Solvents and Diluents - Introduction

The liquids to which attention is directed in the present chapter are, with very few exceptions, not miscible with water. Of water itself it is not necessary to say anything beyond this, that distilled water is best adapted for almost every purpose to which this liquid is applied in the preparation of pigments, as a solvent for gum, honey, etc., and in the practice of painting in water-colours. Next to distilled water may be ranked rain-water collected in the open country, then the softer kinds of water yielded by some streams, springs, and wells. Waters containing more than 20 or 30 grains per gallon of solid matters in solution should be avoided as far as possible. It should be noted that very hard waters tend to curdle or precipitate the particles of colouring matter in the water-colour paints which they may be used to dilute.

Before considering the chief solvents and diluents, a list of the most important of those which have been obtained in a pure state or isolated may be given. Most of these are artificial or laboratory products, the natural liquids employed in the processes of painting being mixtures, not infrequently both variable and complex. In the following table the several definite compounds included are arranged according to their boiling-points, those which boil at low temperatures being placed first: an asterisk indicates that the liquid is miscible with water:

Table Of Solvents

The pinene, sylvestrene, limonene, and dipentene named in the above table are examples of what are now called terpenes. Mixtures of these and of a few other terpenes of less importance constitute what is generally known as oil or spirit of turpentine. Terpenes are very frequent and often very abundant constituents of the volatile, ethereal, or essential oils extracted from plants. Some, however, of these volatile and strong-smelling essences consist mainly of liquids containing oxygen, such as cineol or eucalyptol, C₁₀H₁₈O, which occurs to a large extent in the oils distilled from many species of eucalyptus. Besides the compounds in our list and a certain number of essential oils from plants, we shall have to consider some of the more volatile liquid constituents of natural petroleum and of artificial paraffin oils. The fixed or fatty oils, which are constantly used in painting and in the manufacture of varnishes, have been already discussed in Chapter V.