130 ELECTRO-GILDING. 



of a good colour, much depends upon the state of the solution 

 and battery : it is therefore necessary that strict attention be paid to 

 these, and more so as the gold solution is very liable to change if 

 the relative size of the article receiving the deposit is not according 

 to that of the positive plate. 



The result of a series of observations and experiments, continued 

 daily throughout a period of nine months, showed that in five in- 

 stances only the deposit was exactly equal to the quantity dis- 

 solved from the positive plate. In many cases the difference did 

 not exceed 3 per cent., though occasionally it rose to 50 per cent. 

 The average difference, however, was 25 per cent. In some cases, 

 double the quantity dissolved was deposited, in others the reverse 

 occurred both resulting from alterations made in the respective 

 processes ; for in these experiments we varied, as far as practicable, 

 the state of the solution and the relative sizes of the negative and 

 positive electrodes. 



The most simple method of keeping a constant register of the 

 state of the solution is to weigh the gold electrode before putting 

 it into the solution ; and, when taking it out, to compare the loss 

 with the amount deposited ; a little allowance, however, must be 

 made for small portions of metal dissolved in the solution, from the 

 articles that are gilt, which, when gilding is performed daily, is con- 

 siderable in a year. A constant control can thus be exercised over 

 the solution, to which there will have to be added, from time to 

 time, a little cyanide of potassium, a simple test of requirement 

 being that the gold pole should always come out clean ; for if it has 

 a film or crust, it is a certain indication that the solution is deficient 

 of cyanide of potassium. Care must be taken to distinguish this 

 crust, which is occasionally dark-green or black, from a black 

 appearance which the gold pole will take, when very small in 

 comparison to the article being gilt, and which is caused by the 

 tendency to evolve gas. In this case, an addition of cyanide of 

 potassium would increase the evil ; the black appearance, from the 

 tendency to the escape of gas, has a slimy appearance. This gene- 

 rally takes place when the solution is nearly exhausted of gold, of 

 which fact, this appearance, taken conjointly with the relative sizes 

 of the electrodes, are a sure guide. 



To regulate the Colour of the Gilding. The gold upon the gilt 



article, on coming out of the solution, should be of a dark yellow 

 colour, approaching to brown, but this, when scratched, will yield 

 a beautifully rich deep gold. If the colour is blackish it ought 

 not to be finished, for it will never either brush or burnish a good 

 colour. If the battery is too strong, and gas is given off from the 

 article, the colour will be black ; if the solution is too cold, or the 



