kept covered with coal-dust, is then heated as strongly as possible, and when thinly fluid, a sample is taken to test its qualities.

The metals during melting always absorb oxygen, which is liable to produce blow-holes when the alloy is cast. The nature of the alloy may be ascertained by taking a small quantity as a test, and if this exhibits unsoundness, some makers push a fire-clay pipe to the bottom of the crucible, and drop some pitch through it to the bottom of the metal, and then remove the tube. The pitch is decomposed, giving rise to reducing gases which combine with the oxygen in the metal, and by vigorous stirring before pouring, are removed.

It need hardly be stated that the distinctive methods of working, just described as the German and English methods, are not generally carried out at the present time, a modification being found more rapid, economical, and attended with equally good results.

The separate metals entering into the composition of German silver are not generally used in the free state, but are previously made into binary alloys. Thus, nickel is alloyed with copper by some manufacturers in the proportion of 2 parts by weight of copper to 1 part of nickel, while others use equal weights of these metals. Zinc is used in the form of brass, common mixtures consisting of equal weights of copper and zinc, and 2 parts zinc to 1 of copper.

The crystalline nature of German silver when cast in iron moulds renders the metal somewhat difficult of mechanical manipulation, especially when the amount of nickel present is high, but this may be overcome by careful annealing at a moderate temperature, and by judicious hammering. Of course there are certain proportions which give better results than others, as shown by the author's experiments on the subject already enumerated. The crystalline structure gradually disappears as the plates are reduced by rolling, with occasional annealing as the process proceeds, and when the strips have been rolled sufficiently thin for ordinary stamping

purposes, the cause of brittleness has been practically eliminated, so that the alloy can be worked into any desired form.

It has been found in practice that certain varieties of German silver become more homogeneous by remelting, and can be worked with greater facility, but this is unnecessary if suitable proportions are selected at the commencement, and copper-nickel alloys with copper-zinc alloys used for melting together to form the desired alloy, instead of using the separate metals. It must also be remembered that each time German silver is remelted in a crucible in the ordinary way, a certain amount of oxide is formed, and a greater portion of the zinc than the copper or nickel is volatilised, so that the relative amounts of the constituents are altered. In the case of remelting it is necessary to add a portion of metallic zinc to compensate for the loss, and it is advisable to add this zinc after the fusion of the alloy has been effected. There is reason to believe that the zinc thus added assists in purifying the metal, by uniting with the absorbed oxygen. Whether this is so or not, the author has proved that such an addition is beneficial in many cases, if not in all.

Great care is required in casting the alloy to avoid chilling of the metal, and as a high temperature is requisite to keep the metal sufficiently liquid for pouring into the moulds, the casting shop is kept closed and all spaces which ordinarily admit cold air are stopped up. All the precautions with regard to blacking and heating of the moulds, etc., as described when speaking of brass strip-casting and moulding, apply also to German silver. But as German silver is more easily chilled than brass, it may be necessary, after one mould is filled, to re-heat the remaining portion of metal in the crucible before pouring it into a second mould.

The moulds used for German silver strip-casting are of the same shape as those described for brass-strips, but differ in size. The running sizes of plate-ingots are 16 to 18 inches long, 4, 4, and 5 inches wide, and 1 to 14 inches thick.

The

wire-ingots are about 4 feet 6 inches to 5 feet long, 31 inches wide, and 11 inches thick.

GERMAN SILVER SOLDERS

§ 94. Hard solders, employed for joining the parts of German silver articles together, are generally made of the same metals as those which compose the alloy to be soldered, but in such proportions as to have a lower melting point. In general, the soldering is more perfect the nearer the fusing point of the solder approaches that of the metal of which the article is composed, but the greater is the care required to avoid melting it. In some cases silver solder is used for German silver articles; and German silver solder is also used for soldering articles of iron and steel, on account of its high melting point and great tenacity.

German silver solder is known by different names, such as "argentan solder," "arguzoid,' etc. It is rendered moderately fusible by the addition of a large proportion of zinc to the copper and nickel. The mode of manufacture is similar to that already described for brass solder, and the proportions of the ingredients will depend upon the composition of the alloy which it is required to solder. For the higher alloys, i.e. those rich in nickel, a more refractory solder is advisable than with the cheaper and more fusible alloys.

In making argentan solder the copper and nickel should be melted first, and the zinc added in the free state, or in the form of brass, containing much zinc. The mixture is then cast in thin plates, which are broken into pieces while hot, and crushed to powder in an iron mortar. The facility with which it can be pulverised will depend upon the proportion of zinc. If it is too brittle it indicates too much zinc, and even if somewhat malleable, too much zinc may be present; in either case the defect may be remedied by adding the fresh metal required and remelting. Excess of zinc may be removed by remelting, when some of that metal will

burn off, but this is of course wasteful and expensive. The right composition can always be determined by taking a small quantity and testing it before pouring the main portion. The colour is grayish - white, with a strong lustre.

In order to test the best proportions for soldering articles containing 16 to 22 per cent of nickel, the author obtained a sample of the solder used by a large manufacturer for the above alloys, which, on analysis, gave the following proportions :

The three samples were pulverised whilst hot, and tested by a workman accustomed to use the solder of which the analysis is given above. He considered No. II the best for soldering, but preferred the one he had been accustomed to use. No. III was pronounced porous, from which it was inferred that 57 per cent zinc was too much.

CHAPTER VI

ALLOYS OF TIN

§ 95. Tin and Zinc.-Alloys of these metals can be readily produced by fusion, forming combinations that are generally harder and less malleable than tin, softer than zinc, and more or less crystalline in structure. The colour of the fractured surface depends upon the nature of the mould and the temperature of the alloy at the time of casting. The same observations also apply to the shrinkage upon solidification. Tin-zinc alloys are chiefly employed for casting ornamental objects, and patterns.

The following investigation into the character of tin-zinc alloys was made by Guettier 1 :—

"1. Tin 30, Zinc 70.-Texture of a dull-white colour; an average shrinkage; breaks easily; on fracture shows larger and brighter facets than zinc; the metal is denser at the bottom of the mould; dry to the file; breaks under the chipping chisel; slightly sonorous; and shows an appearance of crystallisation at the surface, with a slight bluish-yellow colour.

"2. Tin 25, Zinc 75.-Texture of a white colour, inclining to blue; slight shrinkage of the bar; bright fracture with large bluish facets like those of zinc; the tin seems to be in larger proportion at the bottom of the button, the same as in No. 1; the surface is covered with a kind of skin, rather

1 Guettier, Guide Pratique des Alliages, 1865.