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.

Ether, often called sulphuric ether, is a very mobile liquid of extreme volatility, and possesses a penetrating odour. Its vapour, given off freely at ordinary temperatures, forms with air a highly inflammable and explosive mixture. Great care is therefore required in using this liquid; no light must on any account be brought near it. It does not mix with water, but floats on the surface, although it dissolves in water to the extent of about 10 per cent. Commercial ether contains water and alcohol along with traces of other impurities. It is seldom necessary to remove the alcohol from it (for varnish-making, etc.), but it can be got rid of by repeatedly shaking the crude ether with water, whereby much ether also is dissolved away. The water present interferes seriously with the use of ether as a solvent for resins, etc., but it may be removed by careful rectification with fused calcium chloride, that substance having previously been allowed to remain in contact with the liquid for a day. A final distillation from a little metallic sodium completes the drying of the ether and also removes, if used in sufficient quantity, the alcohol present.

Great care is necessary in distilling ether to secure, by a current of ice-cold water in the condenser, the condensation of the vapour.

Carbon Bisulphide

This heavy, oily but volatile liquid readily gives off vapour at ordinary temperatures. It is poisonous, and the same care in manipulating it must be taken as that insisted upon in the case of ether. The smell of the ordinary commercial bisulphide is most offensive, but it is now possible to purchase a specially purified sort from which a particularly disagreeable sulphur-compound of nauseous odour has been removed. Carbon bisulphide sinks in water: it is a powerful solvent for many resins, and mixes perfectly with the fixed and essential oils in all proportions.

Acetone also occurs in crude wood-naphtha. It has a penetrating but agreeable odour. It is miscible with water, alcohol, oils, etc., and dissolves many resins, camphor, fixed oils, and allied bodies. It is sometimes serviceable as a solvent for discoloured varnishes on pictures. Commercial acetone is very impure, containing wood-spirit, empyreumatic oils, and water.

Chloroform is another powerful solvent of resins. It has a pungent but sweet taste, is not miscible with water, and is very heavy. Commercial chloroform often contains alcohol and other foreign matters, from most of which it may be purified by redistillation from a little oil of vitriol followed by a second distillation from fragments of quicklime. For making varnishes neither water nor alcohol should be present in chloroform, but there are other impurities which do not interfere with its employment for such a purpose.

Wood-spirit, or methyl-alcohol, is a constituent of wood-naphtha, a product of the destructive distillation of wood. It rarely occurs in commerce in a state even approaching to that of purity. It is miscible with water in all proportions, but not with fixed oils. When free from water it may be used as a solvent for some resins, and for removing discoloured varnish from oil-paintings. Methylated spirit now contains in 100 measures 9 1/2 measures of crude wood-spirit and 1/2 measure of petroleum oil, the remainder being rectified spirit of wine.

Alcohol, or pure spirit of wine, is met with in commerce practically free from all impurities save water. Proof spirit, rectified spirits of wine, and methylated spirit, though of service in cleaning oil-pictures and for many other purposes, ought not to be used in the preparation of varnishes. For this purpose pure alcohol, often called absolute alcohol, is required; but provided that it contains no water the presence of wood-spirit is no drawback to its use. in commerce, nearly absolute alcohol, made both from spirits of wine and from methylated spirit, is obtainable; but it may be prepared by operating upon the strongest available spirits of wine in the following manner: The spirit is distilled in a water-bath until no further strengthening of the alcoholic distillate is secured by repetition of the process; then a dry retort is half-filled with small, clean, hard fragments of quicklime, the strong spirit is poured upon these so as to somewhat more than cover them, and then the whole is left overnight; distillation from a water-bath is then commenced, when it will be found that a spirit comes over which contains no more than one part of water in two hundred. Eeven this small proportion may be removed by redistilling the alcohol from a very little metallic sodium.