This is a wide field and to treat the subject fully would require a volume itself and then not be exhausted as our scientific researchers are discovering new data regarding corrosion of metals as they go along and are advancing new theories as to the paint problem daily. As we cannot wait for them, however, we will have to work on what we know from experience along this line and the author believes that he has not spent almost a life time on this subject without accomplishing some results and establishing some facts.
So much appears to be certain that pure red lead well made and freed from all impurities and containing no vitrified particles will make the best priming coat for iron and steel that is subjected to salt water, provided such red lead has not had an opportunity to over-oxidize the oil with which it has been mixed for application. When pure dry red lead is mixed with pure raw linseed oil within twenty-four hours before application it does not require the admixture with driers or volatile thinners to permit its drying hard, yet elastic on the metal, but will within forty-eight hours become as hard as cement, while if mixed with base material, impure oil, or if permitted to oxidize the oil such red lead paint will not have the preservative qualities that can be had from paints made from other pigments, such as ferric oxide or carbon paints. When these are properly put together from well selected pigments and manipulated with a vehicle that is free from any traces of acidity it only remains for the metal to be prepared to receive the coating before there is any indication of the formation of rust. Rustless or preservative coatings for iron and steel cannot be made in white or light tints for direct application to iron and steel, as white lead (basic lead carbonate) alone does not inhibit rusting and zinc oxide will not assist it, as it does not withstand expansion and contraction of the metal. Sublimed white lead (basic lead sulphate) and sublimed blue lead are not elastic enough either and therefore the first coat or priming paint for structural iron and steel should be composed of red lead, as outlined above, or an oxide of iron paint of a composition we are about to describe, but must be of necessity a red or other dark color. That such a coating will serve equally well as a second and third coat over a red lead priming and as a primer or first coating in place of red lead to be second or third coated with white paint or light tints has been proven by the author by exposing the paints referred to on iron and steel subjected to sulphur gases and to wind and weather for years. This was before researchers had discovered what pigments had inhibitive properties and what pigments promoted the formation of rust through stimulative characteristics. There is no inclination to belittle the work of scientific research, on the contrary, the author believes that much good will yet come from that source, but life at best is short and we are obliged to work upon the basis of past experience until we have found out more about the effects of certain pigments and vehicles on certain metals and vice versa.
Metal Preservative Black can be made by grinding any good well calcined carbon black in well settled raw linseed oil or fire boiled linseed oil (if the latter is not available, use the raw oil in preference to the present day boiled oil), using an extender of light gravity and red lead or litharge as drier in excess, thinning with fire boiled linseed oil and hard gum japan and turpentine for a moderately slow drying paint, while for a paint that will be subjected much to contact with sulphur gases, an addition of very hard gum varnish will make the paint more impervious. These hard gum japan and varnish compounds should have been proven by exposure tests extending through several years in order to feel secure, and an excellent paint of this kind in black can be made as follows: - Grind a soft paste by mixing 12 pounds best gas carbon black, 6 pounds powdered litharge (or 3 pounds each litharge and red lead), 24 pounds finest floated silica or silex, 3 pounds whiting in 55 pounds raw linseed oil and thin this soft paste with 10 gallons fire boiled linseed oil and 1 gallon hard gum japan. This will make 20 gallons of paint, weighing a trifle over 9 pounds per gallon. If hard drying and protection against the infiltration of gases is to be a special feature use a portion of hard gum varnish in place of the fire boiled oil for thinning.
Metal Preservative Red may be made by grinding a base of 40 pounds bright red oxide of 95 per cent purity, 8 pounds red lead, 2 pounds zinc chromate, 25 pounds floated silex or silica in 25 pounds raw linseed oil thinning same with 5 gallons raw linseed oil, 1 gallon hard gum japan and 1/2 gallon turps. This will produce 12 1/2 gallons of paint weighing a trifle over ll 1/2 pounds per gallon. By substituting a long stock of hard gum varnish for part of the 5 gallons raw oil a hard drying product will be the result.
Metal Preservative Maroon. - By substituting for the bright red oxide deep Indian red in similar quantity, a rich maroon paint of same quality as the red will result.
Metal Preservative Brown. - Thirty pounds burnt Turkey umber, 10 pounds French yellow ocher, 5 pounds Indian red, 20 pounds floated silex, 3 pounds whiting, ground in 32 pounds raw linseed oil, thinned with 5 gallons raw linseed oil, 1 gallon hard gum japan and 1 gallon turpentine, producing 14 gallons of a rich brown paint, weighing 11 pounds per gallon. Same remarks as to substituting hard gum varnish for the oil apply here.
Metal Preservative Green of the bronze green type is best made by grinding a paste base as follows: - Ten pounds bone black, powdered, 15 pounds medium chrome yellow, 3 pounds Chinese blue, well oxidized, 5 pounds litharge, 5 pounds zinc oxide, 30 pounds floated silex or silica, 32 pounds raw linseed oil, thinning with 5 1/2 gallons raw linseed oil, 1 gallon hard gum japan and 1 gallon turpentine, producing 14 1/2 gallons rich green paint, weighing about 10 3/4 pounds per gallon. Same final remarks as to the varnish substitution.
A few remarks in connection with metal preservative paints will not be amiss. A paint for metal must necessarily lay down closer to the surface than is the case on wood, where the pores absorb excessive oil, while on iron and particularly on steel the excess in oil must necessarily harden by absorbing oxygen from the air. Hence it is necessary for the paint maker and seller to do all he can towards educating consumers to handle paints on metal more carefully than those for wood and in labeling these paints, cautions should be embodied in the directions on the packages. By doing so the paint maker may save himself a vast deal of annoyance.