BRONZING. 89 



be used, and the processes already referred to be repeated until the 

 desired brown tint is obtained. In this kind of bronze a little 

 Hematitic iron ore, which has an unctuous feel, may be brushed 

 over the face of the bronze, by which a beautiful lustre is imparted 

 to it, and a considerable variety in the shade may be obtained. 

 In the brown bronzes the copper is slightly oxidized on the 

 surface. 



Black Bronzes. A very dark -coloured bronze may be obtained 

 by using a little sulphuretted alkali (sulphuret of ammonia is best). 

 The face of the medal is washed over with the solution, which 

 should be dilute, and the medal is to be dried at a gentle heat. It 

 should afterwards be polished by a hard hair brush. Sulphuretted 

 hydrogen gas is sometimes employed to give this black bronze, but 

 the effect of it is not so good, and the gas is very deleterious when 

 breathed. In these bronzes the surface of the copper is converted 

 into a sulphuret. 



Many metallic solutions, such as weak acid solutions of platinum, 

 gold, palladium, antimony, &c., will impart a dark colour to the 

 surface of medals when they are dipped into them. The medal after 

 being dipped into the metallic solution is to be well washed and 

 brushed. In such bronzes the metals contained in the solution are 

 precipitated upon the face of the copper medal, which effect is 

 accompanied by a partial solution of the copper. 



Green Bronzes. Green bronzes require a little more time than 

 those already described. They depend upon the formation of an 

 acetate, carbonate, or other green salt of copper upon the surface of 

 the medal. Steeping for some days in a strong solution of common 

 salt will give a partial bronzing which is very beautiful, and if 

 washed in water and allowed to dry slowly, is very permanent. 

 Sal ammoniac may be substituted for common salt. Even a strong 

 solution of sugar, alone, or with a little acetic or oxalic acid, will 

 produce a green bronze ; so also will exposure to the fumes of dilute 

 acetic acid, to weak fumes of hydrochloric acid and to several other 

 vapours. A dilute solution of ammonia allowed to dry upon the 

 copper surface will leave a green tint, but not very permanent. 



Electrotypes may also be bronzed green, having the appearance 

 of ancient bronze, by a very simple process : take a small portion of 

 bleaching powder, place it in the bottom of a dry vessel, and suspend 

 the medal over it, and cover the vessel : in a short time the medal 

 will take on a green coating, the depth of which may be regulated 

 by the quantity of bleaching powder used, or the time that the 

 medal is suspended in its fumes : of course any sort of vessel, or any 

 means by which the electrotype may be exposed to the fumes of the 

 powder, will answer the purpose : a few grains of the powder is all 

 that is required. According as the medal is clean or tarnished, dry 



