Iron in Ancient India c9.s3: Manufacture of Wootz—Crucible Process of Making Cast-Steel by Cementation
Iron in Ancient India c9.s3
- Language
- English
Manufacture of Wootz—Crucible Process of Making Cast-Steel by Cementation
c9.s3.1
We would now proceed to describe the traditional Indian method of manufacturing steel—"the ondanique of Marco Polo—which supplied many a true blade girded by Asiatic conquerors of yore, and may be said to have overthrown dynasties and established empires." It has already been said that Indian steel was in great demand in the Middle Ages not only in Asia but also in Europe, and many attempts at imitation were naturally made by Europeans though unsuccessfully. Mr. Henry Wilkinson informs us in this connection that "with respect to the various
c9.s3.2
वङ्ग-कङ्क-रोधिक-वंग-भूपारकेषु च।
विदेहेषु तथाङ्गेषु मध्यमाम-वेदिषु।
मध्यमामेषु चीनेषु तथा कालञ्जरेषु च।
खड्गप्रधाने तज्ज्ञानं खड्गानां शृणु लक्षणं।
खड्गोऽहर्जाता ये दर्शनीयाश्च ते मताः।
काष्ठच्छिदः सार्धकार्ये सर्वशास्त्रविशारदाः।
तीक्ष्णाः वेदमयाः खड्गाः खड्गाः भूपारकोद्भवाः।
चमत्कृतैर्विज्ञेयाः प्रभावतः विदेहजाः।
खड्गेऽपि खड्गाः मध्यमाः मध्यमाः।
खड्गाश्चमत्कृताः वेदिदेशसमुद्भवाः।
सद्योऽमीखड्गाः सुतीक्ष्णाश्चमत्कृताः।
मित्रपाणिप्रशस्ताश्चीनेदेशसमुद्भवाः।
कालिङ्गराः कालमदातेषां वक्ष्यामि लक्षणं।
इत्युत् शार्ङ्गधरपद्धति, खड्गप्रशंसा।
c9.s3.3
attempts at imitation, the least ingenious one is certainly that of etching or engraving a blade of common steel, more to deceive the purchaser. Amongst the numerous failures we may enumerate those of Messrs. Nicholson, O'Reilly, Wilde and others in England as well as that of Monsieur Clouet in France."¹
c9.s3.4
¹ Journal Royal Asiatic Society, 1837, Vol. IV, p. 188.
c9.s3.5
The first to examine wootz chemically was Dr. Pearson, who in 1795 contributed a paper on the subject to the Philosophical Transactions of the Royal Society (Part II). The difference between wrought iron and steel, as is well known, lies in the fact that steel contains more carbon than wrought iron while cast iron contains still more carbon than either wrought iron or steel. But owing to the imperfect methods of analysis prevalent at that time Dr. Pearson states the results of his analysis of the percentage of carbon in a curious fashion. He estimated "the quantity of carbon in the wootz and steel to be nearly equal; and that quantity to be about one-third of the hundredth part or 1/300 of the weight of these substances." He also estimated the specific gravity of wootz as follows:—
c9.s3.6
| 1. | Wootz | .. | 7·181 |
| 2. | Another specimen of wootz | .. | 7·403 |
| 3. | The same forged | .. | 7·647 |
| 4. | Another specimen forged | .. | 7·503 |
| 5. | Wootz which had been melted | .. | 7·200 |
| 6. | Wootz which had been quenched while white hot.. | .. | 7·166 |
c9.s3.7
Dr. Scott who forwarded the specimens of wootz to Dr. Pearson from Bombay described that wootz was used "for cutting iron on a lathe, for cutting stones, for chisels, for making files, for saws, and for every purpose where excessive hardness was necessary." Wootz, however, could not bear anything beyond a very slight red heat, which made it work very tediously in the hands of smiths. There is very little of chemical importance in Dr. Pearson's paper, and he was quite mistaken, as will be shown below, in thinking "we may without risk conclude that it (wootz) is made directly from the ore and consequently that it has never been in the state of wrought iron," the actual process being just the reverse, viz. carburisation of wrought iron. Faraday and Stoddart had also examined wootz.
