A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s5: Chemical Explanation
A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s5
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Chemical Explanation
v1.c18.s5.1
It is difficult to find out exactly the series of chemical reactions that take place during this period. Various impurities in the articles employed make the inquiry doubly complicated. It would, however, do for our purpose to investigate briefly the principal decompositions and formations that occur, and with this view we may suppose the reagents employed to be chemically free from foreign substances.
v1.c18.s5.2
It is well-known that if nitric and hydrochloric acids are brought together, certain reactions take place. If the acids be tolerably concentrated they act upon one another at ordinary temperatures, the mixture becoming yellow and giving off minute bubbles of gas, smelling like chlorine. If the acids be dilute, we have to heat the mixture to bring about the chemical change. The action that takes place consists of two of stages :—
v1.c18.s5.3
First, the oxygen from the nitric acid oxidizes the hydrogen of the hydrochloric acid, thus :— HNO₃ + 2HCl = H₂O + HNO₂ + Cl₂ ... (i)
v1.c18.s5.4
Secondly, the unstable nitrous acid that is formed acts on another molecule of HCl, producing a compound known as nitrosyl chloride and water, thus :— HNO₂ + HCl = NOCl + H₂O ... (ii)
v1.c18.s5.5
The reactions (i) and (ii) when expressed by a single equation may be written as follows :— NO₂OH + 3HCl = NOCl + 2H₂O + Cl₂ (iii)
v1.c18.s5.6
Now, here, instead of free nitric and hydrochloric acids, we have saturated solutions of a nitrate and a chloride, and an action similar to the above may take place under the influence of heat only, instead of water, the hydrates of the metals present in the nitrate and the chloride are formed. Thus, instead of equation (iii) we may have the following action :— NO₂.OK + 3NaCl + 2H₂O = NOCl + 3NaOH + KOH + Cl₂.
v1.c18.s5.7
When the solution boils and these actions set in, the *rungwala* puts the golden articles in it. The chlorine liberated as well as the chlorine in the nytrosyl chloride attacks the gold and reduces it to the form of a soluble chloride, which mixes with the mass of the fluid. The NO of the nitrosyl, which is liberated, is itself a colourless gas, but it rapidly unites with the oxygen of the air and forms the red fumes of nitrogen peroxide which, together with chlorine, are liberated in large quantities during the third stage of colouring.
v1.c18.s5.8
Now we know that if a solution of gold chloride of sufficient strength is formed and a piece of silver is dipped into it, gold is deposited on the surface of the silver, while a portion of the silver is worn away. The reaction is as follows :— AuCl₃ + 3Ag = 3AgCl + Au.
v1.c18.s5.9
This reaction takes place in connection with the surface of the solder which is exposed. A layer of gold is formed upon it entirely hiding the white surface, so that to all appearance the whole article seems to be composed of the same material.
v1.c18.s5.10
In the explanation given above, the use of alum that is added to make up the *jamak* (as the solution employed in the third stage described above is called) has not been shown, and indeed this point we have not been able to clear as satisfactorily as perhaps it is capable of.¹ It seems to undergo no chemical change, and hence does not act in the capacity of a mordant in this case, as it does so widely in the arts of dyeing or calico printing. It is also found that if no alum is added to the jamak, the coating of gold on the solder is produced, but it adheres very lightly to the surface and does not stand such processes as rubbing or brushing. Hence it seems that the only function of the alum is to fix the coating, i. e. to make it adhere firmly to the surface of the solder. Experienced goldsmiths are also of opinion that, but for the alum, the gilding of the pān would not be permanent. The way in which this is done by the alum is to a certain extent a matter of conjecture. We suppose that the action is of a mere mechanical nature. When the particles of gold stick to the surface of the solder, small quantities of alum solution remain in the pores between the separate particles. These afterwards dry up and crystallise, and the minute crystals act as a sort of mortar between the particles of gold and the surface of the solder, as well as between the particles themselves.
v1.c18.s5.11
¹ Mr. Chakravarti all along fails to realise the important part which alum plays in yielding sulphuric acid. Moreover in the above equation chlorine cannot be expected to be set free in presence of alkalies; see below under "the Hindu method of manufacturing calomel."
