834 SILVERING 



If the papers, on which there is silver, are preserved, the silver can be obtained by 

 merely burning them, and may be fused in a porcelain crucible into one lump. In 

 the case of the nitrate-of-silver baths, when too weak for further use, the silver may 

 be precipitated in the form of chloride, by adding hydrochloric acid. The chloride 

 of silver thus obtained may be easily reduced to the metallic state : 1st, by digesting 

 the moist chloride with metallic zinc and dilute sulphuric acid ; the hydrogen which is 

 thus liberated reduces the silver to the metallic state, which remains in the form of 

 a black powder, and when well washed with water may be dissolved in nitric acid, 

 evaporated and crystallised. 2nd, by digesting it by the aid of heat with a caustic 

 alkali and tartaric acid, when it will also be reduced to metallic silver, and will remain 

 as a black powder, which may be treated as above. 3rd, by collecting the precipitated 

 chloride of silver on a filter, washing well with water, and drying ; the dry chloride 

 is then mixed with four or five times its weight of a mixture of carbonate of potash 

 and carbonate of soda, and subjected to a white heat in a porcelain crucible ; the 

 silver will be reduced to the metallic state. This salt is used not only in photography, 

 but in making permanent ink, and as a dye for the hair. 



SILVER, OXIDES OP. There are two oxides of silver : the -protoxide AgO 

 (Ag 2 O) and the peroxide AgO 2 (AgfO). The first is obtained by adding solution of 

 caustic potash, or lime-water, to a solution of nitrate of silver. The precipitate has a 

 brownish-grey colour, which darkens when dried, and contains no combined water. 

 Its specific gravity is 7'143. On exposure to the sun it gives out a certain quantity 

 of oxygen, and becomes a black powder. This oxide is an energetic base ; being 

 slightly soluble in pure water, reacting like the alkalis upon reddened litmus-paper, 

 and displacing, from their combinations with the alkalis, a portion of the acids 

 with which it forms insoluble compounds. It is insoluble in the caustic lyes of 

 potash or soda. By combination with caustic ammonia, it forms fulminating silver. 

 The second, or peroxide, is formed when a very dilute solution of nitrate of silver is 

 decomposed by the voltaic current ; dark grey lustrous needles of the peroxide of 

 silver are formed around the positive pole. See FULMINATING SILVER. 



SILVER, SULPHATE OP, AgO.SO 3 (Ag 2 SO 4 )may be prepared by boiling sul- 

 phuric acid upon the metal. It dissolves in 88 parts of boiling water, but the greater 

 part of the salt crystallises in small needles as the solution cools. It consists of 118 

 parts of oxide, combined with 40 parts of dry acid. 



SILVER, SULPHIDES OP, of which several exist native, may be readily pre- 

 pared by fusing the constituents together. A sulphide forms spontaneously upon the 

 surface of silver exposed to the air of inhabited places. The tarnish may be easily 

 removed by rubbing the metal with a solution of chameleon mineral, prepared by cal- 

 cining peroxide of manganese with nitre. Sulphide of silver is a powerful sulpho-base ; 

 since though it be heated to redness in close vessels, it retains the volatile sulphides, 

 whose combinations with the alkalis are decomposed at that temperature. It consists 

 of 87'04 of silver and 12'96 of sulphur. 



SILVER-LEAF is made by beating silver out very thin, in precisely the same 

 way as gold-leaf is manufactured. See GOLD BEATING. 



SILVERING is the art of covering the surfaces of bodies with a thin film of 

 silver. This is now effected either by applying thin films of silver mechanically to 

 the article to be silvered, or by the electro-metallurgical process. When silver-leaf 

 is to be applied, the methods prescribed for gold-leaf are suitable. Among the metals, 

 copper or brass are those on which the silverer most commonly operates. Iron is 

 seldom silvered ; but the processes for both metals are essentially the same. The 

 white alloy of nickel is now often plated. 



The principal steps of this operation are the following : 



1. The smoothing down the sharp edges, and polishing the surface of the copper ; 

 called emorfler by the French artists. 



2. The annealing ; or, making the piece to be silvered red hot, and then plunging 

 it in a very dilute nitric acid, till it be bright and clean. 



3. Pumicing ; or, clearing up the surface with pumice-stone and water. 



4. The warming, to such a degree merely as, when it touches water, it may make 

 a slight hissing sound ; in which state it is dipped in the very weak aquafortis, 

 whereby it acquires minute insensible asperities, sufficient to retain the silver-leaves 

 that are to be applied. 



5. The hatching. When these small asperities are inadequate for giving due solidify 

 to the silvering, the plane surfaces must be hatched all over with a graving tool ; but 

 the chased surfaces need not be touched. 



6. The blueing, consists in heating the piece till its copper or brass colour changes 

 to blue. In heating, they are placed in hot tools made of iron, called mandrins in 

 France. 



7. The charging, the workman's term for silvering. This operation consists in 



