Jan., 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



Electrotyping. 



Bv Dr. F. Molluo Perkin. 



.\'o one has been long in a chemical laboratory without 

 having learnt that one of the simplest tests to ascertain 

 whether a solution contains copper is to place the 

 blade of a pen-knife into it. If copper is present the 

 blade of the knife becomes covered with a thin coating 

 of copper. Other metals besides copper can be plated 

 out upon another metal by simply immersing it in their 

 solutions. For example, if a silver article is dipped 

 into a solution containing a gold salt, it will become 

 covered with a thin coating of gold. This process of 

 dipping is to a certain extent actually performed in 

 practice, hence one is accustomed to talk of giving an 

 article a gold wash. For example, at one time the 

 method employed for gilding the inside of silver boxes, 

 the bowls of spoons, &c., was to wash them over with 

 a piece of rag w hich had been dipped in the gold solu- 

 tion. When a metal is plated by simply immersing it 

 in the solution of the other metal, then an equivalent 

 of the metal w^hich is being plated upon it goes into 

 solution. Thus, when the blade of a knife is placed 

 into a solution of a copper salt and becomes superfici- 

 ally coated with copper, it is only done at the expense 

 of a portion of the blade which goes into solution. 

 Supposing the solution to consist of copper sulphate, 

 then as copper is deposited out, sulphate of iron takes 

 its place. Thus we can write it in the form of an 

 equation : 



Copper sulphate -j- iron = iron sulphate + copper 

 or by using symbols 



CuSCj 4- Fe r= FeSC, -|- Ca. 



The metal which gees into solution and upon which 

 the other metal becomes plated out is said to be electro- 

 positive to the metal in solution. Zinc is the most 

 electropositive of all metals, and under appropriate con- 

 ditions is able to replace all other metals from the 

 solutions of their salts. 



Now this method of plating or depositing out a metal 

 has only a very limited application. It is used to a 

 certain extent in gold plating, but not for depositing 

 such a metal as copper. The methods employed are 

 electrolytic. It is found if an electric current is 

 passed through a solution of a metallic salt, e.g., copper 

 sulphate, that the ccpper is deposited out upon the one 

 electrode,* and at the other electrode if it is of an in- 

 soluble material, such as platinum or graphite, oxygen 

 gas is evolved. The pole at which the metal is de- 

 posited is called the negative pole or cathode, the one 

 at which oxygen gas is evolved, the positive pole or 

 anode. Fig. i shows such a cell diagramatically. A 

 is the negative electrode or cathode ( — ) and B is the 

 positive electrode or anode (+)■ 



If instead of being made of an insoluble material the 

 anode B consists of a sheet of copper, then as the 

 electric current passes the copper will go into solution. 

 Furthermore, the copper will pass into solution at the 

 same rate as the metal is plated out upon the 

 cathode A; theoretically, therefore, the strength of the 

 solution will remain constant. As a matter of fact, 

 owing to secondary changes, after a time it becomes 

 too concentrated. 



* When two pieces of metal connected with the opposite poles of 

 an electric battery are immersed in a solution, as ^hown in the 

 figure, these pieces of metal are called electrodes. 



Electroplating was first suggested by Elkington in 

 1836, but he did not apparently employ it on an indus- 

 trial scale. It is very interesting to note that some of 

 the articles obtained from the coffins of the Egyptian 

 mummies have been found to be coated with copper; 

 probably, however, the coatings of copper in these 

 cases were produced by simple immersion. On an in- 

 dustrial scale electroplating w-as first introduced by 

 M. H. Jacobi, of St. Petersburg, in 1838. Since then, 

 especially of late years, an enormous industry has been 

 developed. By simple immersion heavy deposits of 

 metal cannot be obtained, but coats of any thickness 

 can be produced by electro-galvanising. In this article 

 it is intended to deal not with plating in general, but 

 with the application of the electric current for producing 

 electrotypes or reproductions; this form of electro- 

 deposition is sometimes called galvanoplastic. 



In all cases of reproduction the article to be repro- 

 duced is made the cathode in a bath of copper sulphate, 

 and a strip of copper the anode. Xow, supposing it is 

 desired to reproduce a medallion, if this be of metal 

 and is made the cathode, copper will be deposited upon 

 it, but the copper will adhere so firmly that it will be 

 i-npcssible to remove it. It is, therefore, necessary to 



coat the medallion with an extremely thin film of son-.e 

 material which will prevent the deposited metal from 

 adhering to the metallic surface. This coating must 

 not be sufficiently thick to obliterate or blur the details 

 of the figures, &-c., upon the article which it is desired 

 to reprcduce. There are several methods which may be 

 employed. If the medallion is of silver cr copper, its siir- 

 face after being carefully cleaned so as to remove dirt 

 or grease, is washed with a solution of sodium sulphide, 

 by which means the surface of the metal is coated 

 w'ith an extremely thin film of sulphide of the metal. 

 This surface is co'nducting, but preventr^ the deposited 

 metal from adhering to the article. Another method is 

 to cover it with a thin coating of black lead (plumbago). 

 The coating must be very thin and should be polished 

 in much the same way as the iron-work of a fire-place 

 is polished. In practice, machines are generally used 

 for polishing and plumbagoing surfaces, as it is not 

 an easy matter to get a perfectly smooth and even 

 surface by hand. 



Having satisfactorily prepared the surface of the 

 article, it is hung by means of a copper wire in the 

 depositing bath and connected with the negative pole 

 of the source of current. The conducting wire where 

 it dips below the surface of the copper solution should 

 be covered with an insulating material, such, e.g., as 

 a piece of rubber tubing. As soon as the circuit is 

 closed and the current passes, the surface of the article 

 becomes coated with a thin film of copper, xN-hich gradu- 

 ally increases in thickness, until a coating of about 

 1 to ^ a millimetre in thickness has been obtained. It 