v1.c18.s5.12
The third process in the course of colouring is by far the most important source of loss of gold in the whole series of manipulations that the metal has to undergo. We have seen that the losses at all other stages are due to causes of a mere mechanical nature and are necessarily small. In all these cases, the gold escapes the vigilant eye of the worker and hence the extent to which it is lost sight of is minute indeed. Here for the first time the gold undergoes a chemical change—it ceases to exist as gold altogether—and is consequently lost for ever to the illiterate goldsmith.
v1.c18.s5.13
We should remark that only a very small part of the gold that is dissolved out in the *jamak*—as the solution of nitre, alum and salt used in the third stage described above is called—is re-deposited on the surface of the solder. A large portion of the gold remains in the solution as AuCl₃. NaCl. 2H₂O. The helpless *rungwala* can do nothing with his valuable fluid but dispose it off for a trifling sum to a class of men called *jamakwalas* who have a crude method of their own to extract some quantity of the gold from the *jamak*. We shall return to the *jamakwala* subsequently. We must now proceed to the fourth or last stage of colouring, during which the requisite colour is imparted to the metal. The articles, after being extracted from the boiling *jamak* solution, are thoroughly washed and rubbed with a brush, after which another bath is prepared for them. This consists of a solution of tamarind pulp, nitre and common salt in water, which is placed in an earthen pot on fire and heated to ebullition. A little sulphur is then added after which the articles are placed in the liquid. The sulphur is added in a finely divided state, obtained by rubbing a stick of sulphur with a little water on a piece of slate. Care is always taken to add an insufficient amount of sulphur at first, for it is found that the shade of colour that is produced in the gold depends solely on the quantity of this ingredient, which accordingly requires exceedingly carefully regulation. The *rungwala* now examines the colour from time to time, adding more sulphur gradually if he wants the colour to deepen. When the requisite shade of colour is reached, he stops adding the sulphur and keeps on boiling for some time more, after which he takes the articles off the solution and gives them a final wash and polish. If too much sulphur is added in this operation the colour becomes deep violet and finally black, and the whole process is vitiated. Under the circumstances the rungwala has to recommence the work from the very beginning.
v1.c18.s5.14
It may be noticed that the acid or the commercial hydric potassic sulphate is never used in this stage, as it is used in the first, instead of the tamarind pulp. The reason seems to be that as nitre is also present in the mixture, the addition of KHSO₄, in the presence of other substances, may result in the formation of nitric acid under the influence of heat.
v1.c18.s5.15
The chemical actions that take place during the fourth or last stage of colouring seems to be extremely difficult of explanation. There can be little doubt, however, as to the fact that the reddish violet colour is due to the deposition of a thin layer of some compound of gold on the surface of the articles. It is the determination of this compound and of the exact nature of the process by which it is formed that presents all the difficulty.
v1.c18.s5.16
The fact that the sulphur is one of the most essential things that are required in the production of the colour would seem strongly to suggest at the first glance that the reddish violet tint is due to the formation of one of the sulphides of gold. It appears that the subject and constitution of the compounds of gold and sulphur is one which has not yet been fully investigated by chemists. But it seems to be a well-established fact that no sulphide of gold is known which is of any other colour than black. This stands in the way of our accepting the hypothesis that the reddish violet colour of gold obtained by the Indian goldsmith is due to the formation of a layer of sulphide on the surface. From the fact that if the colouring is allowed to be overdone *i. e.* if the articles be boiled further after the required tint has been obtained, the gold acquires a deep violet colour, the same conclusion is arrived at.
v1.c18.s5.17
Accordingly, in spite of the fact that the amount of sulphur added regulates the whole operation of the last stage of colouring, we are constrained to search for the reddish violet colour that is developed in the formation of some other compound than a sulphide. The nature of the colour being the same as would be produced by a thin layer of a violet substance on a yellow surface, it follows that the colour of the compound formed must be violet. And the fact that when the colour is overdone, *i. e.* when a thick layer of the coloured substance is formed on the surface, the appearance of the metal is dark violet, abundantly confirms the hypothesis. The only compound of gold that possesses this characteristic colour being aurous oxide Au₂O, we are forced to the conclusion that the colour of gold is due to the formation of a thin layer of suboxide on the surface of the metal.
