2.3S 



♦ KNOW^LEDGE ♦ 



r^EPT. 19, 1884. 



Day Sign for the Month, 



purposes, but when using the cyanide solution a more 

 obstinate material must be substituted. For small objects 

 earthenware pans are useful, but for larger ones enamelled 

 iron is a belter material. 



The process for coppering zinc is very similar. The metal 

 is first freed from grease by immersion in the alkaline solu- 

 tion, and then placed into a vitriol solution, composed of 

 .'j lb. of sulphuric acid to a gallon of water, no hydrochloric 

 acid beiog neces-iary. The remainder of the process is the 

 same as that for depositing on iron, and for like reasons. 



To deposit a film of copper upon a glass object, a coating 

 of gutta-percha (dissolved iu turpentine) is first applied, to 

 enable the plumbago, which is next brushed over-, to adhere. 

 When any difficulty is found on appljini; the plumbago 

 another process may be resorted to. A pound of tallow is 

 melted and added to two ounces of caoutchouc and u pound 

 of asphalte dissolved in a pint of turpentine. The mixture, 

 after being well stirred, has added to it a solution con- 

 sisting of an ounce of phosphorus dissolved in fifteen 

 ounces of bisulphide of carbon. When thoroughly mixed, 

 the object is dipped in it, or a small quantity is brushed 

 over the surface. The object is then dipped into the 

 nitrate of silver solution (referred to on page 152), next 

 into a vessel of clean water, and subsequently into a 

 solution of chloride of gold. After being again washed it 

 is allowed to dr>', when, the wire being attached, it is placed 

 in the bath, the solution being the same as that referred to 

 in previous articles. 



The surface of the copper on being removed from the 

 bath is usually very bright, the appearance, however, being 



speedily impaired by exposure. Consequently the copp^r 

 is generally lacquered or bronzed. A fair copper tint may 

 be easily imparted by warming the deposit, and then appl) - 

 ing pale lacquer with a camel-hair brush. In bronzing, 

 different colours are produced according to the object 

 operated upon. A brown finish may be imparted by 

 moistening the object with water slightly acidulated with 

 nitric acid, and, after allowing it to dry, heating i^ 

 gradually until the required shade is produced. If the 

 object require relief, a dark brown film is in this way pro- 

 duced, and then the prominences are brightened by rubbing 

 them with a piece of cloth or leather which has been dipped 

 into liquid ammonia. 



These efiects, however, are liable to fade, but more per- 

 manent effects may be obtained by bronzing. A deep 

 brown tone is produced by rubbing a little rouge, mixed 

 with a small (juantity of a weak solution of chloride of 

 platinum, over the copper with a soft brush, and allowing the 

 coating to dry. Another brush, a little harder than the 

 first, is then applied briskly until the necessary brightness 

 is obtained. Variations in depth of colour may be produced 

 by rubbing the bronze off the more prominent parts oi 

 the object. A black bronze surface is produced by 

 dipping the object repeatedly in a weak solution of 

 chloride of platinum, or by mixing a small quantity of the 

 chloride with rouge, and then rubbing the mixture over th& 

 object with a .soft brush, and afterwards with a harder one. 

 Ammonia may be used to lighten the projecting portions 

 of the surface, which should, by the way, be heated before 

 the powder is aj>plied. 



