1844. 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



75 



snlution, the galvanic circuit is completed. The metal held in solution and 

 the solvent combined orifiinally from being in different electrical states, the 

 metal being positive, and the solvent negative. When the galvanic circuit is 

 completed, the solution becomes a jiart of that circuit: the electricity then 

 passing through the solution decomposes it, the positive element— the metah 

 going to the negative pole, which is the article ; and the negative element — 

 the solvent — passing to the positive pole, which is the plate of metal sus- 

 pended in the solution, with which it combines. So that for every atom of 

 metal attracted to the negative pole, a corresponding atom is dissolved from 

 the positive, and the solution is maintained in the same state. 



In proceeding to plate an article by the electro-process, great care should 

 be taken that it is free from all grease and o.'iKle ; for this purpose the first 

 operation is to boil it in a solution of caustic alkali, by which any grease is 

 saponified ; it is then scoured with sand and water, after which it is dipped 

 info dilute acid, which removes any oxide that may be on the surface; after 

 rincing in water, it may be placed in the solution. It is sometimes well, before 

 placing the article in the solution, to dip it into a dilute solution of the 

 nitrate or cyanide of mercury, and this may be found necessary when the 

 metal to be operated upon has an affinity for the solvent of the so'ution 

 used. 



It is cenerally believed that metals deposited by electricity are necessarily 

 soft, because pure, but this is not a consequent : from experiments we have 

 frequently made, it appears that the hardness of the deposited metal varies 

 with the intensity of the battery; a battery of three or four pairs intensity 

 producing silver sufficiently bard to scratch ordinary sheet silver, wliilst sheet 

 silver will not scratch it. When the intensity of twelve or fourteen pairs of 

 plales is used, the hardness is so great that burnishing with a steel tool fails 

 ill producing a polish upon it. 



Metals deposited.— The following metals have been deposited by Messrs 

 Elkington by the electro-process— gold, platinum, silver, copper, zinc, anti- 

 mony, arsenic, bismuth, nickel, cobalt, palladium, titanium, cadmium, lead, 

 and tin ; those which are principally adapted to manufacturing purposes are 

 gold, silver, copper, and zinc : and upon these we shall make a few remarks^ 

 pointing out the peculiar adaptation of the electro-process to their use. 



GoW.— Cyanide of jiotassium is the salt we prefer for dissolving this metal 

 for electro-depositing, it is obtained by fusing eight parts'of dry ferro-cyanide 

 of potassium with three parts of carbonate of potash. The gilding solution 

 may be prepared by placing sheets of Kold in a solution of pure cyanide 

 of potassium, and attaching to the negative pole of a galvanic battery a 

 small plate of gold, and to the positive a much larger plate ; when but a small 

 quantity of solution is required, this method answers very well, and makes a 

 pure solution ; but when a large ([uantity is wanted, time will be saved by 

 dissolving gold in nitro-muriatic acid, and precipitating it by magnesia, the 

 oxide thus obtained being again dissolved in cyanide of potassium and water. 

 This solution should be used at a temperature of 130 deg., the arrangement of 

 the battery and articles being the same as already described. 



Silver.— Tim sail of silver, best suited for electro-depositing, is also the 

 cyanide. 



Silver being allowed to remain in a solution of cyanide of potassium, is dis- 

 solved, or take the nitrate of silver, to which, add a solution of cyanide of 

 potassium, the silver combines with the cyanogen, and precipitates as a white 

 powder ; the liquid is then decanted, and the precipitate well washed with 

 pure water : to this more cyanide of potassium is added, which dissolves the 

 precipitate, lorming the double salt of cyanide of potassium and silver ; this 

 constiluies the plating solution. 



The same general arrangement is to be observed for plating with silver as 

 already described, the quantity of battery power, being in proportion to the 

 surface of the articles to be plated, and the intensity agreeing with the density 

 cf the solution. In a lew seconds after the articles are placed in the solution 

 and connected with the battery, they are covered with silver, and arc allowed 

 to remain therein until the necessary coating is obtained, and this is ellected 

 in from lour to six hours. In order to ascertain correctly that the required 

 quantity of silver has been received, every article is weighed previously to 

 being placed in the solution, and again after the process is completed, and the 

 weight entered in a book against each. After the plating is finished, the 

 articles are taken to a lathe, and brushed with brass brushes, when they are 

 ready lor burnishing, this process consists in rubbing the surtace with great 

 force, with a liii^hly polishtd steel or blood-stone tool, until it is as bright as 

 a minor. This is a severe test for any plated article, by whatever process 

 made, and if a most perfect cohesion did not exist, it would at once be de- 

 tected in the burnishing. 



Copper, — It was, as we have already noticed, the deposition of this metal 

 from its sulphate, which suggested to Spencer, in England, and to Jacobi, in 

 Russia, the application of the elenric current to the multiplication of works 

 of art. 



