PREPARATION AND PROPERTIES OF ALLOYS

§ 20. The mode of procedure in the production of any alloy will be largely influenced by the nature of the metals to be operated upon. Some metals are volatile, and readily pass off as vapour when heated a few degrees above their melting points. Others have little tendency to vaporise, and may be raised to high temperatures without sensible volatilisation. When a volatile metal has to be alloyed with a non-volatile metal, and the fusing points of both are approximately the same, combination can be most readily effected by mixing the constituents and melting them together in the same crucible or furnace. This is, however, seldom the case, and, as a general rule, the components of an alloy, one or all of which are volatile, have widely divergent melting points, and then it is requisite for the most refractory constituent to be melted first, and for the others to be added in the solid state. Again, an alloy may contain one or more fixed metals and a volatile one, in which case the more volatile metal is added to the crucible, after the fixed metal or metals have been fused, and raised to a temperature necessary to melt the volatile constituent immediately it is introduced, so that combination may be effected before any serious loss, due to vaporisation, has occurred. Union between the components of an alloy is more perfectly secured by agitation of the contents with a stirring-rod, the most effective in many cases being a wooden or carbon rod, which promotes admixture without the introduction of any substance likely to contaminate the mixture, and modify its properties.

A thing to be guarded against in the melting of all base metals, or alloys containing base metals as essential constituents, is oxidation. Various plans are adopted to avoid loss of metal and injury to the alloy from this cause. The most common one is to cover the metals with carbon, which not only excludes the air admitted to the furnace, but tends to absorb any oxygen liberated from the metals during

fusion. The gas thus formed by union of carbon with oxygen is termed carbonic oxide CO, and this gas being a reducing agent, is capable of taking up another atom of oxygen, forming carbonic acid CO2. Thus, as long as the mixture is covered with carbon, the carbonic oxide formed effectually shields it from oxidation. In the method already referred to of stirring metals with a carbon rod to promote mixture, the same gas, carbonic oxide, is formed, and thus the rod not only promotes union by mechanical agitation, but generates a gas which protects the metals in a great measure from oxidation. In some cases this is not admissible, as commercial metals are impure, and it may be advisable to admit sufficient oxygen, either from the air or by means of a special oxidising agent, added along with the flux, to convert the impurities into oxides, which do not alloy with the metals, but either enter into combination with the flux to form a slag, or rise to the surface as dross or scum. In most cases it is advisable that the covering body should not exert any influence on the metals beneath.

Some manufacturers are in the habit of throwing fat and resin on the heated metals before fusion. These are decomposed by the heat, liberating gases, and when well stirred with the molten metal promote combination by the mechanical agitation imparted by their escape. They also act chemically in removing oxygen, by the union of that element with the carbon and hydrogen set free. When the evolution of gas has ceased, a quantity of carbon remains in a finely divided state, which covers the metals and protects them from oxidation.

Borax is sometimes used to exclude the air, but it is much more costly than carbon, and when it is not required as a flux, its employment is accompanied with some evils. Now borax is composed of the base soda in combination with boric acid, which is only partly saturated with the soda, and the excess of acid unites with any metallic oxide present forming double borates, of a glassy nature. Commercial borax is often very impure, and is adulterated with common

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salt and alum; these impurities are injurious to many metals. Sodium chloride or common salt is also employed for preserving molten metals from oxidation, and also to moderate the action of bodies which cause violent ebullition. Glass is frequently used for a similar purpose, and next to carbon is the least injurious to metals. It is a mixture of silicates, which easily fuses at high temperatures, forming compounds with lime and other bases, so that it acts almost as beneficially as borax when such flux is required. Window glass or green bottle glass is the most useful, but flint glass, which contains much oxide of lead, would be detrimental in many

cases.

The nature of metallic alloys has already been discussed, from which we may assume that certain proportions of the constituents enter into chemical combination, and other portions are simply in a state of mixture or solution, and therefore, on gradually cooling, tend to separate in distinct layers according to their respective densities. This is especially the case when the constituents have widely divergent densities, so that the higher the temperature of the alloy when removed from the furnace the longer will the period of cooling last, and the greater will be the facilities offered for separation. To obviate this defect the metal should be constantly agitated by stirring or otherwise, and poured into the moulds at the lowest temperature consistent with the requisite fluidity; and cooled as rapidly as the nature of the alloy and the purposes for which it is designed will admit. With regard to the melting point of an alloy, it should be borne in mind that it fuses at a lower temperature than that at which the most refractory constituent melts, and sometimes below that of either, which knowledge should guide the operator in so regulating the temperature as not to make the charge unnecessarily hot.

It is a well-known fact that the character of many alloys is altered by repeated remelting, and that the scrap obtained in working cannot be used again without the addition of a certain quantity of new metal. A given mixture may be

employed for the formation of an alloy, which is highly malleable, ductile, and tenacious, and the scrap from the same alloy when remelted may be brittle and unworkable; but when a suitable quantity of new metal is added, the combination may form an alloy even superior to the original one with regard to its good working properties. It is to the advantage of the manufacturer as regards economy, to use as much scrap as possible in alloying, and the quantity thus employed varies from one-third to two-thirds of the weight of the charge. Of course, in using old metal many more impurities are liable to be introduced than with new metal, and although the same impurities may exist in the new metal, the quantities may be insufficient to produce a deteriorating effect, but when augmented from old metal may then rise to such proportions as to entirely alter the physical properties of the alloy. The presence of notable quantities of foreign matter is generally exhibited by increased hardness and a modification of the structure, as seen on a freshly fractured surface.

The difficulty of maintaining uniformity in an alloy after repeated remelting is least when only two metals are mixed together, and increases when the combination requires the presence of three or more metals. Thus German silver requires much greater care in this respect than brass; and soft solder, containing only lead and tin, requires less care than fusible alloy, containing bismuth or cadmium in addition to lead and tin. Those alloys, which contain as an essential constituent a volatile metal, such as zinc or antimony, are generally altered most by remelting, and it is requisite to know, at any rate approximately, what the furnace loss is, so that the defection may be counterbalanced by the addition of the quantity of fresh metal requisite to maintain the right composition. Many errors arise from this cause, as well as from overdoing what is required. Where possible, a chemical analysis is the best means of solving the problem, but as this is out of the question in most cases, a few simple trials with weighed quantities, and careful observations of the results