Silver oxide

“ Silver, however, on being exposed to heat in the same situation, cannot be converted into an oxide; it has no attractive power for oxygen. Hence, when this metal is contained in a bar of lead, the lead only is oxygenated on exposure in a cupel; whilst the silver remains unaltered, but constantly concentrating and sinking, till the lead is all calcined. This is known, to a practised eye, by the increased splendor assumed by the metal.
I do not think the ore of Mine à Burton contains a sufficient quantity of silver to render the separation an object. ”

“ At a very high temperature it is volatile. Melting silver mechanically absorbs 20 volumes of oxygen, which in solidifying it expels, sometimes with sufficient force to throw off particles of metal. Alloyed with 1 or 2 per cent. of copper or with gold, it apparently loses this property. Silver is oxidized neither by exposure at ordinary temperature to dry or moist air, nor by heating in air; but it burns to an oxide when melted upon charcoal in the oxyhydrogen flame, or when exposed to a galvanic current of great intensity, or to ozone. ”

“ After the lead is put into the cupel, it gets immediately covered with a coat of oxide, which resists the admission of the silver to be assayed into the melted metal; so that the alloy cannot form. When a bit of silver is laid on a lead bath in this predicament, we see it swim about for a long time without dissolving. In order to avoid this result, the silver is wrapped up in a bit of paper; and the carburetted hydrogen generated by its combustion, reduces the film of the lead oxide, gives the bath immediately a bright metallic lustre, and enables the two metals readily to combine. ”

“ Silver chloride may be decomposed, with the separation of silver oxide, by heating it with a solution of an alkali, and if an organic substance be added to the alkali the chloride can easily be reduced o metallic silver, the silver oxide being reduced in the oxidation of the organic substance. Iron, zinc, and many other metals reduce silver chloride in the presence of water. Cuprous and mercurous chlorides and many organic substances are also able to reduce the silver from chloride of silver. This shows the rather easy decomposability of the halogen compounds of silver. ”

“ If a piece of pure silver foil is exposed on a cupel to air at the usual temperature of cupellation, it undergoes very little change; it does not even fuse; it loses nothing in weight, and does not oxidise. In fact, even if oxide of silver were formed under these conditions, it could not continue to exist, for it is decomposed into silver and oxygen at a temperature considerably below redness. On the other hand, oxide of silver is not reduced to metal by heat alone, when mixed with an excess of oxide of lead ”

“ It also developed that the minimum silver oxide content decreased progressively as the number of components increased from 2 to 4. The standard catalyst (Hopcalite) finally adopted for production consisted of 50 per cent manganese dioxide, 30 per cent copper oxide, 15 per cent cobaltic oxide and 5 per cent silver oxide. The mixture was prepared by [Pg 193] precipitating and washing the first three oxides separately, and then precipitating the silver oxide in the mixed sludge. After washing, this sludge was run through a filter press, kneaded in a machine, the cake dried and ground to size. ”

“ Silver does not oxidise at any temperature, and its oxide, Ag2O, is decomposed by heat. It is also a very important fact that silver is not oxidised by oxygen either in the presence of alkalis, even at exceedingly high temperatures, or in the presence of acids—at least, of dilute acids—which properties render it a very important metal in chemical industry for the fusion of alkalis, and also for many purposes in everyday life ”

“ Under weathering conditions, the silver sulphide minerals in general are oxidized to form native silver and cerargyrite, which are relatively insoluble and remain for the most part in the oxide zone. Silver is less soluble than copper and zinc, but more soluble than gold; and to some extent it is removed in solution, particularly where the oxidation of pyrite forms ferric sulphate. ”

“ It is prevented by preserving the molten metal from contact with air by covering the surface with non-oxidizing agents, or by traces of copper, bismuth or zinc.
Chemical Properties.—Silver is not oxidized by oxygen, but resembles mercury in being oxidized by ozone. It has no action on water. It is readily soluble in dilute nitric acid, nitric oxide and silver nitrate being formed; it also dissolves in hot, strong sulphuric acid, sulphur dioxide being evolved. Hydrochloric acid forms a surface film of silver chloride ”

