, ETC.] UNDULATORY FORCES. ELECTRO-METALLURGY. 



235 



patent, December 8th, 1843, for depositing either plain 

 or figured copy upon felted fabrics. He passes the cloth 

 nnder either a plain or an engraved copper roller, 

 horizontally immersed in a sulphate of copper solution 

 not containing much free acid, and a deposit takes place 

 upon the roller as it slowly revolves ; the meshes of the 

 cloth are thus filled with metal, and the design of the 

 roller copied upon it. The coppered cloth is slowly 

 rolled off, and passed through a second and closely 

 contiguous vessel filled with clean water ; the roller is 

 properly prepared for a non-adhesive deposit. 



210. Etching Copper. In etching a copper plate by 

 galvanism, we first solder a wire on the back, then 

 varnish the back, and cover the front with a thin layer 

 of engravers' etching-ground ; draw the design upon the 

 front surface with an etching-needle, cutting through 

 this material to the clean surface of the copper. Having 

 completed the etching, hang the plate as an anode in the 

 ordinary sulphate of copper solution, opposite a suitable 

 cathode of brass or copper. The current of electricity, 

 in passing out of the engraved lines into the liquid, 

 causes the copper in them to dissolve, and thus etches 

 the design in the plate. The different gradations of 

 light and shade are produced by suspending cathodes of 

 different forms and sizes opposite the plate to be etched, 

 in different positions and at different distances from it, 

 thus causing the etching to be of different depths in 

 different parts, the deepest action being always at the 

 parti of the electrodes nearest together. 



-11. Glyplioyraphy. This art consists in varnishing 

 the back of a flat and smooth copper plate, laying first a 

 thin coating of white etcliing-ground upon its front side, 

 and then a layer of black etching-ground upon that ; 

 engraving the design upon the coating with different 

 engraving-tools ; then blackleading the whole of the 

 engraved surface, and depositing a thick sheet of copper 

 upon it in a sulphate solution by the battery process. 

 The deposited plate is then removed, its defects corrected, 

 and fixed upon a block of wood A the same manner as a 

 stereotype plate, ready for printing by the ordinary 

 hand press. This process has been patented, and the 

 patent is worked by Mr. Hawkins, Hatton Garden. 



212. Management of Silver Solutions. Silver plating 

 liquids require much more care and attention than the 

 sulphate of copper solution. Articles formed of zinc, iron, 

 or steel, require to be coated with a thin film of copper 

 in the cyanide of copper liquid, before being immersed 

 in tho cyanide of silver solution. Those formed of 

 Britannia metal, tin, or pewter, are taken direct from 

 the hot potash liquid without rinsing in water, and 

 immersed a short time in a cyanide of silver solution 

 containing considerable supplies of free cyanide. A 

 large anode is used, and a current of considerable intensity 

 from a strong battery passed through for several minutes, 

 until the articles receive a thin deposit of silver ; they 

 are then transferred to the ordinary vat to receive the 

 full amount of deposit. Those of lead are first scraped, 

 or otherwise made quite clean and bright by mechanical 

 means, and then treated in the same manner as those of 

 Britannia metal. Articles of copper, brass, or German 

 silver, after being properly cleansed, are dipped into the 

 solution of nitrate of mercury (84), or of cyanide of 

 mercury and potassium (84), then rinsed in a vessel of 

 water, and immediately suspended in the depositing vat. 

 The preparation of those articles by immersion in a bath 

 of cyanide of mercury, was patented by Dr. H. B. 

 Leeson, Juno 4th, 1842, and is in use by the electro- 

 plnturs of Birmingham. If they are immersed without 

 this preparation, the deposited silver will not always 

 adhere firmly. 



L'13. Peculiarities in Practical Silver Deposition. 

 Peculiar phenomena often occur in the electro-deposition 

 of silver, not only upon different metals, but also upon 

 the same metals in different forms or in different con- 

 ditions of surface. For instance 



1st. If two perfectly similar pieces of thin short brass 

 are taken (except that one is perforated all over with 

 finall holes), and both be simultaneously immersed in 

 the same solution to be silvered, and with the same 



battery power applied to each, tho latter, although its 

 amount of surface is reduced by the perforations, will 

 become coated with silver much more slowly than the 

 former. 



