A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s3: The Chemical Operations of the Goldsmith : Cleansing, Colouring and Polishing
A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s3
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The Chemical Operations of the Goldsmith : Cleansing, Colouring and Polishing
v1.c18.s3.1
We now come to a process which, though not so important in the eyes of the goldsmith as some of the processes before described, is yet of the greatest consequence in the enquiry in which we are engaged. This is the process of colouring.
v1.c18.s3.2
If it were possible for our goldsmiths to work with pure gold, the appearance of the material would suffer very little by the various processes of heating, hammering, &c., and a mere mechanical rubbing would throw off the superficial dirt and be sufficient to expose the natural yellow surface of the unalloyed metal. This property of pure gold is due to the fact that no oxides of gold are formed at temperatures to which the metal is exposed during the operations of the goldsmith. Indeed it is well-known that gold does not unite with oxygen at any temperature.
v1.c18.s3.3
But the case is different with copper, the oxide of which is formed at temperatures which are ordinarily attained by the goldsmith in the course of his operations. Hence, if a mass of copper is heated in the goldsmith's fire, it is divested of its bright red colour and acquires a dirty black appearance due to the formation of a coating of cupric oxide. Now, an alloy is nothing but an intimate mechanical mixture of its constituent metals—the molecules of the several metals lying promiscuously as it were. In an alloy of gold accordingly in which copper forms a constituent part, the particles of the latter are freely interspersed between those of gold. When such a substance is heated in air, the gold particles retain their primitive yellow, but the particles of copper, which are exposed to air, are oxidised into CuO. This substance is, as we know, a black amorphous powder and it gives to the whole mass a black appearance, which becomes darker as the metal is hammered over and the powder is thoroughly spread over the surface of the metal.
v1.c18.s3.4
An interesting and rather curious experiment clearly illustrates the above explanation. If a piece of gold containing a little copper be heated in air and care be taken not to rub the metal in any way so as to spread the black oxide of copper over the surface, it will be found that the metal, if free from external dirt or soot, will be still yellow and tolerably bright. The yellow of the more numerous gold particles overpowers the colour of the black oxide of copper. If, however, this black substance be spread over the surface of the metal, the colour instantly changes. The spreading of the oxide is easily brought about by hammering, and thus we can take an apparently bright piece of metal and, by hammering reduce it to a jet black mass.
v1.c18.s3.5
The ignorant goldsmiths explain this blackening by supposing the stain of the black steel hammer to adhere to the surface of gold.
v1.c18.s3.6
We may, perhaps, in this connection mention a particular artifice which is sometimes employed by our goldsmiths to cleanse the surface of a gold and copper alloy, which has been blackened by fire. This method is employed whenever it is desirable in the course of working to expose the bright surface, but where it would be inconvenient, impossible or unnecessary to apply the long and laborious mode of cleansing or colouring which we shall hereafter describe, It is found that if a blackened piece of gold and copper alloy be heated in a charcoal fire to redness and then water be sprinkled on the fire, the alloy at once acquires the bright colour of gold. This phenomenon, though often practised by our goldsmiths, is a puzzle to the most intelligent among them. * * * * * The true explanation of the operation seems to be that, when placed in a charcoal fire, the cupric oxide is reduced to metallic copper. But if it is taken out when hot, it comes into contact with the air again and is oxidised. What the goldsmith does is to suddenly cool down the metal when in the reduced state and not to allow it to come into contact with the open air, until it has cooled sufficiently down, so as not to be liable to re-oxidisation.
v1.c18.s3.7
From what has been stated above, it will be easily seen that when a piece of golden ornament has been completed by the manufacturer, it is of a dirty dark colour and must be cleansed before it can be used. In this country mere cleansing is not sufficient, for the popular taste does not approve the new golden articles appearing and being used in the yellow colour of true gold. The articles must be "coloured" before they can be made over to the customer, by which is meant that a bright reddish colour must be imparted to them. The particular tinge which is liked by different persons is different, some people liking a colour verging very nearly on red or orange, while others have a fancy for a colour which approaches to reddish violet.
v1.c18.s3.8
The process of imparting the requisite shade of colour to articles of gold is one of the most cumbrous and complicated chemical operations in the whole range of indigenous arts and manufactures of India. The numerous chemical reactions that take place in the several stages of the business are of more than ordinary interest; while the immense loss of gold that apparently takes place leads one to wonder why the subject has not as yet been taken up seriously by commercial or scientific men.
v1.c18.s3.9
The process of colouring, so recently as 20 years ago, was universally carried out by the goldsmiths themselves, and in the villages and smaller towns the state of affairs is still the same. But in the metropolis and at the bigger stations of the province, the advantages of the principle of division of labour have lately begun to be felt in every branch of art and manufacture, and the result, so far as this particular subject is concerned, has been the springing up of a class of people whose sole business is to colour ornaments and other articles of gold. These men are known among our goldsmiths as the rungwalas. They do not charge any remuneration for their labour, but retain the fluid in which, during the process of colouring, a quantity of gold is dissolved.
v1.c18.s3.10
We will now follow the rungwala through the course of a complete set of processes for colouring, skipping over the mere mechanical parts of the work, but dealing a little fully with the portions which are of a chemical nature.