Varnish, as a rule, is a rather delicate material and requires a great deal of attention, especially when it is being employed as a vehicle for grinding pigments of one kind or another. And, as our so-called enamels or gloss whites would not have or hold the luster expected from them without being made with large portions of gum varnish, we must consider how the base for the various grades should be manipulated.
No matter whether the base to be prepared is to serve for air drying or baking enamel white, no manufacturer can meet with success in satisfying the trade unless he starts right from the very bottom by having his grinding room as dust free as possible and provided with heating apparatus, so that during the cold season the temperature can be kept at a normal figure, say, about 70 degrees F. The mixing room should be separate from the milling or grinding room and the former frequently dusted. When the base is ground in varnish or in a vehicle with a large portion of varnish it is best run through a water-cooled esopus stone mill until of the required fineness, while for bases with a vehicle of heavy-bodied oil a printing ink roller mill is best suited. Still, with skilful attention and care a good stone mill will serve the purpose in the latter case also. Overheating must be avoided in any case, more especially when varnish constitutes the vehicle. The older method of making interior white enamel, known as china gloss, was to grind French zinc white in white damar varnish, which, as is well known, was simply a solution of damar resin or gum damar in spirits of turpentine (at the rate of 120 pounds gum damar to twenty gallons turpentine). The resin was either dissolved cold by churning it with turpentine in a revolving drum or the solution was made by melting the resin in a kettle at low heat and adding the turpentine, the latter method giving the best product because free of moisture, but slightly yellow from the melting of the resin. Seventy pounds French zinc, ground in thirty pounds (about four gallons) of white damar varnish made a good base for china gloss and was also sold in paste form as French zinc in damar. When used as base for china glossing it was simply further reduced to flowing consistency with more white damar varnish. Another base for white enamel, now practically obsolete, was known as impalpable white in damar, and consisted of forty-four parts white lead of best selection, thirty-three parts French zinc white and twenty-three parts white damar varnish. On account of its good body (covering power) it was a favorite with casket manufacturers and ornamental wood workers, also used it on enameled furniture. The great advance made in varnish manufacture during the past twenty years especially through the development with China wood oil, has had much to do with discouraging the use of damar varnish as a vehicle for white gloss paints. Unless these are made for a special purpose the use of white lead as a pigment base for enamel has been very generally abandoned and French process zinc white and the better grades of American process zinc have the call, while lithopone is gaining ground in the moderate-priced gloss whites. Here is where varnishes with small percentages of China wood oil find their chief use in white paint.
Varnishes with China wood oil, however, are not suited as a vehicle for grinding zinc whites, because a zinc base so ground invariably shows a tendency to pudding" up, and on thinning for use the thickening keeps on, producing a sort of jelly instead of a paint with body. Another varnish not suited for grinding zinc whites is one that is made with manila gum, which has a simular thickening tendency and invariably will give trouble. Where the base is required for quick air drying white enamel it is best to grind zinc white in damar varnish and depend upon a good white enamel varnish for the subsequent thinning, unless the varnish maker can guarantee a quick-drying grinding vehicle free from manila gum and China wood oil. When it comes to white enamel of great durability or for baking purposes, the zinc white can be ground in a special baking varnish or in a heavy-bodied linseed oil, similar to that used in grinding white printing inks.
In Holland, the home of enamel paint making, and in England and Germany the manufacturers prepare a special linseed oil of heavy consistency by boiling without the addition of drying mediums and blowing air through it during the boiling process (known here as blown oil) and permit this oil to age in tanks. German painters know this as standoel, the name being derived from its being allowed to stand undisturbed for long periods. The zinc white is mixed with and ground in this oil or an oil bodied to syruplike consistency by boiling in varnish kettles, known to us as bodied or oxidized linseed oil, such as may be prepared by any expert varnish maker. After grinding, the white enamel base is set away to ripen in well-covered containers for at least two weeks, sometimes much longer, before it is reduced with varnish or other diluents, but of this we will speak later on. The chief point is that the ripening of the paste is of great importance in the final product, and it may be stated right here that much of the trouble found in the working of enamels is due to undue haste in thinning down the fresh-ground base, which is often still hot from the mills.