MATERIALS & METHODS - Painting - Oil Painting - Binders and Diluents - Varnishes
Characteristics - Painting Methods & Techniques - Materials and Equipment - Work Space & Storage - Manufacture of Pigments - Protection of the Picture
From: Kay, Reed. The Painter's Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983.
They are made by dissolving solid resin in a liquid solvent or by melting a resin in an oil. The resins are usually obtained from trees that exude the gummy material when the bark is incised or "tapped," although some resins are fossil remains from dead trees. The resin hardens on exposure to air, producing crystal lumps or "tears," and comes to market in this form, where it is graded according to the size and clearness of the lumps and labeled according to point of origin, as for example, "Dammar resin tears, No. 1 Singapore." More recently artificial resins manufactured for industrial purposes have become available.... Some of these may prove to be worthwhile substitutes for the natural resins in particular painting procedures. Varnishes may be considered in two groups: spirit varnishes and cooked-oil varnishes.
Spirit varnishes (simple solution varnishes) are made by dissolving the resin in a cold solvent, such as spirits of turpentine or alcohol (spirits of wine). The resin used is usually one of the "soft" resins, such as dammar or mastic. It yields a varnish that dries to a solid state through the simple evaporation of its solvent, but then can be returned to its liquid state by reapplying the same solvent. The spirit varnishes dry more quickly than any of the drying oils, since all that is required for their complete hardening is time enough for the solvent to evaporate into the air, whereas the oils must go through the lengthier oxidation and polymerization process.
This type of varnish can be very easily made, in good quality, by the artist in the studio.
Cooked-oil varnishes are produced when certain resins are melted in hot oils, frequently with the addition of metallic driers. They are then thinned with a volatile solvent. These resins are usually harder resins, such as one of the copals, and yield varnishes that, when dry, are much more resistant to the solvent action of thinners. Such varnishes are usually made by large-scale industrial operations. [p. 47-48]
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[Kay, Reed. The Painter's Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983.]