“ SILVER.—This metal, when fused alone, and kept in this state for some time, under a strong oxidizing flame, covers the charcoal with a thin film of dark reddish-brown oxide. If the silver be alloyed with lead, a yellow incrustation of the oxide of that metal is first formed, and afterwards, as the silver becomes more pure, a dark red deposit is formed on the charcoal beyond. If the silver contains a small quantity of antimony, a white incrustation of antimonious acid is formed, which becomes red on the surface if the blast be continued. ”

“ Consequently, if the lead rests on a porous support, which allows the fused litharge to drain away as fast as it is formed, a fresh surface of the lead will be continually exposed to the action of the air, and the operation goes on until the whole of the lead has been removed. Silver or gold exposed to similar treatment does not oxidise, but retains its metallic condition; so that an alloy of lead and silver similarly treated would yield its lead as oxide, which would sink into the support, while the silver would remain as a button of metal. ”

“ If the proportion of gold to that of silver be greater than one to two, then the particles of the former metal so protect or envelop those of the latter, that the nitric acid, even at a boiling heat, remains quite inactive on the alloy. It is indispensable, therefore, that the weight of the silver be at least double that of the gold. 100 pounds of silver take 38 pounds of nitric acid, of specific gravity 1·320, for oxidizement, and 111 for solution of the oxide ”

“ A portion of chloride of mercury will be simultaneously formed, to be decomposed in its turn by the sulphate of silver resulting from the mutual action of the acidified pyrites, and the silver or its oxide in the ore. An addition of quicklime counteracts the injurious effect of too much magistral, by decomposing the resulting sulphate of copper. Quicksilver being an excellent conductor of heat, when introduced in too great quantities, is apt to cool the mass too much, and thereby enfeebles the operation of the deuto-chloride of copper upon the silver. ”

“ The mode we have ourselves adopted for the colouring or whitening of silver goods is somewhat different, and still more simple than even the above. We will proceed to give the details of the process. A mixture of very dilute sulphuric acid is first provided, in the proportion of one ounce to forty ounces of water, and well mixed together; the work, after being heated to a good red heat, is boiled in this, which soon removes the oxide from the surface, and shows the fine white colour of the pure silver. ”

“ Ozone very rapidly decolorises indigo, litmus, and many other dyes by oxidising them. Silver is oxidised by it at the ordinary temperature, whilst oxygen is not able to oxidise silver even at high temperatures; a bright silver plate rapidly turns black (from oxidation) in ozonised oxygen. It is rapidly absorbed by mercury, forming oxide ”

“ Thus, in assaying silver, the oxide of lead or metallic lead used is rarely free from silver; and in the case of arsenic, the acids, zinc or ferric chloride are sure to contain arsenic. The same observation applies to the precipitation of lead by zinc, &c. The errors caused by these impurities are more marked in the determination of material having small quantities of metal than in that of ores which contain larger quantities. ”

“ Fused nitrate of silver is in small cylinders of a dark grey colour, and presenting, when broken across, a crystalline structure. Odour, none; Taste, intensely bitter, austere and metallic; it tinges the skin indelibly black; when perfectly free from copper, it is not deliquescent. Chemical Composition; oxide of silver 70, nitric acid 30, or one atom of oxide and one atom of acid. Solubility. In an equal weight of water, at 60°; it is also soluble in alcohol. The solution readily yields transparent colourless crystals, the primary form of which is a right rhombic prism. ”

“ When metallic sulphides are present in the ore, an excess of oxide of lead helps to keep the sulphur out of the button of metal. In addition to the oxide of lead required as a flux, some will be required to provide the lead in which the silver is to be collected. Oxide of lead, mixed with charcoal or flour, yields, when heated, a multitude of minute buttons of metal uniformly distributed through the mass of the charge ”

