Silver.

Unless alloyed with a large amount of copper, in which case they show green efflorescences similar to those of bronzes, silver objects are almost always covered with a layer of soft silver chloride (horn-silver) of varying thickness, AgCl, or of the harder silver subchloride, Ag₂Cl; and when these compounds form a thick layer, they often show a warty or more rarely a cracked surface. If the layer of chloride is thin, incised designs upon the silver will be visible both before and after removal of the chloride. The two chlorine compounds frequently appear together in distinct sharply defined layers of different colours, that nearer the silver being the layer of subchloride. This is especially well shown on fragments of silver from the Hildesheim silver-find[69]. Upon one fragment[70] the layer of silver chloride was about twice as thick as that of the silver subchloride. Being unable to separate them I determined the silver and the chlorine of both layers together with the following result:

Silver 74·52. Chlorine 21·90.

Now for 2AgCl, Ag₂Cl 74·52 silver would correspond to 18·11 chlorine only, while for AgCl the proportions would be 74·52 silver to 24·15 chlorine. Since the specific weight of silver subchloride is greater than that of silver chloride, these figures prove that the subchloride is also present.

Between the metal and the silver chloride there is often a thin powdery layer consisting of finely divided cupric oxide, or silver sulphide, and occasionally of gold, if, as is frequently the case, the silver is auriferous. The presence of gold may, however, also point to the existence of gilding. The silver chloride often shows a reddish or brown colour on the surface, due probably, in some cases, to the adherence of minute quantities of the earth in which it was found, but partly also to the action of light upon the silver chloride.

Thin black layers upon silver, as also the so-called silver tarnish, result from the formation of silver sulphide, from contact with decaying organic substances which have contained sulphur.

When placed in museums silver objects remain unaltered, and no further chemical changes take place.

Any other changes which have been observed will be gathered from the following extracts.

Church[71] analysed a specimen of silver upon which two layers were distinguishable. The outer semi-metallic layer consisted of metallic silver, with traces of chloride, sulphide, and iodide of silver, together with copper carbonate and a small quantity of gold; the inner layer, which was soft, grey and powdery, had the following composition:

Silver	94·69%
Gold	0·41%
Copper	3·48%
Lead	0·28%
Antimony with traces of arsenic and bismuth	1·21%

As the composition of the sound metallic core was identical, it is evident that physical and molecular changes only had taken place similar to those observed by Warrington[72] as early as 1843.

Silver objects found in Mycene are said by Mitzopulos[73] to show three layers, the outermost of which has a red colour and is not markedly friable, consisting of silver oxide; the second is tough and consists of silver chloride (horn-silver); while the third, that next to the metal, is similar to the outermost layer. Mitzopulos thinks that the chlorine must have been brought by rain water, since there are neither sea nor springs of water in the neighbourhood.

Schertel[ [74] distinguished two layers in fragments of silver from the Hildesheim silver find, the outermost of which proved to be silver chloride:

Silver	75·43% found, 75·31% calculated for AgCl
Chlorine	24·51% found, 24·69% calculated for AgCl

Beneath this layer was a very thin, almost black, brittle layer of silver subchloride:

Silver	87·0% found, 85·89% calculated
Chlorine	12·8% found, 14·11% calculated

Between the metal and the latter layer was a small quantity of dark powder, which Schertel recognized as gold. He thinks that the layer of silver subchloride seems to indicate that the water, which permeated the surrounding clay, contained chlorides, and first converted the copper into copper chloride; that the copper chloride together with the silver then formed silver subchloride and cuprous chloride. Should the subchloride again become chloride, it would be able to attack the silver afresh. The slowness of the process, when the silver and copper in association with it had been converted into chlorine compounds, allowed the gold to be deposited as a fine powder upon the intact metal.

A silver coin rolled out into a thin plate, after remaining in a solution of common salt for six months, was found to have lost 27·7% of its copper, so that the plate became brittle, especially in those parts where it was thinnest.

Bibra[75] gives a similar explanation of the conversion into silver chloride. He believes that the reddish colour which is occasionally seen on silver at a fresh fracture must be due to the presence of cuprous oxide.

The following extract is taken from the section which deals with silver in the work of Berthelot[76] previously quoted:

“Silver chloride is for the most part produced by the sodium chloride dissolved in the subsoil water, which acts in conjunction with the oxygen and the carbonic acid of the air:

2Ag + O + (n + 2)NaCl + CO₂ = 2AgCl, nNaCl + Na₂CO₃.

But this reaction differs from that which takes place in the case of copper in that it does not proceed continuously except in the presence of a considerable quantity of salt water only, as for instance in the sea. In museums the alteration goes no further than corresponds to the minute quantity of sodium chloride contained in the object. On the other hand in an earth which contains salts, the continued presence of water can bring about a more or less marked change, and in some cases even a stable silver subchloride may be formed.”