v1.c18.s5.18
An examination into the nature of the chemical actions that take place during the operation clearly shows that Au₂O is the compound formed. We have seen in investigating the chemical changes during the third stage of colouring that KNO₃ and NaCl being heated together, yield free chlorine together with KOH and NaOH. This chlorine after being liberated in the fourth stage attacks the gold and a small amount of auric chloride is formed. But at this point sulphur is added, and the nascent chlorine, instead of going to attack the gold any more, combines with the sulphur to form one or more of the sulphur chlorides. The supply of fresh chlorine being thus cut off, the heated auric chloride decomposes into aurous chloride and chlorine. The chlorine thus liberated combines with the sulphur, while the aurous chloride is acted on by the KOH and NaOH with the result that Au₂O is formed.¹ This is partly deposited on the surface of the gold producing the required colour and partly mixed with the saline solid matter present in the solution.
v1.c18.s5.19
¹ The explanation is scarcely satisfactory; it may be taken for what it is worth-
v1.c18.s5.20
The series of chemical reactions may be expressed by formula as follows:—
v1.c18.s5.21
(1) KNO₃ + 3NaCl + 2H₂O = NOCl + 3NaOH + KOH + Cl₂; this reaction itself consists of two stages as explained before.
v1.c18.s5.22
(2) 3Cl + Au = AuCl₃
v1.c18.s5.23
(3) 2Cl + S₂ = S₂Cl₂ (SCl₂ may also be formed in the precence of excess of nascent chlorine).
v1.c18.s5.24
(4) Au Cl₃ = AuCl + Cl₂
v1.c18.s5.25
(5) 2 AuCl + 2KOH = Au₂O + 2KCl + H₂O.
v1.c18.s5.26
It will be observed from the above formula that the addition of sulphur is indispensable in this operation If the excess of chlorine were not removed by sulphur, as is done in (3) above, the gold would continue to be dissolved out and a large loss of the metal would arise as in the course of the third stage.
v1.c18.s5.27
Moreover, with a free supply of chlorine, the auric chloride would never be reduced to aurous chloride, and thus no formation of aurous oxide could take place. It is thus seen that, though the sulphur used is not directly contained in the compound which gives the colour, it regulates the whole reaction in the most important stages of its progress.
v1.c18.s5.28
The above hypothesis is supported by the fact that if a little sulphur is added to the fluid obtained in the fourth stage of colouring, it will be found to disslove in the fluid. This could not be the case unless a compound like S₂Cl₂, which can hold sulphur in solution, were formed during the re-action.
v1.c18.s5.29
It has been mentioned above that the loss during the third operation of colouring is considerable. Indeed, from the records of observation given at the end of this section, it is clear that the loss is not only large, but that its extent is also uncertain in the extreme. In some cases it is as low as half a pie per tola, while in others it reaches the ruinous figure of six pies per tola, or nearly one tenth of the whole weight. The average amount of 2½ pies per tola, which is obtained from our figures, is not very different from the idea of experienced *rungwalas* and goldsmiths on the subject. The largeness and uncertainty of loss are easily accounted for by the ever-varying constitution of the bazar articles used in the operation, some of them being very nearly pure, while others contain as much as 25 to 30 per cent of impurities, as well as by the want of any accurate precision as regards the proportion of materials employed. The ingredients are seldom weighed before being mixed together; and the varying amount of water present physically in the salts is an additional source of error and confusion in settling the proportion by mere inspection of bulk.
v1.c18.s5.30
The following figures will give some idea as to the magnitude of the loss in colouring:—
v1.c18.s5.31
| Weight before colouring. | | | Weight after colouring. | | | Loss. |
| Tolas. | Annas. | Pies. | Tolas. | Annas. | Pies. | Pies. |
| 6 | 10 | 0 | 6 | 7 | 1.5 | 10.5 |
| 3 | 2 | 0 | 2 | 14 | 1 | 15 |
| 12 | 12 | 1 | 12 | 5 | 3 | 26 |
| 5 | 3 | 2 | 5 | 2 | 0 | 6 |
| 3 | 0 | 2 | 2 | 12 | 0 | 18 |