This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
Two different enamels are usually employed, viz., one for the ground and one for the top, the latter being somewhat harder than the former. Ground enamel is prepared by melting in an enameled iron kettle 625 parts brown shellac, 125 parts French oil of turpentine, with 80 parts colophony, and warming in another vessel 4,500 parts of spirit (90 per cent). As soon as the rosins are melted, remove the pot from the fire and add the spirit in portions of 250 parts at a time, seeing to it that the spirit added is completely combined with the rosins by stirring before adding any more. When all the spirit is added, warm the whole again for several minutes on the water bath (free fire should be avoided, on account of danger of fire), and allow to settle. If a yellow color is desired, add yellow ocher, in which case the mixture may also be used as floor varnish.
The top enamel (hard) consists of 500 parts shellac, 125 parts French oil of turpentine, and 3,500 parts spirit (90 per cent). Boiling in the water bath until the solution appears clear can only be of advantage. According to the thickness desired, one may still dilute in the cold with high-strength spirit. Tinting may be done, as desired, with earth colors, viz., coffee brown with umber, red with English red, yellow with ocher, silver gray with earthy cerussite, and some lampblack. Before painting, dry out the vats and putty up the joints with a strip of dough which is prepared from ground enamel and finely sifted charcoal or brown coal ashes, and apply the enamel after the putty is dry. The varnish dries quickly, is odorless and tasteless, and extraordinarily durable. If a little annealed soot black is added to this vat enamel, a fine iron varnish is obtained which adheres very firmly. Leather (spattering leather on carriages) can also be nicely varnished with it.