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To Recover Silver from Old present

This method requires only the fol- Plated Goods. Oil of vitriol, together with lowing precautions: The metallic surface 5 per cent. of nitrate of soda, is heated in a must have been quite cleansed from grease or cast-iron boiler, or a stoneware pan, to 212° varnish with spirits of wine-water must be Fahr. The silver-plated clippings are placed poured over the treated surface before judging in a sheet-iron bucket or cullender, which is of the color, as that of the testing fluid is obtain. fastened to a pulley that it may be moved about in the acid. As soon as the silver is removed, the cullender is raised, allowed to drain, then immersed in cold water and emptied, to be again used in the same manner. altered by the metal, and the red precipitate is not distinctly visible until the colored liquid has been washed off. The red spot can afterwards be very easily removed with the finger. By this method the slightest trace of When the acid bath is fresh, the desilvering silver in an alloy may be ascertained. When proceeds very rapidly, and even with heavy an article is suspected to be only thinly plated, plated ware takes but a few minutes; with a very minute drop of the testing fluid should the gradual saturation of the bath more time be used. With no other metal or alloy does is required, and it is readily perceived when this red spot, so characteristic of silver, apthe acid must be renewed. The small amount pear; in some cases the testing fluid only of acid solution adhering to the copper, pre-corrodes the surface of the metal, whilst in cipitates its silver when brought into the wa- others colored precipitates are formed, which, ter. To obtain its complete removal, the however, cannot be confounded with those of clippings, when raised from the desilvering silver. German silver brought into contact bath, and before immersion in water, may be with the testing fluid affords no red spot after dipped into a second bath prepared in the being washed. The spot will, however, have same manner, which is afterwards to be used been strongly corroded. Britannia metal in place of the first. The saturated bath, on yields a black spot; zinc is strongly corroded; cooling, congeals to a crystalline semi-fluid platinum is not attacked; lead gives a yellow mass of sulphate of copper and of soda. The precipitate; tin is strongly affected by the silver is removed by chloride of sodium (com- fluid; when the brownish-colored testing mon salt) which is added in small portions at fluid is washed off, a yellow precipitate is pera time, while the solution is yet warm. The ceived, which adheres tightly to the metal; chloride of silver separates readily, and is copper is strongly attacked, a tarnished surwashed and reduced in the usual manner. The acid solution contains but a very small portion of copper, hardly enough to pay for recovering. face of this metal is brightened by the action of the acid.

Electro-Gilding. The opera- 3721. To Recover Silver from Copper. This process is applied to recover the tion of gilding, or covering other metsilver from the plated metal, which has been als with a coating of gold by the battery, is rolled down for buttons, toys, etc., without performed in the same manner as electrodestroying any large portion of the copper. plating, with the exception of a few practical For this purpose, a dissolving solution is com- modifications. posed of 3 pounds oil of vitriol, 14 ounces 3725. To Prepare Chloride of Gold. nitre, and 1 pound water. The plated metal Dissolve 1 part gold in 3 parts nitro-hydrois boiled in it till the silver is dissolved, and chloric acid (aqua regia); evaporate until then the silver is recovered by throwing com- vapors of chlorine begin to be disengaged, mon salt into the solution. (See No. 3214.) then set the solution aside to crystallize. 3722. Test Fluid for Silver-Plated Aqua regia consists of 1 part nitric acid and Goods. For this purpose a testing fluid is 2 parts (both by measure) muriatic (hydroprepared by adding pure nitric acid to pow-chloric) acid. dered red chromate of potash, and mixing them If aqua ammonia be added to a solution of in such a manner that a part of the latter gold in aqua regia, it precipitates a reddishremains in suspension, the whole being kept yellow deposit, which may be collected, well stirred during the mixing. Equal parts washed, and dried. This is the ammoniuret by weight of each may be taken. The nitric of gold, and must be handled and prepared must be quite free from hydrochloric acid, and with great caution, it being the fulminate of have the proper degree of concentration, be- gold. ing neither too fuming nor too dilute; it 3726. To Prepare a Solution of Gold. should have a specific gravity between 1.20 Add a solution of cyanide of potassium to a and 1.25. When the mixture has been pre- solution of chloride of gold (see No. 3725) pared for a few hours, and been stirred several until all the precipitate is redissolved; but times, the reddish-colored liquid is poured off this gives chloride of potassium in the solution, which is not good. In the preparation | same means. For all the operations of gilding of the solution by this means there are some by the cyanide solution, it must be heated to interesting reactions. As the chloride of at least 130° Fahr. The articles to be gilt gold has always an excess of acid, the addi- are cleaned in the way described for silver tion of cyanide of potassium causes violent (see No. 3705), but are not dipped into nitric effervescence, and no precipitate of gold takes acid previous to being put into the gold soluplace until all the free acid is neutralized, tion. 3 or four minutes is sufficient time to which causes a considerable loss to the gild any small article. After the articles are cyanide of potassium. There is always cleaned and dried they are weighed, and, formed in this deposition a quantity of am- when gilt, they are weighed again; thus monia and carbonic acid, from the deposition the quantity of gold deposited is ascerof the cyanate of potash; and if the chloride tained. Any convenient means may be of gold be recently prepared, and hot, there is adopted for heating the solution. The one often formed some aurate of ammonia (ful- generally adopted is to put a stoneware pan minate of gold), which precipitates with the containing the solution into an iron or tinvanide of gold. Were this precipitate to be plate vessel filled with water, which is kept collected and dried, it would explode when at the boiling point either by being placed slightly heated. By previously diluting the upon a hot plate or over gas. The hotter the chloride of gold, or using it cold, this com- solution the less battery power is required. pound is not formed. After the free acid Generally a battery of 3 or 4 cells is used for is neutralized by the potash, further addition gilding, and the solution is kept at 130° to of the cyanide of potassium precipitates the 150° Fahr. But 1 cell will answer if the solugold as cyanide of gold, having a light yellow is heated to 200°. color; but as this is slightly soluble in ammonia and some of the alkaline salts, it is not advisable to wash the precipitate, lest there be a loss of gold. Cyanide of potassium is ing is as follows: When the articles are generally added until the precipitate is redis- cleaned as described in No. 3705, they are solved; consequently much impurity is formed weighed, and well scratched with wire brushin the solution, namely, nitrate and carbonate es, which cleanse away any tarnish from the of potash with chloride of potassium and surface, and prevents the formation of airammonia. Notwithstanding, this solution bubbles. They are then kept in clean water works very well for a short time, and it is