Nor should a carriage lead be ground in too large a percentage of oil. For coats between priming and color but comparatively little oil is needed, and washing out with benzine or turpentine entails an unnecessary amount of labor. Hence, it should be insisted upon that carriage painters' lead be ground moderately stiff in oil, so that protracted washing-out may be avoided on the one hand, and extended mixing and breaking-up operations shunned on the other. A practical and, at the same time, a conclusive test of fineness is furnished by taking two pieces of plate glass 8×8 inches in size, setting them securely in blocks of wood, and then smearing a couple of small flakes of the lead, rubbing the pieces of glass together. Continue rubbing with a firm, even pressure until a uniform distribution of the pigment and a thorough impact is established. The glasses should then disclose the nature of the grinding. To learn the drying power of the lead, take the palette knife and slick a small quantity over the glass and set aside, noting the time consumed in drying. A lead ground in the proper proportion of oil for carriage work should, as taken from the keg and smeared in a thin film over the glass, dry in twelve hours so that the finger may be passed over it without sticking.
What has here been said in reference to chemical purity or strictly pure as a necessity in the white lead product does not apply to all the pigments so useful to the vehicle painter. For reasons here shown lead extenders and lead compounds should be emphatically objected to. A disavowal of their worth as carriage painting pigments, however, in no wise lessens the significance of the fact, as already pointed out, that a strictly pure lead is very often an expensive, if, indeed, it be not a worthless, lead to buy. Chemically pure is not invariably an accurate gauge of quality. A chemically pure lead that has not fineness to recommend it lacks an essentially vital quality. In respect to the pigments and colors following in the wake of white lead it has been made plain on many a hard-fought field of experiment that the color consumer, the practical painter, the workman far removed from the analytic gentleman of the laboratories, is chiefly concerned in getting a pigment or color adapted to his needs more completely than any other available one. It may not be chemically pure as the chemists would construe the term; but if it responds satisfactorily to a practical test, it is then serving the painter's practical need. As declared by the writer, in an article published some time ago, "a color or pigment may be pure in the sense that it is not adulterated, and still fall short of being chemically pure. It is the duty of the consumer to avoid buying, under the label 'strictly pure,' an adulterated color. The real color contained in such a product is then costing him considerably more than would a color in a state of purity." The chemist and the practical painter do not agree oftentimes upon what may be called adulterants. Once upon a time, as the fairy books say, at a painters' convention the chemist employed to make an analysis of chrome yellow stated in substance that practically everything outside of the chromate of lead should be classed as an adulterant or as a matter out of place. The practical painter who has looked up the subject of chrome yellow manufacture could tell the chemist in this case that he has signally failed to take into consideration the necessary constituents of the different shades of chrome yellow. As, for example, acetate and nitrate of lead, bichromate of potash and bichromate of soda, sulphate of soda, etc., are constituents of a pure chromate of lead. And our friend, the chemist, would tell us that a chromate of lead composed of some of the above ingredients is not a chemically pure article. What the carriage painter, the consumer, will find it of value to ask himself is this: Does a given pigment or color suit the requirements of my business? If in doubt as to the utility of the given pigment or color, then an immediate practical test should be resorted to. It is not the purpose of the writer to belittle the position or the usefulness of the chemist. The value of a chemical analysis in the detection of adulteration and in explaining how a color is made is cheerfully acknowledged; but after the chemist's deduction must follow the practical test. In conducting a practical test the foremost aim of the painter should be to consider the color or pigment to be tested in relation to the object for which it is intended. Shade, brilliancy, working property, durability, etc., are entitled to a careful and chief consideration in a test for quality. And a test for quality, if conducted painstakingly and thoroughly, will disclose the real value of the material to the consumer. When extenders are added to a pigment for the sole purpose of enriching the manufacturer at the expense of the consumer, the practice becomes adulteration, pure and simple. If, however, such extenders are used to, and actually do, increase a pigment's usefulness, fortifying it in a way and to an extent that it needs to be fortified, the painter will not attempt to question its commercial value.
The study of the pigments which the vehicle painter calls to his uses is a feature of business deserving the most rigid attention. Carried on watchfully and with a vigilant regard for details, it cannot well fail to increase paint shop profits.
LIQUID MATERIALS.
In the consumption of liquid materials the vehicle painter has no use for extenders. Unfortunately, however, the thrifty and shifty sons of adulteration, after the manner of Marco Bozzaris, in the Fourth Reader, are struggling, tooth and nail, to adulterate linseed oil and the turpentine product in a way to defy detection.
What the cathode ray is to a certain branch of science, pure raw linseed oil is to carriage and wagon painting. Back in a somewhat indefinite period of the past, linseed oil pre-empted the chief claim in the domain of paint and varnish, and its right to a royal office in that domain has never yet been successfully disputed, notwithstanding the fact that a flood of substitutes and counterfeits have been turned loose upon the market. In the language of another, "Raw linseed oil is the king of the paint realm. There are lots of usurpers in the field but they are short lived. The true homage of the brotherhood of the brush continues to be paid to the old stand-by. It is the gold of the paint shop currency."
In the basic stage of carriage and wagon painting, pure raw linseed oil is conceded to be the life of the pigment. Impure or adulterated linseed oil—the spurious, fraudulent article, if you please—has more to do with the premature decay of paint and varnish than one at first thought might concede. During the process of painting there are numerous complications which, by the harsh reality of scientific analysis, could be directly traced to the insidious effects of an adulterated brand of oil. Investigations conducted by competent experts have shown that the self-assertiveness of adulterated oil is determined, not so much by apparent unfavorable effects upon the under coats, but rather from its resistless attack upon the lustre and durability of the finishing varnish. Some of the oils used to adulterate linseed oil are pronounced by such authorities as Hurst and Terry to be good driers, although, as in the case of rosin oil, they may seemingly dry good upon the surface only to soften up later on. And provided these adulterant oils are not good driers, the people engaged in floating them along the avenues of trade have simply to add a certain proportion of drying japan to O. K. them in this respect.
The vehicle painter's practice of using raw linseed oil insures him somewhat against oil adulteration, as it is much more difficult to adulterate the raw linseed product than the boiled and have the fraud go undetected. A raw linseed oil when fresh and new is of a bright yellowish-green color, and as it grows older it becomes paler in color and perhaps a little brighter. When spread on a surface in a thin film and exposed to a pure dry air it will harden quite solidly in from forty-five to fifty hours. It ranks as reliable drying oil, promptly solidifying when acted upon by peroxide of hydrogen or by subnitrate of mercury. A non-drying oil refuses to show a change of this kind. Combining powerfully with oxygen, it offers, when dry, a stronger resinous character than any other oil.
Probably the chief adulterants of linseed oil should be listed as rosin, mineral, and fish oil; cottonseed oil being looked upon with less favor than formerly, while hempseed oil, owing to its pronounced tendency to change color, is not much in evidence at present.
Rosin oil is strictly an unreliable drier. It toughens the working property of paint and is deficient in all the essentials which should distinguish a good paint oil. Its low flash point, as indicated by Hurst,—300° to 330° F.—together with its strong rosin odor when heated, would appear to make it an easily-detected adulterant. Deodorizing processes have of late served to fortify this oil, and fish oil as well, against detection by the sense of smell. Fish oil, chiefly the product of the menhaden fishing industry flourishing so vigorously along the Atlantic coast, has naturally an offensively fishy odor, particularly when heated. Its main recommendations as a linseed oil adulterant are tersely summed up by Terry as follows: The rapidity with which it oxidizes, and its good body, render it not unsuitable as a vehicle for paint.