NO picture should be varnished before it is at least six months old, for I have already stated that the process of drying with Linseed Oil Colors is progressive, and keeps on for years, and if a picture be varnished too soon, cracks are bound to result. Only those types of varnishes should be used which can be readily removed and the three types that can be readily removed are Sandarac, Mastic and Damar.
Sandarac is a varnish which dissolves in Alcohol and dries perfectly within three hours.
Mastic is a pale gum which dissolves only in Turpentine and dries hard and dust-free overnight.
Damar is similar in its characteristics to Mastic, but not quite as hard.
Pictures with a heavy impasto, and particularly those in which the slow drying pigments are used, should not be varnished inside of a year—or, preferably, in two or three years. All Linseed Oil and Poppy Oil paints dry from the surface down and wrinkle like a desiccated apple. If a curved needle is inserted into these wrinkles, it is very often found that the interior is still liquid or semi-liquid. I have found globules of graphite and lamp black mixed with Linseed Oil, to remain soft after many years. When a picture becomes thoroughly hard and it has dried completely, the film becomes as tough as a sheet of glue, and it cannot be punctured. It is then called “needle-proof.” A clot of Raw Sienna in a picture by Josef Israels ten years old was not yet “needle-proof.”
It would therefore appear that the best time to varnish a picture is after it has hardened up uniformly and completely, even though this takes a year or two.
Mastic is very largely used, but has the one great defect that you never know whether it is going to dry with a gloss or whether it is going to dry flat, and sometimes a picture varnished with Mastic will be partially glossy and partly matte. To overcome this, a small amount, not exceeding 10%, of Lavender Oil or Linseed Oil may be added.
On a bright, clear day, Damar Varnish usually dries with a gloss, but on a damp day, when the picture is moist, Damar will also dry flat.
Sandarac Varnish may be very easily removed with Alcohol, and both Damar and Mastic may be easily removed with Turpentine or a mixture of Turpentine and Benzine (Petroleum Naphtha).
For the restoration and renovation of paintings, it will at once be seen that a varnish that can be easily removed is by far preferable to the Copal Oil Varnishes, and Amber Varnishes which dry with a hard, tough insoluble film, and it is at times impossible to remove these; and, where strong solvents or much attrition is used, it sometimes happens that the surface glazing or delicate tints are removed at the same time.