24 



When the ink is well dried, the white parts are cleaned again by 

 polishing the plate with cotton and pounce, or any other polishing 

 powder : a ball of cotton, or any other matter, covered with a thin 

 piece of caoutchouc or skin, can be used for this purpose. When 

 polished, the plate is ready to receive the electro. chemical-coating 

 of rold, which will protect the white parts. 



Gilding. The gilding is'obtained by any of the various processes 

 of electrotyping which are known. The only indispensable condi- 

 tion is, that the surface obtained by the precipitation must not be 

 liable to be attacked by any weak acid ; a solution answering this 

 purpose is made of ten parts (by weight) of ferrocyanide of potas- 

 sium, one part of chloride of gold, and 1000 parts of water, used 

 with a galvanic battery. During the gilding the plate must be 

 turned in several positions, in order to regulate the metallic deposit. 

 In some cases the gilding may be made more perfect, if the plate is 

 covered with a thin coating of mercury before being put in the 

 gilding solution. 



When the plate is gilded, it must be treated with the boiling 

 caustic potash, by the process already indicated for the preparatory 

 engraving, in order to cleanse it from all the dried oil or ink which 

 fills the hollows. The plate is then washed and dried, and when the 

 oil employed has been thickened with the lamp-black, the surface 

 of the plate is rubbed with crumb of bread, in order to cleanse and 

 take off the black remainirg ; then, the white parts being covered 

 and protected by a varnish not liable to be attacked, and the black 

 parts being uncovered and clean, the plate can be bitten in by aqua- 

 fortis, according to the ordinary process used by engravers. 



This operation must be done upon the stand, and not by immers- 

 ing the plate in the solution. 



Before this last biting-in, if the preparatory engraving has not 

 succeeded well, and the plate still wants a sufficient grain, it can be 

 given by the various processes of aquatint engraving. 



Before submitting the plate to the operation of printing, in order 

 to insure an unlimited number of copies, it is necessary, as before 

 stated, to protect it by a slight coating of copper, which is obtained 

 by the electrotype process ; otherwise the printing would soon wear 

 the plate. This coating must be kept very thin, lest the fineness of 

 the engraving, and the polish of the white parts, should be de- 

 stroyed. In this state the plate can be delivered to the printer. 



After a certain number of impressions have been obtained, it will 

 bo perceived that the coating of copper is worn in some places ; 



