Shellac or lac is a resinous substance which, in India, flows from certain trees in the form of lucid tears, in consequence of punctures made upon their branches by a small insect.
It is found in commerce in three forms - stick lac, seed lac and shellac. Stick lac is the substance in its natural state investing the small twigs of the trees, which are generally broken off in collecting it. When separated from the twigs and partially cleansed it is known as seed lac. Shellac is the seed lac after it has been melted, purified and formed into thin cakes.
Shellac is very apt to be adulterated with common resin, and hence, unless when a pale lacquer is required, most artisans prefer seed la. When lac is mixed with a little resin and colored with vermillion or ivory black it forms sealing wax.
Shellac is soluble in alcohol but not in turpentine. It is also soluble in alkaline solutions, including ammonia. A solution of borax in water dissolves it readily, and the resulting solution has been used as a cement, as a varnish, and as a basis for indelible ink. It is much used by batters as an insoluble cement.
Much trouble is generally experienced in obtaining clear solutions of shellac. If a mixture of 1 part shellac with 7 parts of alcohol of 90 per cent, is heated to a suitable temperature, it quickly clears, but as quickly becomes turbid again on cooling. The only practical method of freeing the solution from what some writers call "wax," and others "fatty acid," which is present in shellac in the proportion of 1 to 5 per cent., and is the cause of the turbidity, has hitherto been the tedious process of repeated filtration. M. Peltz recommends the following method: Shellac 1 part is dissolved in alcohol 8 parts, and allowed to stand for a few hours. Powdered chalk is then added in quantity equal to half the weight of shellac in the solution, and the latter is heated to 60° R. The greater portion of the solution clears rapidly, and the remainder may be clarified by once filtering. Carbonate of magnesia and sulphate of baryta were tried in the same way, but were not found equally efficacious.
When bleached by the ordinary process, shellac affords a polish for light woods, etc., that is brittle and liable to peel off, while the presence of a trace of chlorine causes metallic inlaying to become dim. These defects may be avoided by a different mode of bleaching, namely, by adding fine granulated bone-black to the solution of shellac in 90 per cent, alcohol, until a thin, pasty mass is formed, and exposing this for several days to direct sunlight, occasionally shaking it thoroughly and filtering when sufficiently bleached.