This section is from the book "Chromatography; Or, A Treatise On Colours And Pigments, And Of Their Powers In Painting", by George Field. Also available from Amazon: Chromatography, or A Treatise on Colours and Pigments, and of Their Powers in Painting.
Fineness of texture is gotten by grinding and levigating extremely, but is only perfectly obtained by solution, - and this few pigments admit of; - it merits attention, however, that colours ground in water in the state of a thick paste, and others, such as gamboge, in strong solution in water and liquid rubiate, etc. are miscible in oil, and dry therein firmly; and in case of utility or necessity, any water-colour in cake, being rubbed off thick in water, may then be diffused in oil, the gum of the cake acting as a chemical medium of union to the water and oil without injury. And pigments, which cannot otherwise be employed in oil or varnish, may be thus forced into the service, and add to the resources, of the painter in oil. In such case, however, the steel palette-knife should be employed with caution.
With respect to Desiccation or Drying, the well-known additions of the acetate or sugar of lead, * litharge, † and sulphate of zinc, called also white copperas and white vitriol, either mechanically ground or in solution, for light colours; and japanner's gold size, or oils boiled upon litharge, for lakes, or in some cases verdigris and manganese for dark colours, may be resorted to when the colours or vehicles are not sufficiently good dryers alone: but it requires attention, that an excess of dryer renders oil saponaceous, is inimical to drying, and injurious to the permanent texture of the work. Some colours, however, dry badly from not being sufficiently edulcorated or washed, and many are improved in drying by passing through the fire, or by age. Sulphate of zinc, as a dryer, is less powerful than acetate of lead, but is preferable in use with some colours, upon which it acts less injuriously: but it is supposed, erroneously, to set the colours running; which is not positively the case, though it will not retain those disposed to move, because it wants the property the acetate of lead possesses, of gelatinizing the mixture of oil and varnish. These two dryers should not be employed together, as frequently directed, since they counteract and decompose each other by double election, - forming two new substances, the acetate of zinc, which is an ill dryer, and the sulphate of lead, which is insoluble and opaque. The inexperienced ought here to be guarded also from the highly improper practice of some artists, who strew their pictures while wet with the acetate of lead, or use this substance otherwise in its crystalline or granular form, without grinding or solution, which, though it may promote present drying, will ultimately effloresce on the surface of the work, and throw off the colour in sandy spots.
* This is the Saccharum Satumi of the old chemists, and the Saturnus glorificatus of the alchymist, celebrated for its uses in forming pastes for artificial gems, for drying oils. &c.
† The Lythangyros of old authors, who speak also of Litharge of Silver and Litharge of Golds both of which are oxides of lead produced in the separating and refining of those metal;., the first being of a lighter, and the latter of a deeper and yellower colour: which colours they owe to the degrees of their oxidation and the fuel employed therein, the yellow being the more oxidated and the better drier of the two. This oxide sometimes contains a small portion of iron.
It is not always that ill drying is attributable to the pigments or vehicle, - the states of the weather and atmosphere have great influence thereon. The oxygenating power of the direct rays of the sun renders them peculiarly active in drying oils and colours, and was probably resorted to before dryers were added to oils, particularly in the warmer climate of Italy, in which the very atmosphere is imbued with the active matter of light to which the drying property of its climate may be attributed. The ground may also advance or retard drying, because some pigments, united either by mixing or glazing, are either promoted or obstructed in drying by their conjunction. The best practice in this respect is to sponge the picture previously to painting thereon with soft water, and in damp weather with weak aqueous solution of the acetate or sugar of lead and wipe dry.
The various affinities of pigments occasion each to have its more or less appropriate dryer; and it would be a matter of useful experience if the habits of every pigment in this respect were ascertained; - siccatives of less power generally than the above, such as the acetate of copper, and the oxides of manganese, to which umber and the Cappagh browns owe their drying quality, and others might come into use in particular cases. Many other accidental circumstances may also affect drying; and among these none is more to be guarded against by the artist than the presence of soap or alkali, too often left in the washing of his brushes, which, besides other ill effects, decompose and are decora-posed by acetate of lead and other dryers, which set their fat oil free and retard drying, in streaks and patches on the painting: in all which cases, however, the odium of ill-drying falls upon some unlucky pigment. To free brushes from this disadvantage, they should be cleansed with the oils of linseed and turpentine. Dryers should be added to pigments only at the time of using them, because they exercise their drying property while chemically combining with the oils employed during which the latter become thick or fatten. Too much of the dryer will, as before noticed, often retard drying.
To all other good qualities of pigments it would be well if we could in all cases add that of being innoxious; - as this however cannot always be, and good pigments are by no means to be sacrificed to the want of this property, while no pigment that is not imbibed by the stomach will in the slightest degree injure the health of the artist; common cleanliness, and avoiding the habit of putting the pencil unnecessarily to the mouth, so common in water-painting, are sufficient guards against any possibly pernicious effects from the use of any pigment.