c9.s3.8
The first complete analysis of ‘wootz’ was made by Mr. T. H. Henry in the middle of the nineteenth century. The results of his analysis are given below:—
c9.s3.9
| Carbon combined | .. | 1·333 | 1·340 |
| " uncombined | .. | ·312 | .. |
| Silicon | .. | ·045 | ·042 |
| Sulphur | .. | ·181 | ·170 |
| Arsenic | .. | ·037 | ·036 |
| Iron by difference | .. | 98·092 | |
| | | 100·000 | |
c9.s3.10
Steel may be prepared in two ways, firstly, by removing part of the carbon of cast iron before it is converted into wrought iron; or secondly, by carburising wrought iron or making it combine with the requisite quantity of carbon. The second process is called the "process of cementation" and has been discovered in England only in the eighteenth century, while Indian iron was prepared by this process from time immemorial. As has already been mentioned, Sir Robert Hadfield on examining photomicrographically some ancient Ceylonese iron specimens of the fifth century A.D. found that a chisel showed distinct signs of carburisation varying on the two faces "from saturation point (·9 per cent) to about ·2 per cent carbon on the outside edge." He further remarks: "the presence of martensite and hardenite suggests the important information that the chisel has been quenched."¹ Sir Robert concludes: "the author believes this to be the first time there has been put up on record evidence of the art of cementation having been known 1500 to 2000 years ago, as shown by these specimens; probably therefore, such knowledge could be traced back still further." Ceylon, having been only a part of Greater India, we may safely conclude that the process of cementation and quenching was known in India "1500 to 2000 years ago," if not earlier.
c9.s3.11
¹ Journal of Iron and Steel Institute, 1912, Vol. 85, No. 1, p. 164.
c9.s3.12
Judging from the conservatism of the people we would expect to find the same process in working in the eighteenth or nineteenth century, though it must be confessed, on an infinitely smaller scale, as the scientific spirit of enquiry and industrial enterprise seem to have almost left India to find a new habitat in Europe at the close of the seventeenth century. We have, however, got excellent account of steel manufacture in Southern India from the accounts of travellers like Dr. Voysey (1795), Buchanan (1800) and Heyne (1794-1814). The process of Indian steel-making may be summarised in the following way—wrought iron is first obtained by the *direct method*, viz. heating the ores of iron with charcoal in small blast furnaces (the blast being admitted by means of hand bellows) without the intermediate formation of cast iron as has already been described. The wrought iron is then cut into small pieces and taken in crucibles along with certain kinds of wood and leaves of plants and then heated in charcoal blast furnaces with the lids closed. The blast is continued from four to six hours when the steel is obtained in the molten condition. Water is then sprinkled or poured on the metal, which is thus hardened on being quenched and the steel obtained in a crystalline condition.
c9.s3.13
We give below Heyne's account of steel-making in Mysore which was published in 1814.¹ "In order to convert the iron into steel each piece is cut into three parts, making fifty-two in the whole, each of which is put into a crucible, together with a handful of the dried branches of "tangedu" (cassia auriculata) and another of fresh leaves of "vonangady" (convolvulus laurifolia). The mouth of the crucible is then closely shut with a handful of red mud, and the whole arranged in circular order with their bottoms turned towards the centre in a hole made on the ground for the purpose. The whole is then filled up with charcoal, and large bellows are kept blowing for six hours, by which time the operation is finished. The crucibles are then removed from the furnace, ranged in rows on moistened mud, and water is thrown on them whilst yet hot. The steel is found in conical pieces at the bottom of the crucibles, the form of which it has taken. The upper or broader surface is often striated from the centre to the circumference. The iron ore used is magnetic iron sand common all over the coast and even found on the sea-beach near Madras." The steel thus obtained is heated again in closed crucibles so that the excess of carbon is burnt out. Dr. Heyne also publishes a letter from Mr. Stoddart that even up to his time, not to speak of Indian steel of the Middle Ages, " Indian steel was decidedly superior to any other description of steel."
c9.s3.14
¹ Heyne's "Tracts on India, historical and statistical," p. 359.
