821 



ELECTRO-METALLURGY. 



ELECTRO-METALLURGY. 



822 



adhesion between the newly deposited metal and the object to be 

 covered. In electrotyping, however, the reverse is the case, for adhe- 

 sion between the deposit and the mould would be fatal to success. In 

 order therefore to be able to detach the metal, it is customary to 

 a newly polished article to the air for twenty-four hours before 

 placing it in the depositing cell. In the case of electro-gilding and 

 plating, when the object has been made chemically clean, it is immersed 

 while wet in the silvering bath, represented in plan in the following 

 figure, in which c z is the battery, P P are rods connected with the 

 negative pole, to which rods the copper wires for holding the articles 



to be plated are twisted : z z are" silver plates attached to the positive 

 pole. The silvering solution contained in the bath may be prepared by 

 dissolving cyanide of silver in a solution of cyanide or of ferrocyanide 

 of potassium, although in some cases solutions containing hyposulphite 

 or sulphite of silver are used. A good working solution should con- 

 tain jLth of its weight of silver, but there is an inconvenience attending 

 the use of cyanide of potassium, arising from the formation of car- 

 bonate of potash, which accumulates and interferes with the regularity 

 of the process. If, however, cyanide of calcium be used, an insoluble 

 carbonate of lime is formed, which sinks as fast as it is produced. The 

 plated articles have a dead white or chalky surface, but may be bur- 

 nished to a brilliant lustre. If, however, a small portion of bisulphide 

 of carbon be added to the bath, the deposited metal has the lustre of 

 polished silver. The thickness of the deposit is regulated by weighing 

 the article* before they are placed in the bath, and once or twice 

 during the process. A deposit of 1J to 14 oz. of silver per square foot 

 of plated surface is about the thickness of ordinary writing paper, and 

 is said to wear well. The solution requires frequent stirring, and the 

 strength of the buttery is regulated by the experience of the workman. 

 In electro-gilding, the article is made chemically clean, and placed in 

 a bath of cyanide of gold dissolved in cyanide of potassium. Tin; 

 gilding rii<lntiii does pot usually contain more than 1 per cent, of gold. 

 M. Ruolz brings the cyanide of gold into solution by means of the 

 ferrocyanide or the ferridcyanide of potassium ; he lias also used the 

 double sulphite of gold and soda, and some other similar compound . 

 A solution of the double chloride of platinum and potassium in caustic 

 potash has been used for electro-platinising. Patents have been taken 

 out for electro-deposits of silicium, titanium, tungsten, chromiin 

 molybdenum. Indeed attempts, more or less successful, have been 

 made to obtain deposits of nearly all the metals. 



"-/itailiiy. In this case the mould is formed of a 



melted composition of glue and treacle, which has, when cold, suffi- 

 cient elasticity to enable the pattern to be removed from it with 

 facility, however much " undercut " the device may be. From this 

 elastic mould a cast ia taken, in a peculiar waxen composition ; and 

 this cast, when made conducting, is suspended in a tank containing a 

 solution of sulphate of copper. A layer of copper is then deposited on 

 the surface of the model, of such a thickness that when the waxen 

 model is melted out from within it, the copper may be able to retain 

 its shape unsupported. The result is, that the intide of the copper 

 shell represents a mould of the out/tide of the article to be manu- 

 factured. The exterior of the copper is protected by a resisting or 

 non-conducting composition, and it is then suspended in a tank con- 

 taining a solution of silver (or gold, as the case may be) ; the galvanic 

 current causes the deposition of the gold or silver on the inside of the 

 copper mould ; deposition on the outside being prevented by the 

 resisting composition. When a sufficient thickness of the precious 

 metal is thus obtained, the copper mould is exposed to the action of an 

 acid, which gradually eats it away without injuring the gold or silver 

 beneath, and the result is the production of a vessel or ornamental 

 article of pure silver or gold, of the same pattern as the original wax 



Electro-sinking. A. solution of sulphate of zinc may be used for de- 

 jxwitiug that metal on iron, much in the same way as the sulphate of 

 copper. Only a feeble current is required, and the operation may be 

 performed on a lart-e scale. In this way the iron links of the Charing- 

 CroM suspension bridge which go into the abutments on each side the 

 river were coated. Each link in 24 feet in length. 



Electro-deposit, as a protection or as an ornament. The deposition of 

 a thin metallic layer ou a body prepared for its reception being thus 

 rendered practicable, the applications of the method have become very 

 numerous. It is in most eases as a protection, as an ornament, or as a 

 copy or type, that these applications are intended. 



As a protection against damp, &c., Mr. Dent has proposed electro- 

 gilding for chronometer springs, instead of the usual method of "blueing," 

 which he thinks affects their elastic force. A similar suggestion has 

 been made for protecting magnetic bars and needles by the same 

 means. It is surprising how admirably natural objects can be pre- 

 served by a metallic coating, and yet preserve all their characteristics. 