“ I.—The salts of silver are colorless when the acids, the elements of which enter into their composition, are not colored, but they generally blacken on exposure to light. It is easy, therefore, to blacken silver and obtain its oxide; it is sufficient to place it in contact with a sulphide, vapor of sulphur, sulphohydric acids, such as the sulphides or polysulphides of potash, soda, dissolved in water and called eau de barège. The chlorides play the same part, and the chloride of lime in solution or simply Javelle water may be used. It is used hot in order to accelerate its action. ”

“ To disengage this, where the quantity is sufficient to repay the expense, the lead, after it has been smelted, is subjected to the action of what is called a refining furnace. A continued blast of fresh air is thrown upon its surface by means of large bellows, while the lead is kept in a state as intensely hot as possible. This by degrees converts the lead into a yellow scaly oxide or dross. The oxide, thus formed, is driven off from the melted metal as it rises, and the silver is 208left alone at the bottom, in a metallic state. ”

“ The smelting processes are mostly based upon the capacity of metallic lead, as well as its oxide and sulphate, to separate silver under fusion from its combinations, the liberated silver alloying itself with an excess of lead and accumulating in the metallic bath in the hearth of the furnace. ”

“ It appears to be formed in every case where common oxide of silver is dissolved in ammonia, and the entire solution of all solid matter is no evidence of its nomexistenee, for the compound is itself soluble in ammonia, though not so much so as the oxide. When there is an excess of oxide, unless it predominate in a great degree, the undissolved portion will be found to contain fulminating silver, and when the whole is dissolved, by heating the solution, it is thrown down. ”

“ All metals, except iron, tin, antimony, and tellurium, have a tendency to decompose this compound, and separate it into oxygen and water. Some metals are oxidized during the decomposition, but gold, silver, platinum, and a few others, still retain their metallic state. If either silver, lead, mercury, gold, platinum, manganese, or cobalt, in [Pg 299] their highest states of oxidation, are put into a tube, containing this peroxide of hydrogen, its oxygen is liberated with the rapidity of an explosion, and so much heat is excited that the tube becomes red hot. ”

“ Silvering is generally effected cold, except in the case of small articles. Iron, steel, zinc, lead, and tin are better if previously copper-plated and then silvered hot. The cleaned articles are first treated in a nitrate of mercury bath, being kept continually in motion.
With excess of current the pieces become gray, and blacken. In the cold bath anodes of platinum or silver should be employed. Old baths are, in this case, preferable to new. They may, if required, be artificially aged by the addition of 1 or 2 parts in 1,000 of liquid ammonia.
If the anode blackens, the bath is too weak. ”

“ Ziervogel's method of extracting silver by roasting the sulphuretted ore to produce silver sulphate, leaching this with hot acidulated water, and precipitating with copper, is the simplest and cheapest of all; but it requires very skilful and delicate roasting, and ores comparatively free from lead, antimony, arsenic, and zinc. The three latter tend to cause volatilization of silver; the sulphide of antimony and lead cause a sintering of the roasting charge; copper dioxide, or too high a temperature in the furnace, leads to the formation of metallic silver, instead of the desired sulphate. ”

“ The current of air in connection with the heat of the furnace immediately begins to purify the silver by oxidizing the lead, and forms litharge, which passes off through the channel provided in the mouth of the cupel; as this proceeds, fresh silver-lead alloy is added, to keep the level of the metal always at the same height. ”

“ It is the most accommodating of metals. You can hammer it in the cold until it becomes as hard as soft iron. Then, if you need to have it soft again, it will become so by melting. It takes a fine polish and is not affected, as silver is, by the fumes which are thrown off by burning coal; and so keeps its color when silver would turn black. Salt water does not hurt it in the least, and few of the acids affect it. Another good quality is that it conducts electricity excellently. ”