The terms 'siccatives' and 'dryers' are applied to three classes of substances. Perhaps the most correct or appropriate application of these words is to those metallic compounds which are used in order to increase the rate at which the drying oils harden, but in the literature of the subject we often find that drying oils which have been thus treated, and likewise certain resinous solutions, are spoken of as siccatives. In the present chapter we describe the dryers proper only, referring our readers to the chapters on oils and on varnishes for the necessary particulars concerning the other materials which may be included in the group under discussion.

Lead and several of its salts have been long and widely used as dryers. Metallic lead in the form of foil, litharge or lead protoxide, minium or red lead, lead peroxide, sugar of lead or lead acetate, the basic lead acetate, and white lead itself, have all been used in this way, chiefly for the purpose of making linseed or other painting oil dry more quickly. Some of these compounds, particularly sugar of lead, have been introduced into the very picture itself. It was a common practice to employ powdered sugar of lead or a solution of this salt in water to hasten the drying of vehicles and of slow-drying pigments which have been ground in oil. I have seen one of the results of this commingling of sugar of lead with the medium or the paint in the production of an immense number of small spots in the picture, sometimes appearing through the surface-varnish in the form of a white efflorescence. This efflorescence consists at first of lead acetate in crystals, but these soon attract carbonic acid from the air and become lead carbonate, which, in its turn, is changed into lead sulphide by the action of sulphuretted hydrogen. This tendency of the lead compounds to yield brown or black lead sulphide is, indeed, the great drawback to any use of these substances as dryers.

When oil is left in contact with them, and especially when heat is applied to the mixture, some of the lead dissolves, forming, with the fatty acids of the oil, lead-soaps. These soaps are distributed uniformly throughout the oil, and help to make it dry and harden quickly. The same action occurs when white lead is ground as a paint with oil, and has been urged as an objection to the use of those white leads which contain hydrate of lead, a compound which acts upon oil more quickly and thoroughly than the carbonate of lead.

It will be seen, however, that while there may be reasons for permitting the use of a single lead pigment which possesses this peculiar property, there can be none for introducing into every part of a picture oils or other materials which contain a metal, like lead, so liable to cause discoloration and darkening, when other and perfectly innocuous substances are available for producing the same siccative effects. On this account we omit further reference to the lead compounds, which have been and are still employed in the preparation of strongly-drying oils, etc., but pass on to the Manganese compounds, of which the dioxide, the hydrated protoxide and sesquioxide, the borate, the oxalate, and the linoleate are the most important.

Manganese dioxide, the black oxide, MnO2, is used in the form of a powder obtained by grinding the mineral pyrolusite. As the effectiveness of this compound is made complete only by the use of oil of vitriol, which needs subsequent neutralization with lime, it cannot be recommended as a material for rendering linseed oil intended for painting, or for making picture-varnish, more drying. The difficulty of preparing the manganese hydrates above mentioned constitutes an objection to their employment for this purpose. But the borate, the linoleate, and the oxalate of manganese may be obtained in commerce in a state of sufficient purity for our present purpose, and it is to them that we wish to direct attention. Borate of manganese may, moreover, be so easily prepared, that it is worth while to give here the necessary directions. One pound of pure manganese sulphate is dissolved in six pints of distilled water, the solution being filtered if cloudy. A few drops of the liquid are now to be tested with caustic soda solution - the precipitate formed should be white; if it show a greenish, yellowish, or greyish hue, iron is probably present, and it will be necessary to treat the whole of the solution with caustic soda until a white precipitate falls, and then to filter it again.

In order to produce manganese borate, a boiling saturated solution of pure borax is added to the manganese sulphate solution until no more precipitate falls. The precipitate is collected on a filter and washed with hot distilled water until the wash-waters show no turbidity when a solution of barium chloride and a few drops of dilute hydrochloric acid are added to the last portion coming through the paper. The borate of manganese is then dried in a warm place, and finally in the water-oven. One grain of it, warmed with linseed oil, is sufficient to render an ounce of the latter highly drying (see Chapter V.). The oxalate or the linoleate of manganese may be used in the same way, and there are also met with certain manganese 'resinates' which may be employed similarly. Cobalt resinate is also a powerful dryer.

¶ The quantities of lead or manganese compounds necessary to confer the drying character on oil are very small. With .2 to 1.0 per cent. of the weight of the oil the maximum effect is so nearly attained that any further addition is unnecessary. If the manganese in the form of resinate or linoleate be dissolved in ten times its weight of turpentine, a siccative having very little colour but possessed of great power is obtained; this, when added to the oil in the proportion of from 1 to 20, 1 to 50, or even 1 to 100, confers the drying character upon it without any further heating. Moreover, if this oil be allowed, under carefully adjusted conditions, to absorb enough additional oxygen for it to enter on the second period (p. 53), it dries with great rapidity. Oil of this kind, prepared with the smallest possible amount of a manganese salt, is much to be commended for the use of the artist, since it assures to the finished painting a longer life than any other preparation of this kind.

Borate of lime and borate of zinc have been employed not only for rendering oils more quickly drying, but also in admixture with some of those oil-paints which dry with difficulty. Being colourless they are well adapted for use with white pigments, such as oxide of zinc. Several of the siccative materials sold under various fancy names consist of mixtures of these borates with carbonate of zinc or oxide of zinc, manganese compounds being also sometimes added. Another dryer in common use is white vitriol or sulphate of zinc. Its siccative character is very slight. Most of the other siccatives employed by artists owe their efficacy to lead, or are resinous preparations. Siccatif de Courtrai is a very dangerous mixture, heavily loaded with compounds of lead; Siccatif de Haarlem is a resinous preparation, which produces, on drying, a hard, brilliant, and tough film. This acquires, in course of time, a deep yellow hue, which, however, hardly affects the colour of the paints with which the siccative has been employed, because of the small proportion used.

¶ Here it may be mentioned that spirit of turpentine may be regarded as a fairly efficacious dryer, especially if the picture in which it has been employed is exposed to sunlight. This property of turpentine (terpenes) is not shared by the hydrocarbons of petroleum and of coal-naphtha, such as petroleum spirit, benzene, and toluene. This difference in chemical activity must be borne in mind if we would correctly estimate and foresee the results of employing these several liquids as vehicles or diluents.

It may be well to remark in this place that many of the volatile solvents, described in this chapter, are dangerously inflammable, and some are of a poisonous character.