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.
Once gave them strength and saved the freshness of the colours from the contamination of the oil: which advantages oil-painting obtained by its gradual transition from the methods of fresco and distemper, at a time, too, when even the crayon and silver style were employed upon cartoons and cloths prepared with terrene surfaces, upon which we may reasonably infer the first oil paintings were also executed.
It was two hundred years after Armenini that the compounding of varnish with drying oil was adopted in this country, as a regular system of painting; W. Williams, an artist of Bath,! promulgated it as a great discovery, which he sold as a secret in a guinea pamphlet, printed in 17S7: and to this circumstance, added to the facility of its use, may, perhaps, be owing the avidity with which it was adopted. Accordingly, half a century ago, the gellied vehicles which received the cant appellation of macgilp and gumlion were the favourite nostrums of the initiated in this countrv, and have maintained a preference with many artists to this day. These compounds of one part or more of strong mastic varnish with two of linseed or other oils rendered drying and coagulable by the salts and oxides of lead, were, according to the preceding intentions, improvements upon the simple oil vehicle used on impenetrable grounds, by diluting it, and giving it a gelatinous texture, which enabled it, while flowing freely from the pencil, to keep its place in painting and glazing; but their principal intention was missed by the weakening of the oil without preserving the colour or transparency of the vehicle, - defects which arise from the feeble body of mastic, its softness, and a degree of disposition to darken and cloud by age.
The defective colour of the macgilp formed with oil rendered drying by boiling or maceration on litharge, both before and after drying, was in some measure remedied by gumtion, composed of not more than an eighth of the acetate or sugar of lead, with simple oil and strong varnish, which is subject to less change ultimately, particularly when the varnish abounds in the compound. In the using of sugar of lead, if the acid abound, which it does usually in the purer and more crystalline kinds, its power of drying is weakened, and it may have some injurious action upon colours, such as those of ultramarine and lakes. In this case a small addition of some of the pure oxides of lead, such as litharge, ground fine, will increase the drying property of the sugar of lead, and correct its injurious tendency; but too much litharge, or more than the oil will dissolve, will give a lasting opacity to the vehicle injurious to transparent colours, although these substances do not occasion the blooming of varnishes, as commonly supposed, for this depends upon the precipitation of the resin by the action of a moist atmosphere on the varnish.
Ibbotson discovered a medium resembling these, which he prepared by dissolving mastic in linseed oil and mixing it up with sugar of lead and water. By grinding these together he formed a paste which resembled nearly the Venetian medium, dried hard without cracking, and had their impasto; but the line of his pictures betrays the too free use of his vehicle.
Sir William Beechey formed a vehicle of this class by stirring the acetate, or sugar of lead, into an aqueous solution of yellow or turpentine soap, in which, by double chemical affinity, the lead and turpentine united in a magma which set up linseed oil, and an acetate of soda separated by pressure of the palette-knife, and passed off in solution with the water. The advantage in this case arises from the perfect solution and intimate union of the materials.
Wilson, our Coryphaeus in landscape painting, used at one period of his practice a simpler compound, consisting of equal quantities of linseed-nil and oil of turpentine, thickened by exposure to the sun and air till it became resinous and half evaporated, to which he afterwards added a portion of melted bees'-wax; and it is probable, from similarity in the texture of their pictures, that Sir Joshua Reynolds used also the same vehicle; indeed be is said to have greatly prized linseed-oil inspissated by age. According to Lanzi, Correggio's vehicle was a mixture of two parts oil with one of varnish, but of what oil and varnish we are not informed; and it is an indispensable condition for obtaining a gelatinous texture of this vehicle that the oil be rendered drying by an oxide of lead, and that the varnish be of mastic or a similar resin.
Little advantage appears to have resulted from other attempts to improve these vehicles, by substituting for mastic those soft resins and balsams which are but native or factitious compounds of weak resins with essential oils, similar to that of turpentine; or from the introduction of spirit, wax, etc. The advantages in such cases, if any, have been merely in the working; hence many judicious artists of the present day have resorted to the use of copal varnish, and rejected mastic and the weak resins, contending with the difficulties of working copal for the advantages of strength, fine texture, and the greater transparency and permanence of its colour; while the resistance it opposes to re-solution by spirit, and other menstrua after drying, fits it for receiving even spirit varnishes, which may afterwards be removed without injury, and favours in other respects the texture and durability of the work: it has, nevertheless, the defect of cracking when it has been prepared with an essential oil, or used without sufficient drying oil to temper it. The late original and eminent landscape painter, Constable, employed drying oil mixed with thick copal varnish, and diluted with thin turpentine oil, to which he sometimes added bees'-wax: and we have seen heads painted by another eminent artist, Mr. Joseph, of Cambridge, of the most fascinating transparency and texture, with oil copal varnish alone. Copal, in every mode of dissolving, swells or augments in bulk more than any other resin, like glue in water, and contracts proportionably in drying; and it is this which disposes it to crack, in which respect it is inferior to mastic, when unsustained by a drying oil.
As a general sketch of the progress of vehicles is rather a light to shew the way, than a hand to guide in practice, we will subjoin such observations of a more particular and tangible nature as experience has supplied concerning the materials of vehicles and varnishes; and, first, of oils.