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.
It is true that other soft resins are sometimes substituted for that of mastic, and that very elaborate compounds of them have been recommended and celebrated, but none that possess any evident advantage over the simple solution of mastic in rectified oil of turpentine. Correggio and P'armigiano, according to Armenini, used a varnish of common white resin mixed with naphtha. Other old masters are said to have employed mastic and sandarac dissolved in nut, poppy, or linseed oils, and this seems evident from the difficulty of removing varnishes from very old pictures. Mastic varnish is easily prepared, by digesting in a bottle during a few hours, in a warm place, one part of the dry picked resin with two parts or more of the oil of turpentine. A sufficient quantity of this, cleared, varnish to gelatinize or set up either of the before-mentioned drying oils of linseed, constitutes the transparent macgilp of the painter. etc. If, instead of drying oil, the simple pure linseed oil be used with about an eighth of acetate or sugar of lead dissolved in water, or ground fine, we obtain variously the opaque mixture called gumtion.