Oils

Oil of Spike Lavender. Oil of Spike Lavender is obtained by distillation from the flowers of a species of lavender, Lavandula spica. Its specific gravity varies from .905 to .918: it dissolves in three volumes of 70 percent alcohol. It contains about 30 to 40 percent of an alcohol, linalol (C10H18O), about 5 percent of an ester, linalyl acetate, a considerable quantity of cineol (eucalyptol), and a small proportion of terpenes. It is often adulterated with rosemary oil. When spike oil is exposed for a long time to the air, the part which does not evaporate gradually thickens, owing to the oxidation of its constituents. This thickened oil, although usefully employed in the application to porcelain of enamel colours, is useless in oil and spirit-fresco painting. To preserve spike oil in its thin and mobile state, the precautions recommended in the case of turpentine (p. 114) should be taken. Spike oil is a powerful solvent of resins; it is one of the components of Gambier-Parry's spirit-fresco medium.

Oils of Eucalyptus. Oils of Eucalyptus are derived from many species of eucalyptus, and differ from one another widely as to the relative proportions in which their components exist, and also in their odour. Cineol, or eucalyptol, is the chief and most characteristic constituent of the majority of them, occurring to the extent of 50 to 65 percent in the oil from E. globulus, the commonest of all kinds. On the other hand, the oil from E. maculata, var. citriodora, sometimes contains no less than 95 percent of citronellal, an alcohol. This oil is of particularly agreeable odour, as are also the oils of E. cneorifolia, E. corymbosa, and E. dealbata. For a diluent in oil-painting it is probable that any one of these oils may be chosen with equal propriety, our choice being guided by the odour of the sample.

Oil Of Lemon. Though the terpenes known as dextro-and lævo-limonene constitute the main portion of this oil, yet its characteristic odour is partly due to the presence of 6 to 9 percent of the aldehyde citral mentioned above. This compound is occasionally separated from lemon-oil; by its removal the remainder becomes richer in terpenes and more fitted for the use of the painter.

Oil Of Orange. This essential oil, which, like that of lemon, is obtained by expressing the rind of the fruit, is remarkably rich in limonene (dextro-limonene). This, with small quantities of other terpenes, makes up over 95 percent of the oil. Although the expressed oil possesses a yellow colour, it may be obtained absolutely free from colour and of agreeable odour, by distillation under reduced pressure.

Oil Of Rosemary. This oil varies in specific gravity between .9 and .918. It contains about 20 percent of an ester, bornyl acetate (C12H20O21), and 6 percent of borneol. Its other constituents include lineol and two terpenes.

Cymene. There are twenty or more varieties of the hydrocarbon expressed by the empirical formula C10H14. The best known of these is the agreeable lemon-scented liquid contained in cumin oil from the seeds of Cuminum cyminum. Its full scientific name is paramethylisopropyl-benzene, while its constitution is shown by the formula CH3.C6H4.CH(CH3)2. This hydrocarbon serves the same purpose as a diluent as the terpenes, but is too costly for general use. Doubtless some of the other forms of the C10H14 hydrocarbon might be similarly employed. The range of their boiling-points lies between 168° and 204° C. Some are solid at ordinary temperatures.