The term "enamel" is very much abused, as it is often being applied to any gloss paint. Properly speaking, enamel paints are or should be made to resemble in appearance the finish given to articles of metal that have been enameled by the furnacing process. Enamel paints are made in two forms - air drying and baking - but in either case cannot be produced by combining pigments with drying oils and drying mediums alone. Gum varnishes constitute an essential part of enamel paint, and the harder the resins or gums the better will be the wear of the resulting product. The old-fashioned way of making white enamel for interior decoration has been to grind French process zinc white in clear damar varnish, adding a small portion of anhydrous white sugar of lead, previously ground fine in bleached oil or in damar varnish, thinning the paste so ground with either damar or very pale rubbing varnish to a consistency that flowed well from the brush and leveled down uniformly onto the surface. This style of enameling was known as china glossing, and is still in practice for moderate priced work. In the chapter on grinding white bases for enamel paints we have pointed out that 70 pounds French process zinc and 30 pounds white damar varnish will, when ground on a water cooled mill produce the proper base for china gloss, and we may add that in order to produce good drying and hardness, 1/2 pound white sugar of lead, ground fine in oil or varnish, should be incorporated with this paste base previous to reducing it with 14 gallons varnish that may be either damar varnish or a good white mixing varnish of approved quality, but in neither case must the varnish be too heavy in body, and above all the paste base must have an ample time to cool after coming from the mill. The quantities of base and varnish mentioned should produce 20 gallons of china gloss weighing not over 10 pounds per gallon, while the general run of interior enamel white average between 11 and 12 pounds per gallon when the pigment is pure French zinc. For interior white enamel that is to be rubbed and afterward polished, a hard gum varnish is required, because that made with damar varnish is most too slow and does not stand well when rubbing with oil and pumice, but requires rubbing with pumice and water. A high grade interior enamel white or porcelain finish that will stand rubbing with oil and makes a fine polished surface that cannot be surpassed can be made on the following formula: - Forty-five pounds French zinc in damar as above. 6 gallons palest hard rubbing varnish, 1 gallon pure spirits of turpentine. This will make a little over 9 gallons of enamel weighing 11 pounds per gallon. Beat the base up with the turpentine first, then gradually add the varnish. A trifle of Prussian blue will give a porcelain effect, but must not be overdone. The mixing of enamels must never be done in a room with a temperature below 70 degrees F., and all enamels must be carefully strained after mixing, having the apparatus clean as possible.
When it is desired to tint white enamel paints to certain color effects only the very purest and strongest colors should be selected and ground to the utmost fineness in oil or varnish, preferably the latter, when an appreciable percentage is to be used. Before adding the color to the white base it should be thinned to the consistency of varnish and carefully strained. Colored enamel paints for interior work are best made from pure pigments ground fine in varnish that may be pale or dark, according to the pigment that is embodied with it. The colors selected should be free from any extending material and as light in gravity as possible. For instance, for black, the finest quality of carbon black; for red, pure toner of the desirable shade; for yellow, pure chrome yellow; for green, chemically pure chrome green; for blue, Chinese or Prussian blue are most preferable. One quarter pound of finest carbon black, dry, and a similar quantity of dry toner for red, 3/4 pound dry chrome yellow, 3/4 pound dry C. P. chrome green and 1/2 pound Prussian blue, dry, to make one gallon when thinned with sufficient varnish will be ample. A hard drying mixing varnish is sufficiently good enough unless the enamel paint is to be rubbed, in which case a rubbing varnish is required.
For a Bathtub Enamel the best base is a mixture of equal parts white lead and zinc white, and should be ground in oil and thinned with a good pale hard gum varnish as follows: - Forty pounds base as above, in oil; 6 pounds pure spirits of turpentine, 54 pounds varnish as above. This will produce an enamel of excellent body for tin or zinc lined bathtubs, the result of above formula being 8 1/2 gallons.
Exterior or Weatherproof Enamel should be composed of the base described in the chapter on grinding the white bases for enamels as practiced in Holland; that is, grinding French zinc white in heavy bodied or highly oxidized pale oil and ageing the paste for some time before mixing it with hard gum varnish. An exceptionally high grade of exterior white enamel can be made by grinding 65 pounds condensed zinc white in 28 pounds of the heavy oil and 7 pounds pure spirits of turpentine, permitting this paste to stand for a week or more, then placing same in a mixer, and to every 40 pounds of the paste add 60 pounds of varnish made from XXXX kauri gum at the rate of 12 gallons linseed oil to 100 pounds gum, the grinding of the zinc in the heavy oil furnishing elasticity and wear, the gum varnish the high gloss. This formula will produce 9 gallons weighing about 11 pounds. If this exterior enamel is to be tinted, oil colors will serve the purpose very well, but only such as are permanent to strong light must be selected.
Marine or Waterproof Enamels must have in their composition a hard gum varnish that has proven itself impervious to the action of water, and it stands to reason that linseed oil cannot be its chief constituent. It should be made of palest kauri gum, with not too large a proportion of oil, the pigment for a white enamel of this kind being composed of zinc white ground in linseed oil. Fifty pounds of this base mixed with 6 1/2 gallons of the varnish would produce 8 3/4 gallons of paint. If the varnish does not make the paint dry sufficiently hard in reasonable time, a concentrated white drier may be used for part thereof to the extent of 1/2 or 3/4 gallon. Colored marine or waterproof enamels may be produced with pigments of the desired color, ground in oil, reduced to brushing consistency with a mixture of 1 part japan drier and 9 parts hard drying water resisting varnish.
Baking Enamels are as a rule produced from pigment ground fine in fire boiled linseed oil and reduced for application with a hard gum varnish that carries a large percentage of oil. These enamels must flow out evenly, and when being stoved must not show any imperfections such as pin holes, brush marks, ridges, etc., which can only be prevented from appearing when the enamel is of full body. This body must be produced by the varnish, as it cannot be done by the pigment. As white lead cannot stand any high temperature in stoving (baking), the pigment must necessarily be zinc oxide or lithopone white for white baking enamel, and the oil, as well as the varnish, must needs be very pale. Damar varnish makes a good white baking enamel, but is rather brittle and very sensitive to chipping. A good formula for a white porcelain-like baking enamel for galvanized iron sheathing or tin that will not readily scratch or chip after baking for five or six hours at a temperature of 170 to 180 degrees F. may be made as follows: - Seventy pounds French process zinc ground in 30 pounds fire melted gum damar varnish as the base, thinned, after standing forty-eight hours, with 6 gallons pale gum varnish that requires thirty hours to dry if used by itself. This produces 11 gallons weighing a trifle over 13 pounds, and may be reduced at the place of operation with equal parts turpentine and damar varnish if desired. The white can be tinted to any desired effect. Colored baking enamels are made with pigments ground in oil and reduced with baking varnishes of more or less dark color.
The manufacture of Black Baking Enamels is essentially a problem for the Varnish Maker because to make them properly requires, that the asphaltums be incorporated with the oil, etc. by heat.