Scientific American 7, 13.2.1869
Prepared for the Scientific American.
The waste material of the beating of metals (an art which took its rise in the fourteenth century, in Nuremberg, Germany) was thrown away till 1750. In that year a mason in Fuerth, by the name of Huber, conceived the fortunate idea to grind this material called "Schabig" on a stone, and to sell the metallic powder thus abtained as a color. The goldbeater Marting Helsinger succeeded subsequently in imparting to the powder various lusters by exposing it to different degrees of heat; and in 1781, Courrier, a Frenchman, discovered the mode of preparing gold bronze from leaces, consisting of an alloy of zinc and copper. Although this bronze powder was offered for one florin (fifty-one cents, currency) per pound, it was but little in demand; but since preparation of various colors, from red down to nearly white, is no longer a secret, the manufacture of bronze powders has attained considerable importance, and is now practiced in several towns in Bavaria and Westphalia, and in the capitals of France and England. The refuse of goldbeating being no longer sufficient, special alloys are flattened. When in Fuerth, Bavaria, in 1864, we counted not less than fourteen bronze powder establishments. In Munich and Nuremberg the value of this article is said to reach yearly $255,000 in currency.
The process of flattening metals for the purpose of reducing them into powder is carried on in a manner similar to that of goldbeating. When obtained in a thickness sos as to permit the transmission of the rays of light, the leaves are rubbed through an iron sieve of exceedingly small holes by means of a wire brush, the powder thus produced is then allowed to pass through a mill under addition of some oil, and finally it is heated to a certain degree, according to the color desired.
Prof. Wagner, a chemist well known in this country, has ascertained that all bronze powders consist chiefly of a fatty matter, oxygen, copper, and iron. The composition used for light shades consists of 83 per cent of copper and 13 per cent zinc; for deep ones, of 94 to 90 copper, and 6 to 10 zinc; for copper red, pure copper is used. The amount of copper in various colour was found to be the following:
In French copper red, 97.32 per cent; orange, 94-44 per cent; light yellow, 81-29 per cent.
In English orange, 90-82 per cent; deep yellow, 82-37 per cent: light yellow. 80-42 per cent.
In German copper red, 98-92 per cent; violet 98-81 per cent; orange, 95-30 per cent; deep yellow, 81-55 per cent; lemon, 82-34 per cent.
Wagner discovered a small per centage of iron in the English bronzes, but tin, silver and nickel, or smalt, carmine and indigo, as often asserted, were not met with in any.
Recently various methods have been suggested in order to avoid the dividing of the metal leaves by means of a brush. They are partly founded on mechanical, partly on chemical principles. It was, for instance, attempted to prepare the powder by means of files, but it was discovered to be angular and without luster. When, however, passed through rollers, it gained its original luster again. In Germany, this method has not met with any approval, but it is said to be employed in England.
In 1850, Rostaing proposed to divide metals in their melted state by means of a centrifugal machine, and Fuchs announced that he succeeded in preparing bronze powder by amalgamation. The highly injurious effects of mercury vapors do, however, not allow the introduction of this latter method.
Copper powder may be prepared chemically in various ways which results in forming, with one single exception, crystalline and brittle products, which, in crushing, are converted into a dull powder. In reducing oxide of copper with rhigoline and gasoline, the two lightest products of the distillation of petroleum, Prof. Wagner, for the first time, obtained copper in minute scales. In conducting the process, it is necessary that the metal be left to cool in the vapors of these hydrocarbons. The bronze color is thus obtained in somewhat dark, but may perhaps be changed into brighter hues, by passing vapors of zinc or cadmium over them. In one instance where gasoline containing sulphur was used, the copper bronze exhibited a fine irisdescent appearance.
It is only within the last decade that various substitutes for the above described bronze powders have been brought to the noteice of consumers. We mention
1. The Tungsten bronzes. Of these the "tungstate of oxide of tungsten and soda" is the most important. It forms beautiful crystals of a golden-yellow color and gold luster. The potassa salt, discovered by Laurent, forms violet needles with copper lusters, and possesses great similarity with sublimed indigo. The lithian salt appears in prismatic scales and leaves of the color of slightly tempered steel. In glowing the potassa salt, a brilliant dark blue steel color may be obtained. The tungsten, or wolframium bronzes first appeared at the World's Fair in London, in 1862, and they then attracted considerable attention. The soda compoun appeared under the denomination of saffron bronze, the potassa compound under that of magenta bronze. At the exhibition at Paris, in 1867, these bronzes were only present in small quantities. The reason for this fact is stated by Prof. A. W. Hofman as follows:
"It appears, that in order to cover well, and reflect the light with intensity, it is necessary that the smallest particles of the bronze powders should possess the porperty to split in lamellæ. If their crystalline structure shows this glimmerlike character, their covering capacity remains the same when reduced to a finer state. If these bodies, however, crystallize in cubes, they are in being crushed, not reduced into lamellae but again in cubes. A certain quantity of such a powder covers a much smaller surface, than an equal weight of bronzes consisting of scales. They also reflect the light not in the same degree as purely metallic bronzes."
2. The tin bronze, or Mosaic gold. The variety may, as regards it brilliancy, well compete with the lighter bronze colors. It is also more durable. Kletzinski proposes to prepare it, by subliming the amorphous sulphide of tin, which is obtained in boiling a tin-salt solution with dilute oil of vitriol and saturating the liquid with the gas of burning sulphur. The sulphid of titanoim also deserves attention; it forms scales of a brass color.
3. Chromium bronze, or chloride of chromium, forms brillian violet follæ, which, in transmitted light, appear blood red. It may be rubbed into the skin like all bronzes.
4. Crystallized iodide of lead, a beautiful yellow substance, is proposed for decorative purposes; gold-inks, shell-colors, as a mass for pencils, for the painting of fabrics, wall paper, for filling glass pearls, etc.
5. Organic bronze colors. To these belong the derivates of the haematoxylin, already extensively employed in the manufacture of bronze paper, the numerous tar-pigments, of which the corallin is one of the most recent discoveries, the murexide and the green hydrochinon.