GALVANIC GILDING. 77 



the following points. "VYe must know the proportion, 1, of 

 gold to the potassa-salts contained in the gold solution ; 2, of 

 the gilding surface to the strength of galvanic current, and 

 the strength of the gold solution ; 3, of the surface of the 

 anode to the gilding surface, and to the content of gold in 1 

 decilitre-solution ; 4, of the surface of the anode and of the 

 gildino; surface to the strength of the current. 



The gold solution is thus made. 1 pt. gold is dissolved in 

 aqua regia and evaporated (at a gentle heat) to dryness, the 

 residue treated with an aqueous solution of 1 pt. caustic po- 

 tassa, this mixture then with an aqueous solution of 2| pts. 

 Cyanide of potassium, and 1 pt. caustic potassa, and the whole 

 warmed and filtered. 



He found that, for successful gilding, the above proportion 

 between the gold and potassa in the solution should remain 

 constant ; and if (as when a platinum anode is used) the 

 quantity of gold diminishes by precipitation, the force of the 

 galvanic current must be increased by adding more cells, and 

 increasing the surface of the anode, since this force is di- 

 minished in a dilute solution. The finest gilding is obtained 

 when 1 decilitre of solution contains from 1 to J gramme of 

 gold. A reddish gilding is obtained by diminishing the gal- 

 vanic force, such as lessening the number of cells, or the 

 surface of the anode, or increasing the gilding surface. 



Leuchtenberg rejects gold and silver anodes, because there 

 is not as much of them dissolved as is precipitated on the 

 cathode, and because they become coated with cyanides. He 

 therefore employs a platinum anode, and determines the con- 

 tent of gold in solution, before and after gilding, by chemical 

 analysis, in order to ascertain how much gold has been pre- 

 cipitated on the cathode. This is done by evaporating a 

 decilitre to dryness, moistening with sulphuric acid, heating 

 to redness, and extracting by water, when metallic gold 

 remains. 



In the galvanoplastic establishment of St. Petersburg, about 

 18 tons (363 ctr.) of copper are annually precipitated, 300 

 pud of silver, and about GOft) gold, thrown down in a month. 

 g2 



