A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s4: The Processes of the Rungwala
A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s4
- Language
- English
The Processes of the Rungwala
v1.c18.s4.1
The shop of a Calcutta *rungwala* is one of the most miserably furnished working-places in the whole metropolis. A charcoal fire, a pair of bellows, a few crude pieces of indigenous earthern-ware, two or three coarse pots of China, together with a few other little things, are all that greet the eager sight of the observer whom business or curiosity leads to his dark and ill-ventilated den.
v1.c18.s4.2
The *rungwala* has to wait till a sufficient number of golden articles are accumulated in his hands. He does not generally commence the process until he has got at least ten tolas of gold for colouring. Because the cost, the trouble and the time required, as he says, in going through a complete series of operations are such as to prohibit his doing the work with a smaller amount of gold at a time. Ordinarily a "khola", as the amount of gold coloured at one operation is called, consists of about 20 to 30 tolas of gold.
v1.c18.s4.3
Having got the amount of gold sufficient for working a *khola*, the first thing that the *rungwala* has to do is to cleanse the articles of all external dirt as well as the blackess due to the formation of CuO. With this view he first heats them in a charcoal fire, thus causing all the oil, charcoal dust, and other foreign substances on the surface to burn off. He next puts an earthen vessel on his fire and boils in it about a seer of the unripe fruit of tamarind (Tamarindus Indica), a sustance containing considerable amount of tartaric and other acids in the free state. Taking out these boiled articles and squeezing them so as to extract the extremely acid pulp he forms a thick syrupy fluid. In this fluid he boils the articles till the dilute acids dissolve away the cupric oxide, leaving a yellow surface of pure gold. It is found that the solution of tamarind pulp becomes distinctly blue in vitrue of the formation of copper salts.
v1.c18.s4.4
In the place of this rather clumsy mode of cleansing, which is largely practised even in the metropolis, some of the more adventurous rungwalas use a rather more clever and neater method. Instead of preparing a large amount of tamarind pulp, they obtain from the grocer's an ounce or two of a dull white crystalline substance which they call "acid," and a solution of this serves equally well, if not better, for their purpose. To the question as to what the substance called "acid is, they cannot give a satisfactory reply. And even the grocer who sells it cannot enlighten the inquirer more than to this extent that the article is a *gād* (scum) or a bye-product in the manufacture of some acid. Having subjected the substance to a rough chemical analysis, we came to to conclusion that the substance should be nothing more nor less than KHSO₄ or hydric potassic sulphate, an acid salt and a bye-product, as we know, in the process of manufacturing nitric acid. The reaction of this substance on the copper seems to be this:— 2 KHSO₄ + CuO = K₂SO₄ + CuSO₄ + H₂O.
v1.c18.s4.5
The normal salt di-potassic sulphate, copper sulphate and water are formed.
v1.c18.s4.6
What takes place in the gold as regards the particles of copper takes place also in the solder used as regards the particles of both zinc and copper that are on the surface.
v1.c18.s4.7
The oxides of copper and zinc are acted on by the dilute acids of the tamarind or the hydric potassic sulphate as the case may be, and are dissolved away in the form of soluble salts. The reaction in the latter case is:— 2 KHSO₄ + ZnO = ZnSO₄ + K₂SO₄ + H₂O.
v1.c18.s4.8
As the solder contains a large amount of silver, its appearance after the copper and zinc have been dissolved away, is almost perfectly white, which appears rather prominent by the side of the bright yellow surface of gold.
v1.c18.s4.9
The next operation of the *rungwala* is perhaps intended to carry on the operation of cleansing to a further extent. He takes about half a pound of common salt, and mixes it with an equal amount of ordinary potash alum, purchased from the bazar.
v1.c18.s4.10
The mixture is then reduced to the form of a fine paste. The *rungwala* now covers each of the articles to be coloured with a thin layer of this paste of salt and alum and then places them on the fire. After the paste has dried up it is washed off with water, and generally the colour of the gold appears a little bit improved. The reason of this lies in the fact that the few particles of cupric oxide that remain on the surface are not wholly dissolved out by dilute acid in the first operation are got rid of by this process.
v1.c18.s4.11
After having been subjected to these two processes, the article acquires almost perfectly the beautiful yellow of pure gold; but as has been stated already the *pān* or solder appears at the same time as white as pure silver. The contrast of these two different colours placed in close contiguity is rather marked; and the next operation of the *rungwala* is intended to remedy this defect. In order to make the solder and the metal appear of the same colour, he actually covers the surface of the solder with a layer of gold taken from the article itself. This is done in the course of the third process, which we will describe rather fully, as it is the most important of these stages of manipulation at which loss of gold takes place in considerable amount.
v1.c18.s4.12
Having placed an open earthen vessel on his fire and having put some water in it, the *rungwala* prepares a mixture consisting of about four parts of nitre (say 1 lb.), one part of common salt and 1 part of alum (say ¼ lb each).¹ He then puts the mixture into the vessel, taking care always that the water in the vessel be not more than what is necessary for dissolving about half the amount of salts added. The solution is then heated till it boils evolving large quantities of gases and fumes, amongst which chlorine can be easily detected by its unmistakable corroding odour.
v1.c18.s4.13
¹ Cf. p. 73 under "killing of mercury and gold," where a mixture, among other ingredients, of alum, common salt and saltpetre technically called a *vida*, is used.
v1.c18.s4.14
In this boiling solution the goldsmith places the cleansed ornaments so as to keep them wholly immersed. He examines them from time to time and after a while finds that a layer of shining gold has formed on the surface of the white solder. When this deposit is sufficiently thick and the whole article presents the appearance of being formed out of a homogeneous material he takes them out and, washing them well, prepares them for the next operation.
Linked texts (1)
- Rasaratnasamuccaya — Vāgbhaṭa (alchemist), Sanskrit