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Coloring Finishing And Painting Wood

The pigments used in enamels are ground very fine, in varnish, preferably, and sometimes in linseed-oil or in japan.

In the case of white enamels of high grade, only the very best French-process zinc white of the green or white seal grades is used, and it should be ground in a very transparent or pale enamel-varnish. Zinc white (ZnO) is selected because it is somewhat whiter than white lead, which is a hydrated carbonate of lead having the formula —2PbCO3Pb(OH)2. It is also preferable because it can be ground to great fineness. Zinc white readily unites with the varnish-resins, and has certain desirable physical characteristics which are beneficial in the application of the enamel by making it flow on the surface more satisfactorily. It is not wise to use zinc white ground in ordinary linseed-oil when making white enamel, because linseed-oil frequently turns somewhat yellow on long exposure and this discoloration spoils the snow-white effect of the finish.

Many manufacturers use an elastic, long-oil varnish as a vehicle, even in the production of white and ivory enamels. The varnishes selected for this purpose are special enamel-varnishes, however, and are almost transparent because they are made from pale resins and bleached linseed-oil. Until rather recently zinc white was used almost exclusively as the pigment to give opacity to enamels. Lithopone, which is about 70 per cent barytes (BaSO4), together with zinc sulphide (ZnS) varying from 26.60 to 29.51 per cent, and zinc oxide (ZnO) in small amounts of from 0.57 to 3.30 per cent, is now made in pure form—free from other metals which tend to discolor it. Lithopone is often used with zinc oxide in many of the cheap enamels, especially.