In concluding this slight view of some of the general characters of alloys, it remains to consider the influence of heat, both as an agent in their formation, and as regards the degree in which it is required for their after-fusion; the lowest available temperature being the most desirable in every such case.
"Metals do not combine with each other," says Dr. Turner, "in their solid state, owing to the influence of chemical affinity being counteracted by the force of cohesion. It is necessary to liquefy at least one of them, in which case they always unite, provided their mutual attraction is energetic. Thus, brass is formed when pieces of copper are put into melted zinc; and gold unites with mercury at common temperatures by mere contact." *
The agency of mercury in bringing about triple combinations of the metals, both with and without heat, is also very curious and extensive. Thus, in water-gilding, the silver, copper, or gilding metal, when chemically clean, are rubbed over with an amalgam of gold containing about eight parts of mercury; this immediately attaches itself, and it is only necessary to evaporate the mercury, which requires a very moderate heat, and the gold is left behind. water-silvering is similarly accomplished.
* Turner's Chemistry, p. 559.
Cast-iron, wrought-iron, and steel, as well as copper and many other metals, may be tinned in a similar manner. An amalgam Of tin and mercury is made so as to be soft and just friable; the metal to be tinned is thoroughly cleaned either by filing or turning, or if only tarnished by exposure, it is cleaned with a piece of emery-paper or otherwise, without oil, and then rubbed with a thick doth moistened with a few drops of muriatic acid. A little of the amalgam then rubbed on with the same rag, thoroughly coats the cleaned parts of the metal by a process which is described as cold-tinning; other pieces of metal may be attached to the tinned parts by the ordinary process of tin-soldering.
In making the tinned-iron plates, the scoured and cleaned iron plates are immersed in a bath of pure melted tin, covered with pure tallow; the tin then unites with every part of the surfaces; and in the ordinary practice of tinning culinary vessels of copper, pure tin is also used. The two metals, however, must then be raised to the melting heat of tin; but the presence of a little mercury enables the process to be executed at the atmospheric temperature, as above explained.
In Mr. Mallett's recently patented "processes for the pro-tection of iron from oxidation and corrosion, and for the prevention of the fouling of ships," one proceeding consists in covering the iron with zinc.
The ribs or plates for iron ships are immersed in a "cleansing hath" of equal parts of sulphuric or muriatic acid and water, used warm; the works are then hammered, and scrubbed with emery or sand, to detach the scales and to thoroughly clean them; they arc then immersed in a "preparing bath" of equal parts of saturated solutions of muriate of zinc and sal-ammoniac, from which the works are transferred to a fluid "metallic hath," consisting of 202 parts of mercury and 1292 parts of zinc, both by weight;* to every ton weight of which alloy, is added about one pound of either potassium or sodium, (the metallic bases of potass and soda,) the latter being preferred. As soon as the cleaned iron works have attained the melting heat of the triple alloy, they are removed, ha\ing become thoroughly coated with zinc
* Being in the proportion of one atom of mercury to forty atoms of zinc.
"The affinity of this alloy for iron is, however, so intense, and the peculiar circumstances of surface as induced upon the iron presented to it by the preparing bath are such, that care is requisite lest by too long an immersion the plates are not partially or wholly dissolved. Indeed where the articles to be covered are small, or their parts minute, such as wire, nails or small chain, it is necessary before immersing them to permit the triple alloy to dissolve or combine with some wrought-iron, in order that its affinity for iron may be partially satisfied and thus diminished. At the proper fusing temperature of this alloy which is about 680° Fahr., it will dissolve a plate of wrought-iron of an eighth of an inch thick in a few seconds." *
"The palladiumizing process. - The articles to be protected are to be first cleansed in the same way as in the case of zincing; namely, by means of the double salts of zinc and ammonia, or of manganese and ammonia; and then to be thinly coated over with palladium, applied in a state of amalgam with mercury." †
In the opinion of eminent chemists and metallurgists, all the metals, even the most refractory, which nearly or quite refuse to melt in the crucible when alone, will gradually run down when surrounded by some of the more fusible metals in the fluid state; in a manner similar to the solution of the metals in mercury, as in the amalgams, or the solutions of solid salts in water. The surfaces of the superior metals are, as it were, dissolved, washed down, or reduced to the state of alloys, layer by layer, until the entire mass is liquefied.
