Used extensively in nineteenth-century English and Irish art museums. Its nature and application to restored paintings is commented on in some detail in the classical, The Chemistry of paints and Painting*, by Sir Arthur Church a London National Gallery curator of the nineteenth century.
The formula was is a mixture of Stand Oil (boiled linseed oil) and Dammar or, sometimes, Mastics resin. It sometimes contained a portion of hard copal resin. The resulting viscous liquid was difficult to apply to a painting’s surface. The preferred diluent was Spirits of Turpentine, as distinguished from Mineral Turpentine.
It is derived from pine trees and distantly related to Venice turpentine an oleo-resin extracted from certain classes of fir. Venice turpentine was occasionally introduced into nineteenth-century varnishes to benefit from its free-flowing qualities.
Gallery varnishes darkened rapidly, obscuring the subject matter of Old Master painting in art museums. Very few paintings today still retain their original gallery varnish. These were largely removed from paintings on public show during conservation projects carried out after World War ll.
During conservation conservators often saved the Gallery Varnish in the liquid solvent used during its removal. With the admixture of Cassel Earth pigment it was often applied to furniture and other art objects where an appearance of ageing was required.
Coach and trains painting’ techniques specific required a resilience to climate and atmospheric conditions to ensure their survival. Train carriage painters used Standoil for painting carriages, it being an important technique where the adoption of oil varnishes provided a durable, glossy, and smooth finish for the protection of coaches that were, up to the second world war manufactured mostly in timber.
The medium adopted by craftsmen, who specialised in this technique was of a heat-thickened linseed /copal oil, It provided level brushstrokes and created a more flexible paint film than raw linseed oil.
The technique resulted in a enamel-like finishes It was a hand-over from coach painting techniques passed on from traditional horse-drawn vehicles of an earlier age.
Mastic resin was commonly used with linseed oil, then subjected to a high-temperature cooking process, at around 300 C., for the purpose of increasing its viscosity. The process relied on the painter art to achieve the resulting uniformed smooth lucid finish.
Coach painters decorating train carriages applied decoration, often freehand,and guided chalk covered threads to paint decorative strips often several meters in length using “pencils”. They were frequently used in applying gold leaf lines in signwriting.
The nineteen fifties, especially in Ireland is the last known use of the archaic Frenchexpression, pincel or, perhaps the Latin penicillus. In coach painting the ‘pencil’ was a fine brush, the handle formed from a short birds quill into which was set chisel-edged sable hairs often 5 cm or more in length.
The coach-painters preserved their pencils, when not in use, in open-ended metal containers laid out in a bed of animal fat-based greases, used in machinery-based lubricants. This technique conserved their collection of precious pencils from degrading while allowing their creative instruments to retain their form and shape.
*The chemistry of paints and painting A.H. Church – Third Edition – MCMI – pps 112-123https://www.academia.edu/48988796/Copal_Varnishes_Used_on_18th_and_19th_century_Carriages