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Petroleum-Spirit

When native petroleum and the similar materials obtained in the distillation of bituminous shales, etc., are submitted to fractional distillation, the more volatile portions which come over first constitute the liquids variously known as benzine, gasoline, benzoline, ligroine, petroleum-naphtha, petroleum-ether, and petroleum-spirit. This liquid consists entirely of hydrocarbons, some of which belong to the paraffin series, while others are naphthenes. Their boiling-points are all under 170° C., while some of them boil as low as 50°; indeed, commercial samples of petroleum-spirit often begin to enter into ebullition at a lower temperature even than this. The series of petroleum products may be roughly grouped thus:

Petroleum-spirit boils below 170° C.; specific gravity, .6 to .7.

Lamp-oil, kerosene, photogen, or paraffin-oil, boils between 180° and 220°, and has a specific gravity of 78 to .82.

Solar-oil, lubricating-oil, vaseline, and paraffin-wax, are heavier products, with a range of specific gravity from .83 up to .94. Their viscosity increases with their density until the semi-solid vaseline and the solid paraffin-waxes are reached. The latter substances have been described already, the former are not available in painting: in fact, their presence even in traces in petroleum-spirit - an extremely useful solvent and diluent - should be carefully guarded against. They neither escape by evaporation nor harden in the lapse of time. Thus petroleum-spirit remains alone for further consideration.

As a solvent for resins, and as an extremely volatile and very thin liquid for diluting oily vehicles and paints in the process of oil-painting, the variety of petroleum-spirit which boils between 50° and 70° C. is the most suitable. It contains hydrocarbons represented by the formulæ C6H12 and C6H14. It must be used with great caution on account of its easy inflammability and the readiness with which it gives off a vapour, which, when mingled with atmospheric air, is highly explosive. It may be used for many purposes in lieu of benzene (from coal-tar naphtha), being much cheaper and quite as efficient. A drop of this variety of petroleum - spirit on paper evaporates very quickly, leaving no greasy stain.

Another variety of this petroleum-spirit is obtained by collecting apart the fractions which boil between 100° C. and 130°. These contain heptane (C7H16), octane (C8H18), heptylene (C7H14), and octylene (C8H16), and other hydrocarbons. This mixture is less volatile than that just described, it dries more slowly, and is a less energetic solvent.

A third variety boils between 130° and 170° and is available for many of the purposes for which turpentine-oil is employed. It is not advisable, in my opinion, to use fractions having a higher boiling-point than 170° C. as additions to the pigments and vehicles of oil-painting, for, though their slow drying is sometimes an advantage, there exists the danger of their incomplete evaporation from the painted surface. If they remain even in traces in the finished work after it has been varnished, they may give rise to the same accidents as are caused by the treacherous though seductive asphaltum.

It should be remembered that the various petroleum liquids just described do not resinify, nor do they leave any permanent stain or mark upon paper which has been moistened with them.

In the Table of Solvents on p. 107 three liquids containing oxygen and related to the terpenes find a place. Two of these, cineol or eucalyptol, and geraniol, represented by the empiric formula C10H18O, are alcoholic in constitution; while the third, known as citral, belongs to the aldehydes. These and several other allied oxidized compounds, as well as a few ethereal salts known as esters, enter largely into the composition of certain essential oils occasionally used in oil-painting. We will introduce a few remarks concerning some of these compounds under the names of the essential oils of which they are important components.


Last Update: 2011-01-23