For all purposes of solid deposit, for electrotyping, &c., the solution of the 

 sulphate is found to answer best, and is the cheapest; the method of operat- 

 irg with it is now generally well umierstood. A considerable improvement. 



however, may be made in this solution, (when used for the purpose of covering 

 metals) by the addition of caustic potash or soda, which should be added by 

 small quantities until the precipitate formed by it, is no longer re-dissolved 

 by the solution. The ordinary sulphate of copper contains one atom of acid 

 for one of copper, so that, in decomposing by the electric current one atom 

 of this salt, we have one atom of copper only (le[.{jsited ; but by the addition 

 of caustic alkali, a part of the acid is taken up by the alkali, leaving two or 

 three atoms of copper combined with the remaining acid, forming a different 

 salt of copper. This we find superior to the ordinary sulphate, and in practice 

 effects a saving of battery power, besides the deposit being obtained in a 

 shorter time. 



The arrangement of battery best adapted for solid deposits of copper is the 

 single cell. A plate of amalgamated zinc is put into a vessel of unglazed 

 earthenware, or any other porous substance, containing dilute acid, and placed 

 in the copper solution, the articles to be coppered are attached to the zinc 

 plate. In this arrangement crystals of sulphate of cO[iper should be sus- 

 pended in the solution, to compensate for the metal deposited : but when the 

 neutralized solution above described is employed, it is necessary now and then 

 to add caustic alkali to maintain the proper qualities of this salt. 



But the simplest of all single cell arrangements, and which we find the best in 

 practice, is to wrap the amalgamated zinc in a double sheet of ordinary brown 

 paper, tied round with string, the joints being cemented with glue; care 

 should be taken in this, as well as in every other single cell arrangement, that 

 the surface of zinc be equal to the surface of the article or articles to be 

 coated ; the better plan being to suspend before each article a corresponding 

 surface of zinc. 



From the qualities and cheapness of copper and its salts, it is generally used 

 for all purposes of electrotyping and solid deposition; and it may also be 

 used as a coating for the protection of iron from rust, besides rendering it 

 higl-.ly ornamental. 



The alkaline salt of copper best suited to the coating of iron, is the double 

 salt of cyanide of potassium and copper ; it is thus prepared : — Take pure dry 

 crystals of sulphate of copper, dissolve them in water and precipitate with the 

 ferro-cyanide of potassium, wash the precipitate and dissolve it in cyanide of 

 potassium ,and water. After the iron has been properly cleaned, which is 

 ctl'ecied by allowing it to remain for a short period in dilute sulphuric acid 

 heated, it is placed in the cyanide solution, heated to about 12U deg., and 

 connected with the battery ; in from two to five minutes it will lie found com- 

 pletely coated ; the iron should then be scoured with sand, and placed in the 

 sulphate solution ; if any portion should have been imperfectly coated in the 

 alkaline solution, it will immediately turn black in this, in which case it 

 should be cleaned and returned to the alkaline ablution for one or two 

 minutes. 



By this process every article of iron work, either cast or wrought, may be 

 firmly coated with copper, and afterwards bronzed, the articles retaining all 

 the strength of ihe iron, with the beauty and indestructible qualities of the 

 copper ; and with the aid of our patents for the production of works of art 

 belbre described, copies of the beautiful marbles and bronzes of the antique 

 may be successfully produced. 



Zinc. — All metals, in oidinary circumstances, possess a certain definite gal- 

 vanic character, and when any two are together exposed to an exciting fluid, 

 there is immediately generated a galvanic action ; the electro-positive metal 

 is gradually destroyed, whilst the negative is protected by the action which 

 destroys the positive. Thus zinc and copper placed in connexion, in an ex- 

 citing fluid, lorm a galvanic battery; the positive metal, zinc, being destroyed, 

 and the negative, copper, protected. The ellect of this action may be noiiced 

 upon iron railings which have been connected with the stone work of build- 

 ings by lead, a metal negative to iron ; after long exposure they will be found 

 much more wasted upon the parts touching, or adjacent to the lead, than in 

 any other ; this arises wholly from the galvanic action induced by the two 

 metals in contact, and at the expense of the more positive. Zinc is galvani- 

 cally posi«wc to all ordinaiy metals, and, from this property, protects them 

 when in contact with them ; whilst the zinc wastes away, but very slowly ; 

 because " When exposed to air or placed in water, its surface becomes covered 

 with a grey film of suboxide, which does not increase ; aud this film is better 

 calculated to resist the mechanical and-chemical effects of other bodies than 

 the metal itself." Thus, while zinc has the property of protecting other 

 metals by its electro-positive. state towards them, its own decay is prevented 

 by its oxide, which exists only as a thin film, is insoluble in water, and is not 

 easily removed. These properties of zinc have long been known ; but have 

 not until now been practically taken advantage of for two reasons:— 1st. The 

 impussibiliiy of procuring /lure zinc, except at a cost so great as to prevent 

 its use. And, 2ndly. The impossibility of applying pure zinc in a melted 

 stale to iron. Impure zinc is of little or no value as a protection to other 

 metals, because the impurities it contains being all electro-negative to it, are 

 by their galvanic action operating to its destruction ; and there is no method 

 of procuring the metal pure, but by distillation or deposition. But supposing 

 pure zmc to be obtained, the difficulty is still to be oTercome, of applying it, 

 xcept by deposition. 



