Shellac is obtained from the branches and twigs of several species of trees in India, where it is deposited by insects that feed on the sap of the trees.
The crude material
Is refined into a number of grades for various purposes. The two best available on the market are orange shellac, which comes in the form of thin, translucent orange-brown flakes, and bleached or white shellac, which looks like pulled molasses candy. The less-refined grades (seed-lac, garnet-lac, and button-lac) are a deep blood red; formerly this lac was used to make a red dye, and before synthetic dyes superseded it this was the principal object of its cultivation, shellac for varnish being a by-product.
Both white and orange shellac are entirely insoluble in turpentine and mineral spirits, but yield cloudy solutions in alcohol. The cloudiness is due to waxes that are imperfectly soluble in the alcohol but become clear after the varnish film has dried. Many of the more powerful solvents will also dissolve shellac but are not commonly so used.
As a varnish
Shellac dries rapidly to a hard, tough, flexible film, and is useful for varnishing floors and furniture. The surface under normal brush application shows a characteristic slightly rough or orange-peel effect.
Shellac is not used extensively in permanent painting on account of its tendency to turn dark with age; some investigators also report severe cracking after five or ten years when it is used as a final picture varnish. However, when it has been diluted with pure alcohol to an extremely thin solution, its yellowing is not of much significance, and it may be used as a sizing for porous surfaces and as an isolating layer between films of paint in certain techniques (especially in tempera painting); it is also sold as an inexpensive fixative for charcoal and other drawings.
This solution, however, is not to be employed as a retouch varnish or in any clear coating over a painting, for in such uses its yellowing would soon be apparent. Shellac in any work of art must always be well covered by pigmented layers. Its complete insolubility in mineral spirits and turpentine makes it valuable as a size in ordinary wall or decorative painting. Although dry shellac is mixed or melted with other resins and miscellaneous materials to make sealing wax and other industrial products, the varnish is generally not improved by admixture with other resins and oils.
Good grades
Some good varnish are sold in cans and bottles by reliable makers who state the contents and weight of cut on the label. A specially refined, de-waxed shellac cut with pure alcohol, and quite clear, was formerly called French varnish; but this name now has little meaning. Many inferior shellac substitutes are commonly sold; some of these contain materials added to imitate the cloudiness of true shellac; they are to be avoided, even for household use.
Fresh bleached shellac is soluble in water solutions of mild alkalis; borax is the usual material used for this purpose. These solutions are used as sizes and stiffeners in industry, and for paper, cloth, straw, etc. If shellac varnish purchased in a can is only partially used, the remainder is best stored in a glass bottle, as it is liable to darken if kept for any length of time in a metal can that has been opened. Shellac that has been thinned often becomes nondrying on storage and is best discarded. Dry white shellac becomes insoluble on long storage; fresh stock should be used.