Another fact, and one of practical im portance to the manufacturer, is that an alloy, when remelted, even if composed of only two metals, will not exhibit the same characteristics as at first. A malleable alloy may become brittle, a ductile alloy unworkable, etc. This metamorphism is even more marked when the remehed alloy is composed of several metals. The ordinary remedy employed for this defect is to add more raw metal; but this has to be done with great care as to proportions, which can only be ascertained by experiment. Skill in the remelting of scrap often constitutes a valuable trade secret, such as for a long period was the case with German silver, tombac, Jem mapes brass and other then popular alloys.
Fusion of metals to make an alloy is not the only method to produce such a result. Another method much in use, especially in the fine arts, is the electrolytic method. The metals to be combined are placed in the form of water solutions in the electrolyte and the current plates then in alloy upon the ob jects attached to the cathode. In this way jewelers produce the "red gold," "green and "gold" of other intermediate tints in al loys of gold, silver and copper from an elec trolytic bath. Other means of producing al loys are by pressure upon the mixed powders of the metals, or by the intimate mixtures of their vapors, but these are employed only in laboratory investigations. Gold and quick silver will coalesce and form a natural alloy or amalgam at ordinary temperatures the moment they touch each other, a fact of the highest importance in gold mining. Discovery of this circumstance and its employment as the process)) is commonly credited to a Spanish miner named Bartolomeo Medina of the 16th century (1557). In point of fact, it was known to the Arabs, Romans and Greeks, indeed is of the highest antiquity. It was commonly employed as a means of recov ering gold from auriferous gravel or sand by the Arabian miners and metallurgists of the Middle Ages; the source whence both the Span iards and Portuguese got it. There exist un questionable evidences of its employment in Spain and Portugal centuries before Medina's time. These evidences are brought together in a 'History of the Precious Metals' (New York 1904, p. 134). Medina's claim was in fact a claim on behalf of the Fuggers, who farmed and monopolized the Almaden quicksilver mines of Spain, and thus held the American gold and silver mines in subjection. The process which may have been discovered by Medina is the "patio process," a complicated metallurgy, in which the amalgamation of gold and quick silver plays but a subordinate part.
To Make a Metallic Alloy.— The first con sideration is the temperature at which the metals will melt in the furnace. This is as follows for various metals, in degrees Centi grade: Mercury (quicksilver), —39.4; phos phorus, 44; sulphur, 114.5; tin, 231.68; bismuth, 268.3; tellurium, 282; cadmium, 320; lead, 330 to 335; zinc, 419; arsenic, 450; antimony, 629.5;
magnesium, 632.7; aluminum, 654.5; silver, 960.5; gold, 1061.7; copper, 1080.5; silicon, 1100 to 1300; nickel, 1400 to 1450; iron, 1550 to 1600; platinum, 1775, and manganese, 1800 to 1900. Ordinarily when the melting points of the con stituents of the proposed alloy are nearly equal, the alloy is made by fusing first the metal which has the higher melting point and adding the other to the molten mass. When the melt ing points are widely apart a difficulty may be encountered in the vaporizing of one of the constituent metals at the fusing point of the other. In this case it becomes necessary to form preliminary alloys which are afterward combined. As an illustration, German silver may be instanced. This is an alloy of nickel, copper and zinc. The nickel melts at at which temperature zinc would turn to vapor. So an alloy of nickel and copper is first made and then a brass of copper and zinc. The copper-nickel alloy has a much lower melting point than pure nickel and the brass has a higher melting point than zinc. The two pre liminary alloys are then fused together with out the loss of the zinc. Oxidation during the fusing process is a serious obstacle in pro ducing some alloys. This may be obviated by the use of a flux, like borax; or by metallic deoxidizers, like powdered aluminum, which not only reduce the oxides but increase the fluidity of the mass and therefore the intimacy of the mixture. Great care is needed also in the pouring (casting) of the alloy at just the right temperature. If the heat be too great the structure will be coarse-grained; if the degree is too low the less fusible constituents are liable to crystallize out and the ingot be un sound and crack under working. The rate at which the cooling after pouring is permitted to take place is also of prime importance. Slow cooling produces a weak alloy of coarse structure; quick cooling yields a strong alloy of fine-grained structure but inclined to be brittle. The molds employed must be of ma terial suited to the prospective alloy. They are usually of iron, brass, iron and sand, sand and clay, or plaster of Paris. Most alloys are subjected to after-treatment with heat and cold, in annealing, chilling, tempering, etc., and by mechanical processes, as hammering, rolling, etc. A remarkable circumstance con cerning alloys is that a very small proportion of another element will often entirely alter the character of a metallic composition. One part of carbon in 2000 of iron will convert it into steel; the same minute proportion of tellurium in bismuth converts it into a finely crystalline metal; one part of phosphorus in copper will make it hot-short; while the same almost negli gible proportion of bismuth will destroy its usefulness for many purposes for which, when rid of this intrusion, it is eminently fitted.