240 Prof, von Kohell upon Galvanography. 



causes them to receive the copper without difficulty. From a 

 careful examination it will be always found that a spot which, 

 after the action has continued some time, still remains bare, has 

 not the requisite roughness of surface, or that too much resin has 

 been dissolved in the oil of turpentine employed in the painting. 

 In order to make the silver powder adhere properly, the places 

 where it is to be applied may be very slightly damped with oil of 

 turpentine. 



4, Of the removal of the galvanic plate and its treatment till 

 taking impressions from it. — When the deposit of copper has at- 

 tained a sufficient thickness to form a plate from which impres- 

 sions may be thrown oflT, the two plates in their united condition 

 are to be taken out of the apparatus, wiped dry, and screwed 

 tight in a vice between two boards, mi order to separate them by 

 the use of a file. A broad file is to be applied to the edges, and 

 when the galvanic copper that laps over on the back of the sil- 

 vered plate comes off in flakes, it is a sign that we have come 

 down to the painted plate. If these flakes are, as is ordinarily 

 the case, very pliable, the metal thrown down is known to be of 

 good quality ; while, on the contrary, should the flakes turn out 

 brittle, we know that the copper plate precipitated is so too, and 

 therefore liable to be broken if too thin.* 



When the edges are filed off, the two plates are to be separated 

 by inserting the thumb nail or a piece of wedge-shaped wood at 

 one corner. In general the plates may be separated without dif- 

 ficulty ; sometimes indeed they fall apart. If the deposited plate 

 is too thin, it is easy to increase its thickness, by carefully coating 

 the picture side with melted wax, and laying strips of lead cross- 

 wise across it, and bending them back so as to be in contact with 

 the back of the plaie towards the edge. Thus prepared, the 

 plate is to be replaced in the apparatus till the metal attains the 

 substance desired. The lead strips are easily detached, and the 

 wax is to be removed by carefully wiping it ofi" the plate when 

 made hot. 



Sometimes the picture remains tolerably perfect on the silvered 

 plate after the removal of the copper one, but generally spea;king 

 the color adheres to the hollows formed in the latter. It is to be 



* The specific gravity of good pliable galvanic copper was found =■ 89, while a 

 very brittle plate was only = 87. 