 In tlu's way even the exquisite framework of the insect's wings is 

 exhibited almost as distinctly as in the natural state. Mr. Smee says 

 (' Elements of Electro-metallurgy,' p. 222), " The beauty of electro- 

 coppered leaves, branches, and similar objects is surprising. I have a 

 case of these specimens placed on a black ground, which no one would 

 take to be productions of art. . . . When I state that the numerous 

 hairs covering the leaves of a mclastmna, and even the delicate hairs of, 

 the salvia, are all perfectly covered, the botanist must at once admit 

 that these specimens have rather the minuteness of nature than the 

 imperfections of art." 



In the preservation of all such fruits and vegetables as apples, pears, 

 gourds, grapes, cucumbers, and potatoes, by electro-coppering, a pin is 

 inserted before the immersion, and on this pin being afterwards 

 removed, the hole left by it furnishes an outlet for the sap and 

 moisture from within ; and it is found that in many cases the inside 

 dries up completely without deranging the shape of the copper enve- 

 lope. Ornamental baskets, whether made of wicker or of wire, are 

 coated in a similar manner. So likewise are lace and other articles 

 made of woven fibres. Indeed, should a taste for productions of this 

 kind spring up, the means are at hand for extending the art almost 

 indefinitely ; for by a preliminary steeping in a solution of phosphorus, 

 or some other conducting agent, almost any substance may be brought 

 into a state to receive a deposition of metal in a delicate film ; and 

 this film may be of copper, silver, gold, platiua, or nickel. Porcelain 

 and earthenware might without much difficulty be brought under 

 similar operations. 



Uectro-depoiit as a means of c<ij>yiny. Examples have already been 

 given of this branch of the art ; and we may here remark, that the 

 i Irrtro-process has been recommended not only for copying engravings, 

 but for making the copper-plate itself on which an eugraving is to be 

 executed. The copper-plates prepared for engravers generally contain 

 a small portion of other metals, which render both the engraving and 

 the etching somewhat uncertain : by the substitution, therefore, of 

 plates produced by electro-deposition, in which the copper is quite 

 uucontaminated with other metals, an advantage is anticipated. To 

 produce these plates, a copper-plate is prepared iu the usual way and 

 suspended in a copper solution, by which a film of any desired thick- 

 ness may be produced ; and by a previous adjustment of the plate, the 

 new portion may be separated from the old in the form of a distinct 

 plate, susceptible of after-preparation for the engraver. Or, the 

 copper-plate, instead of being made by deposition upon another plate 

 of the same material, may be produced on a flat surface of wax or 

 plaster properly prepared. 



"nil- ill //.'//;/ and r>ir/rar!iiy. The voltaic current may be used 

 for biting in the plates of the engraver instead of dilute nitric acid 

 as usually employed. For this purpose both sides of the plate are 

 covered with engravers' varnish, and the artist etches with a fine 

 point the design on one of the surfaces, cutting through the varnish 

 so as to expose the metal beneath. The plate is then connected 

 with the potitive wire of the battery, and on completing the circuit 

 the oxygen and acid set free on the surface of the plate will attack 

 those portions which have been laid bare by the tool of the engraver. 

 As a plate etched in the common way is treated with the acid three 

 or four times, to produce different grades of tint ; so is likewise the 

 plate etched by the galvanic method. Mr. Smee says that " the 

 advantages of galvanism for etching are, the absence of poisonous 

 nitrous fumes, which are evolved in the ordinary process ; the greater 

 uniformity of action which takes place than when acids are used ; and 

 the rapidity of biting, which may be regulated to the greatest nicety. 

 The lines may be of any depth, and are sharper and cleaner than when 

 acid is used ; and lastly, no bubbles are evolved, which the engraver well 

 knows are apt to tear up the ground, or to cause unequal action." 



The terms Electrotint and Qlyphognphy have been applied to two 

 methods of etching by galvanism, in which the device is produced in 

 rather a peculiar way. The methods are adapted, one for plate- 

 printing, in which the design is in intaglio ; and the other for surface- 

 printing, as in common typography. For the former of these two 

 methods, a paint or pigment is used, possessing the properties of 

 working freely, laying on without spreading, and resisting the action 

 of sulphate of copper : a composition of white wax, lard, lamp-black, 

 olive oil, and turpentine, is said to answer well for the purpose. The 

 plate employed i not of copper, but of some mixed metal presenting 

 a white surface, such as one of the modem kinds of German silver. 

 Brushes made of fitch, hog's hair, camel's hair, and sable hair, cut at 

 the ends to various shapes, are employed; as also points of wood, 

 ivory, and metal. The artist sketches his design on the dull white 

 surface of the plate, and then paints the subject in full by means of 



