232 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS This aim is never reached, success coming only in degrees. Blending, as described above, occurs immediately the ingredients are homogeneously mixed. As the molecules leave the surface of the blend, so they become separated and blending effects diminish. Conditions of dispensation greatly affect the odour of the perfume as the nose receives it, concentration, surface and humidity all playing an important part. Ageing As is well known, the odour of a perfume changes with age. Regrettably, the changes are not always for the best, but this is due to faulty formulation. Blending is due to physical effects and occurs immediately the mixture is completed: ageing is the result of chemical changes and proceeds to equili- brium with time. With mixtures of alcohols, acids, esters, aldehydes, acetals, ketones, ethers, phenols, amines and terpenes, chemical action and inter- action will take place with resultant changes in odour. Ester formation, ester exchange, acetal or hemi-acetal formations, Schiff base formation, aldol condensation and a host of other reactions can be anticipated. In addition, chemical breakdown and oxidation must be considered. l•'ixation Fixation of a perfume is achieved by the selection of high-boiling sub- stances which have powers of attraction for the more volatile ingredients. The very blending of ingredients is in fact a form of fixation. To return to the main subject, namely, pressurized packs, let us consider typical spray perfumes to be packed in glass containers, adequately protected by a plastic coating. The type of spray employed will depend upon the use to which the product will eventually be put. Three important types are: Wet spray for the genuine Eau de Cologne. Semi-wet spray for the sophisticated toilet water. Dry spray for the true handkerchief perfume. The above spray characteristics are determined by the composition of the propellant, by the ratio of alcohol to propellant, by the water content and, finally, by the type of valve employed. A typical formula for a perfume spray is: Alcohol 90 per cent 60 Dichlorotetrafluoroethane 35 Dichlorodifluoromethane 5 Straight colognes may have between 3 and 5 per cent perfume, sophisti- cated colognes 4 to 8 per cent, and handkerchief perfumes between 8 and 15 per cent. This refers to the alcohohc solution, propellant absent. It xvill be noticed that these suggested amounts of perfume are higher than the

PERFUMES IN PRESSURIZED PRODUCTS 233 recommendations made in the literature. This is not sales talk, but common sense. One of the most common mistakes made in pressurized perfumery is to consider handkerchief perfumes and toilet waters as space sprays. They can, of course, be sprayed into the atmosphere, but normally they are applied to surfaces such as skin or fabric. Even apart from considerations of price, it is often an advantage to include water in an alcoholic perfume, particularly where colognes are concerned. In the pressurized product it is difficult to include as much as 15 per cent of water. With higher amounts, problems of miscibility with the propellants arise. It has been stated many times that a given amount of perfume in a pressurized system will produce odour effects many times stronger than those given by the same amount of perfume in a non-pressurized system. This increase in strength of odour only applies to a solution examined in the form of a fine spray, which is not the way we normally use our perfume. It is more realistic to retain the normal amount of perfume relative to the alcohol and to ignore the propellant as a constituent. After all, if we use 2 per cent of perfume, 31 per cent alcohol and 67 per cent propellant, and spray this on to a surface, the propellant is almost immediately lost and we are left with a 6 per cent solution of perfume in alcohol. Another misleading statement which occurs in the literature is the suggestion that drastic differences in odour are to be found between pressure packed perfume and conventional packs. Take an alcoholic solution of a perfume and divide it into two portions. Place one in a normal bottle and a second in a pressur- ized bottle together with propellant. If these two solutions are applied to at surface, in less than no time the propellant is lost from the pressurized type and one is left with two identical solutions which, if smelled, give the same odour impression. It is only when the pressurized solution is sprayed into the atmosphere and smelled that the odour effects are different. It is therefore necessary to appreciate the difference between a space spray and a surface spray, and many writers have assumed that handkerchief perfumes and toilet waters are sprayed into the atmosphere, and not on to surfaces. if the perfume is in fact space sprayed, then drastic odour effects caa indeed be expected. When sprayed, loss of propellant can be presumed and small particles of alcohol and perfume are left which strike the recep- tors of the nose partly in 'vapour form, but also in liquid or condensed form. Not unnaturally, the odour effects differ from those appreciated when smelling the perfume in vapour form from a surface and, of course, the possibilities of irritation are considerable. There is no doubt that a perfume giving a well-blended effect whea smelled from a surface may give very different odour impressions in the spray form. Many perfumes make use of one particular ingredient around which the general bouquet is built. Properly formulated, this one