To manufacture enamel paints for the cold parts of engines is not a difficult task as it is only necessary to select a vehicle, that will, on the drying of the paint, make it an easy matter to wipe off any lubricating grease or machinery oil without injury to the gloss. As engines are usually made of smooth castings and these well filled and surfaced, any well ground color of the desired shade will answer for the ordinary decorative enamel, when reduced to brushing consistency with a good mixing varnish, the percentage of color required depending upon the shade desired. Thus for a black enamel, 4 oz. of best grade carbon black, ground in oil may be added to one gallon of a quick drying mixing varnish, and will then cover, while it would require about 3 pounds of English vermilion to obtain fair covering. To produce a tinted enamel for engines, such as a warm gray, would require 5 lbs. zinc in oil or varnish tinted with lampblack and a trifle of yellow to one gallon of the mixing varnish and so on.
Unless specified, no paint maker of the present day would undertake to use English vermilion or orange mineral and eosine vermilion in this connection, as he has the red toners of the paranitraniline type at his command, one half pound of which, if pure, will when ground in varnish and added to enough mixing varnish to make one gallon of paint, cover very well over a suitable priming coat or ground.
When it comes to enamel paint for the heated portions of engines, the matter of preparing same is quite different, the material being much higher in cost and requiring caution in selection, when it comes to such colors as green, red, blue, etc. The temperatures being rather high, heat proof colors must be selected and the varnish must also be one, that will bake hard and yet retain its elasticity under the extreme conditions of heat and cold. Green oxide of chromium is really the only green pigment, that will withstand this, so that an olive or bronze green enamel can be made by mixing enough carbon black with chrome oxide green to produce the effect and if needed, a small portion of alizarine red lake will assist in producing the proper tone. But in selecting the chromium oxide green, that known as Guignet's green must not be used, as high temperatures will destroy its fine color by removing its water of hydration. Toluidine red is a fast red, that is heat and alkali proof, and while very high in price, when chemically pure, will stand reduction with best china clay or infusorial earth, so as to bring cost within competitive limits. Imitation of cobalt blue, will stand quite high temperatures without change, and if a small portion of zinc oxide can be added without obtaining too light a shade of blue, will be the best pigment for a blue engine enamel. Whether these heat proof enamel paints are made in the solid colors or in tints on a base of zinc white, they should not be made to air dry too rapidly, not under 24 hours in the ordinary temperature of a room, otherwise they will not last.