The oldest drier is probably litharge, a reddish-yellow powder, consisting of lead and oxygen. Formerly it was ground finely in oil, either pure or with admixture of white vitriol and added to the dark oil paints. Litharge and sugar of lead are used to-day only rarely as drying agents, having been displaced by the liquid manganese siccatives, which are easy to handle. E. Ebelin, however, is of the opinion that the neglect of the lead compounds has not been beneficial to decorative painting. Where these mediums were used in suitable quantities hard-drying coatings were almost always obtained. Ebelin believes that formerly there used to be less lamentation on account of tacky floors, pews, etc., than at the present time.
Doubtless a proposition to grind litharge into the oil again will not be favorably received, although some old master painters have by no means discarded this method.
Sugar of lead (lead acetate) is likewise used as a drier for oil paint. While we may presume in general that a siccative acts by imparting its oxygen to the linseed oil or else prepares the linseed oil in such a manner as to render it capable of readily absorbing the oxygen of the air, it is especially sugar of lead which strengthens us in this belief. If, according to Leuchs, a piece of charcoal is saturated with lead acetate, the charcoal can be ignited even with a burning sponge, and burns entirely to ashes. (Whoever desires to make the experiment should take 2 to 3 parts, by weight, of sugar of lead per 100 parts of charcoal.) This demonstrates that the sugar of lead readily parts with its oxygen, which though not burning itself, supports the combustion. Hence, it may be assumed that it will also as a siccative freely give off its oxygen.
Tormin reports on a siccative, of which he says that it has been found valuable for floor coatings. Its production is as follows: Pour 1 part of white lead and 1.5 parts each of litharge, sugar of lead and red lead to 12.5 parts of linseed oil, and allow this mixture to boil for 8 to 10 hours. Then remove the kettle from the fire and add to the mixture 20 parts of oil of turpentine. During the boiling, as well as during and after the pouring in of the oil turpentine, diligent stirring is necessary, partly to prevent anything from sticking to the kettle (which would render the drier impure) and partly to cause the liquid mass to cool off sooner. After that, it is allowed to stand for a few days, whereby the whole will clarify. The upper layer is then poured off and added to the light tints, while the sediment may be used for the darker shades.
If white vitriol (zinc sulphate or zinc vitriol) has been introduced among the drying agents, this is done in the endeavor to create a non-coloring admixture for the white pigments and also not to be compelled to add lead compounds, which, as experience has shown, cause a yellowing of white coatings to zinc white. For ordinary purposes, Dr. Roller recommends to add to the linseed oil 2 per cent (by weight) of litharge and 1/2 per cent of zinc vitriol, whereupon the mixture is freely boiled. If the white vitriol is to be added in powder form, it must be deprived of its constitutional water. This is done in the simplest manner by calcining. The powder, which feels moist, is subjected to the action of fire on a sheet-iron plate, whereby the white vitriol is transformed into a vesicular, crumbly mass. At one time it was ground in oil for pure zinc white coatings only, while for the other pigments litharge is added besides, as stated above.
As regards the manganese preparations which are employed for siccatives, it must be stated that they do not possess certain disadvantages of the lead preparations as, for instance, that of being acted upon by hydrogen sulphide gas. The ordinary brown manganese driers, however, are very liable to render the paint yellowish, which, of course, is not desirable for pure white coatings. In case of too large an addition of the said siccative, a strong subsequent yellowing is perceptible, even if, for instance, zinc white has been considerably " broken" by blue or black. But there are also manganese siccatives or drying preparations offered for sale which are colorless or white, and therefore may unhesitatingly be used in comparatively large quantities for white coatings. A pulverulent drying material of this kind consists, for example, of equal parts of calcined (i. e., anhydrous) manganese vitriol, manganous acetate, and calcined.zinc vitriol.
Of this mixture 3 per cent is added to the zinc white. Of the other manganese compounds, especially that containing most oxygen, viz., manganic peroxide, is extensively employed. This body is treated as follows: It is first coarsely powdered, feebly calcined, and sifted. Next, the substance is put into wire gauze and suspended in linseed oil, which should be boiled slightly. The weight of the linseed oil should be 10 times that of the manganese peroxide.
According to another recipe a pure pulverous preparation may be produced by treating the manganic peroxide with hydrochloric acid, next filtering, precipitating with hot borax solution, allowing to deposit, washing out and finally drying. Further recipes will probably be unnecessary, since the painter will hardly prepare his own driers.
Unless for special cases driers should be used but sparingly. As a rule 3 to 5 per cent of siccative suffices; in other words, 3 to 5 pounds of siccative should be added to 100 pounds of ground oil paint ready for use. As a standard it may be proposed to endeavor to have the coating dry in 24 hours. For lead colors a slight addition of drier is advisable; for red lead, it may be omitted altogether. Where non-tacky coatings are desired, as for floors, chairs, etc., as well as a priming for wood imitations, lead color should always be employed as foundation, and as a drier also a lead preparation. On the other hand, no lead compounds should be used for pure zinc-white coats and white lacquering.