Thus nickel, although it barely fuses alone, enters into the composition of German silver by aid of the copper, and whilst it gives whiteness and hardness, it also renders the mixture less fusible. Platinum combines very readily with zinc, arsenic, and also with tin and other metals; so much so, that it is dangerous to melt either of those metals in a platinum spoon; or to solder platinum with common tin solder, which fuses at a very low temperature: although platinum is constantly soldered with fine gold, the melting point of which is very high in the scale.
+ Mechanics' Magazine, 1842, vol. 36, p. 41, where the details of this zincing process, and of the preparation of the varnishes and paints used for additional protection, are minutely transcribed from the specification of the patent.
Again, the circumstances that some of the fusible bismuth alloys melt below the temperature of boiling water, or at less than half the melting heat of tin, their most fusible ingredient, show that the points of fusion of alloys, arc equally as difficult of explanation or generalization as many other of the anomalous circumstances concerning them.
This much however may be safely advanced, that the alloys, without exception, are more easily fused than the superior metal of which they are composed; and extending the same view to the relative quantities of the components, it may be observed that the hard solders for the various metals and alloys, are in general made of the selfsame material which they are intended to join, but with small additions of the more fusible metals. The solder should be, as nearly as practicable, equal to the metal on which it is employed, in hardness, colour, and every property except fusibiliiy; in which it must excel just to an extent that, when ordinary care is used, will avoid the risk of melting at the same time, both the object to be soldered and likewise the softer alloy or solder by which it is intended to unite its parts.
It would appear as if every example of soldering in which a more fusible alloy is interposed, were also one of superficial alloying. Thus, when two pieces of iron arc united by copper, used as a solder, it seems to be a natural conclusion that each surface of the iron becomes alloyed with the copper; and that the two alloyed surfaces are held together from their particles having been fused in contact, and run into one film. It is much the same when brass or spelter solder is used, except that triple alloys are then formed at the surfaces of the iron, and so with most other instances of soldering.
And in cases where metallic surfaces are coated by other metals, the latter being at the time in a state of fusion, as in tinned-iron plates and silvered copper; may it not also be conceived. that between the two exterior surfaces which are doubtless the simple metals, a thin film of an alloy compounded of the two does in reality exist? And in those cases in which the coating is laid on by the aid of mercury, and without heat, the circumstances are very similar, as the fluidity of mercury is identical with the ordinary state of fusion of other metals, although the latter require higher temperatures than that of our atmosphere.
When portions of the same metal are united by partial fusion, and without solder, as in the process described as burning together, and more recently known as the "autogenous" mode of soldering, no alloy is formed, as the metals simply fuse together at their surfaces.*
Neither can it be supposed that any formation of alloy can occur, where the one metal is attached to the other by the act of burnishing on with heat, as in making gilt wire, but without a temperature sufficient to fuse either of the metals. The union in this case is probably mechanical, and caused by the respective particles or crystals of the one metal being forced into the pores of the other, and becoming attached by a species of entanglement, similar to that which may be conceived to exist throughout solid bodies. This process, almost more than any other in common use, requires that the metals should be perfectly or chemically clean; for which purpose they are scraped quite bright before they are burnished together, so that the junction may be next approaching to that of solids generally.
And, lastly, when metals are deposited upon other metals by chemical or electrical means, the addition frequently appears to be a detached sheath, and which is easily removed; indeed, unless the metal to be coated is chemically clean, and that various attendant circumstances are favourable, the sound and absolute union of the two does not always happen, even when carefully aimed at. †
It is time, however, that we proceed to the description of the methods of forming the ordinary alloys, the subject of the succeeding chapter.
* Delbruck's patent autogenous soldering process, which will be described in the chapter on soldering, is here alluded to.
† The table of the metals and alloys, pp. 285 - 286, contains many illustrations of these two pages; but it should be observed, the energies of the respective unions depend very considerably on the chemical affinities of the metals concerned.
The Parts I. and II. of Electrotype Manipulation, by Mr. Charles V. Walker, 1841, may be likewise consulted by those interested in electro-plating, gilding, and etching.