laboratory reaction already known is turned to industrial use. Such is the case with heliotropine, formerly known as piperonylic aldehyde or piperonal; terpineol, or white lilac; anisic aldehyde, or hawthorn, etc.
Twenty-five or thirty years ago it was believed that with a few well-known exceptions, such as those of bitter almonds, anise, mustard, and some others, the essences were constituted of hydrocarbons, C10H16 in indefinite numbers, all isomeric and similar to spirits of turpentine. Our views on this subject have been considerably modified. It has been found that the hydrocarbons or terpenes contained in essences may be referred to well-defined species possessing characteristic reactions and derivatives, some of them crystallizable, by which they may be distinguished. Oxidized principles have been isolated in essences related to the fatty series, capable of facile transformation into cyclic derivatives, which may be regarded as connecting links between the fatty and the aromatic series. Frequently a natural essence represents a harmonious mixture of various combinations. Oil of bergamot, having a composition of this character, possesses an odor vastly more characteristic than any of its components taken separately. And it sometimes happens that compounds suitable for fine perfumery, if they were pure, are spoiled by the presence of disagreeably smelling substances. The elimination of such principles, or the refining of such perfumes, has given rise to a second branch of our chemical industry.
The discoveries that have so far been made are quite insufficient to explain the composition and odor of essences. We find certain substances, like linalool and geraniol, common constituents in essences of the most different characters, and are hence forced to recognize that in a great number of cases they are only the vehicle, the substratum, of the really characteristic perfume; and we begin to suspect the presence of still rarer principles corresponding probably with a more differentiated, more specialized organism, and related to the specific characteristics of the vegetable cell.—Translated for the Popular Science Monthly from the Revue Scientiftque.