Oxidation of Turpentine

Besides these observations another will have been made - different specimens of spirit of turpentine will have been found to differ much as to the rate at which these changes have taken place. Some samples, even in half-full bottles, remain clear and limpid for long; others become thick, opaque, and sticky in a few weeks. Such changes are undesirable in a solvent, diluent, or painting medium, on many grounds. The resin formed is an unsatisfactory one - soft, sticky, and contractile. The liquid decreases so greatly in mobility, and increases so greatly in viscosity, that its utility in thinning oil pigments, and in making fine touches, is greatly impaired. And this thickening of the liquid is accompanied by the production of acid substances and of water, which affect injuriously the ease of working and the stability of the picture. Spirit of turpentine should disappear by evaporation quickly and completely from the painting into which it has been introduced. Now, if it be easily oxidizable, even if it be kept from experiencing change before it is actually employed, it will, during the very time in which it is being used, attract oxygen; so that though a great part of it will escape by evaporation, the remainder will resinify on the canvas itself, adding a sticky deposit to the drying oils and hard resins which may have been used as the painting medium.

It is clear, from all the above considerations, that the greatest care ought to be taken in selecting, in the first instance, such a sort of spirit of turpentine as will resist oxidation under ordinary conditions. Even an inferior spirit may be used, with a minimum of disadvantage, if immediately after distillation it be poured into a number of small bottles, so as to fill each of them completely; they should be at once closed with sound corks. In this way the contents of a bottle may be used up very soon after it has been opened. Another precaution may be taken: A few small fragments of hard quicklime may be placed in each bottle to absorb any moisture produced by oxidation, and also the acid bodies which are formed at the same time. Even with the choicer samples of spirit of turpentine, which pass much less easily into resins, this precaution is desirable; but in this case the employment of many small bottles is unnecessary, and it will suffice to put a few hard pieces of lime, free from powder, into a pint or quart bottle, and then to fill it with the spirit.

The clear liquid may be poured off as required for use, any disintegrated particles of lime sinking readily to the bottom of the vessel.