2nd. If a wire-gauze cylinder of a Davy lamp be sus- 

 pended side by side with a piece of thin tubing of tho 

 same metal and of the same dimensions, the latter will 

 become coated much more rapidly than the former. 



3rd. If two pieces of the same metal iron for in- 

 stance be immersed, to bo silvered, in tho ordinary 

 cyanide solution, or to be coppered in the hot cyanide 

 of copper and potassium liquid, each containing exactly 

 the same amount of surface to be coated, but one being 

 in the form of a thin sheet, and the other in that of a 

 thick plate or solid block of metal, tho former will be- 

 come coated much more rapidly than the latter. 



4th. The edges and points of articles, whilst being 

 plated, exhibit a greater tendency to a crystalline de- 

 posit than the flat parts, and this tendency is sometimes 

 manifested in depositing silver upon table-knives and 

 forks. It is the knowledge of these and many other 

 peculiarities of different metals and articles met with in 

 practical working, and of the means of overcoming their 

 attendant difficulties, which constitutes one of the chief 

 differences between the practical operator and the scien- 

 tific man. 



214. Management of "Bright Solution." A bright 

 solution is much more difficult to manage than the 

 ordinary silvering liquid ; if it be not worked constantly 

 and in a uniform manner, it will lose its power of yield- 

 ing bright metal. If any of the articles which are being 

 plated in it are disturbed, or removed from tho liquid 

 and replaced, that one will not now receive a bright 

 deposit ; and the disturbance of the liquid, by remo. in.; 

 it, will oftentimes cause all the neighbouring articles 

 to lose their brightness. If too much "brightening 

 liquid" (144) bo added, the solution will bo considerably 

 injured : many silver solutions have been irretrievably 

 damaged in tlu's way. A bright solution requires a 

 battery current of large quantity and low intensity to 

 work it, and the dissolving plates in it are generally of 

 a darker colour than those in the ordinary silvering 

 liquid; tho silver deposited from it is much harder 

 than that deposited from the ordinary plating solution, 

 and has very much the appearance of fused metal ; tho 

 bright appearance commences at the upper part of the 

 articles, and travels downwards ; it soon after commences 

 also at their lower extremities and travels upwards, 

 until the bright portions meet each other. If thero 

 .ire very small holes in the surface of the articles, dull 

 streaks appear above them. 



215. Adding Cyanide of Potassium to Plating LiqtiiJs. 

 It is necessary to add a little cyanide of potassium 

 occasionally to every cyanide of silver plating liquid, 

 probably because the solutions absorb carbonic acid from 

 the atmosphere, which converts some of the cyanide of 

 potassium into carbonate of potash, and sets hydro- 

 cyanic acid gas free. A further portion of tho potas- 

 sium salt may also be decomposed by some means, with 

 formation and escape of ammonia ; tho necessity of add- 

 ing a little fresh cyanide is indicated when tho dissolving 

 plate begins to change from its ordinary pure whito 

 appearance to a dull yellowish-gray colour; it is best 

 added in the evening after plating about ualf-au-hour 

 before stirring the solution. 



216. If the solution be too strong, i.e., if it contain 

 insufficient water, but has silver and cyanide of potas- 

 sium in treir proper relative proportions, it conducts 

 freely, deposits rapidly, and gives a rich deposit of a 

 fine silky lustre ; but it is more difficult to manage than 

 a weaker liquid, especially in hot weather, because, from 

 the less mobility of its particles, it is very apt to settle, 

 by working, into strata of different densities its upper 

 part becoming exhausted of silver and full of freo 

 cyanide, and its lower part becoming nearly saturated 

 with that metal, and destitute of free cyanide j the con- 

 sequence of this is, that tho upper parts of the dis- 

 solving plates waste rapidly, whilst the upper parts of 

 the article receive either very little deposit, or one of a 



