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.
The last operation of painting is varnishing, which completes the intention of the vehicle, by causing the design and colouring to bear out with their fullest freshness, force, and keeping; supplies, as it were, natural moisture, and a transparent atmosphere to the whole, while it forms a glazing which secures the work from injury and decay.
As it is expedient that the constitution of a picture, with regard to its materials, should be as homogeneous as possible, the proper varnish for a picture, painted with a vehicle of which mastic forms a principal ingredient, would be a mastic varnish; and this has accordingly been the prevailing practice. Antiently the mastic was dissolved in expressed oils, and not in oil of turpentine, as at present; the former, though indisposed to crack and bloom, in the manner of the latter, was nevertheless liable to become dark and discoloured by time, and difficultly removed from a picture in proportion as the oil abounded therein; in which respect copal oil varnish, though more durable than mastic varnish, is also in a less degree defective. When copal and the harder varnishes have been too soon used upon such pictures, the unequally contracting and expanding, and the various dispositions of the varnish and ground to move by heat or force, have usually cracked and damaged such pictures; but where copal has been the cement and body of the vehicle in a picture, varnishes of copal, and the harder resins, have been, for the above reason, properly preferred to mastic varnish in varnishing such pictures, and have been found less liable to chill or bloom.
But of all resinous substances in use for preparing of varnishes, the lac of India, which is the basis of the strong and beautiful lacquered works of the East, affords the hardest, most tenacious, and durable varnish, that of amber not excepted. The darkness of its colour was, nevertheless, an insuperable bar to its use in painting: the discovery, however, by which this subtance is entirely deprived of colour and impurities, has rendered it by far the most perfect of varnishes, and is bringing it into use in the art; so that when the difficulties usually attending the employment of new agitating it during several days with dilute sulphuric acid, and subsequently washing the oil with a little powder, or milk of lime.