c9.s3.15
The well-known traveller Dr. Buchanan who visited Southern India in 1800 also gives a similar account of steel-making.¹ " In each is put one-third part of a wedge of iron, with three rupees' weight of the stem of " tayngada " or cassia auriculata and two green leaves of the " hugivany " which is no doubt a convolvulus or an Ipomea with a large smooth leaf; but never having seen the flower, I could not in such a difficult class of plants attempt to ascertain the species. The mouth of the crucible is then covered with a round cap of unbaked clay and the junction is well luted." The crucibles are then dried and heated in a charcoal furnace fed with hand-bellows. The operation is conducted in four hours. When the crucibles are opened, the steel is found melted into a bottom with evident marks on its superior surface of a tendency to crystallization, which shows clearly that it has undergone a complete fusion. The steel is subjected to heat a second time in closed crucibles until the metal is obtained in a suitable condition.
c9.s3.16
¹ Buchanan's " Travels from Madras through Mysore, &c.", p. 308.
c9.s3.17
Syed Hossain Bilgrami in his lecture on " Iron industry in the Hyderabad and Deccan " before the Iron and Steel Institute has given a similar narrative of the crucible process of steel-making in the Nizam's Dominions, the wood used " being a species of cassia, very common in the country and known as cassia tora and the leaves those of calatropis gigantea."²
c9.s3.18
² Syed Hossain Bilgrami : Journal of Iron and Steel Institute, 1899, Vol. II, p. 75.
c9.s3.19
A similar method of manufacturing cast-steel was prevalent in Ceylon from early times, though the native method of steel-making has almost been beaten out of the country by foreign competition. Mr. Ananda K. Kumarswamy writes in his Mediæval Singhalese Art: " the steel is made in clay crucibles, each about eight inches long, two inches in diameter, and a quarter of an inch in thickness. In the crucible is put a piece of iron, with some chips of *cassia auriculata*, in the proportion of 12½ oz. iron to 5 oz. wood, in the case examined. The crucible is covered with a lid, having small holes pierced for the escape of gas; six crucibles thus prepared are embedded in the charcoal and a fire started. Very soon the woods burn off, and while this goes on the blowing is stopped. Then the blast is kept up continuously." The steel obtained in the course of a few hours is highly crystalline.
c9.s3.20
CRUCIBLE PROCESS OF MAKING CAST-STEEL—AN INDIAN DISCOVERY.
c9.s3.21
From the accounts given above it is evident that the traditional Indian method of making steel was the crucible process of making cast-steel in a fused condition by cementation, which process should in reality be regarded as an Indian discovery. The chemical action that takes places, as suggested by Heath, is that during the application of heat to the closed crucible the dry wood and green leaves would yield charcoal as well as an abundant supply of hydrocarbons. The *joint action of carbon and hydrocarbons* on the iron greatly facilitates the formation of steel as the European method of cementation by means of *charcoal alone* used to take six or seven days, and even fourteen to twenty days, while the Indian process takes only four to six hours. Though cementation has been mentioned by Agricola in his *de re metallica*, and though Reaumer in 1722 described the process of converting bar-iron into "blistered steel," it was only in 1800 that Mr. D. Mushet took out a patent for converting malleable iron into cast-steel by fusing it in a closed crucible in contact with carbon. "Now this specification," says Mr. Heath, "unquestionably comprises the principle of the Indian process which adopts the use of dry wood, which is a substance containing the coaly or carbonaceous principle." This opinion is confirmed by Dr. Percy who says, "It is curious that Mushet's process, so far as relates to the use of malleable iron in the production of cast-steel, should in principle, and I may add even in practice too, be identical with that by which the Hindoos have from ancient times prepared their wootz. I cannot discover any essential difference between the two." ¹ Dr. Percy, however, did not take into consideration the action of carburetted hydrogen gas produced by the destructive decomposition of wood in the Indian process, though as a matter of fact in the year 1825 Mr. Charles Mackintosh actually took out a patent for converting malleable iron into steel by exposing it to the action of carburetted hydrogen gas in a closed vessel, at a very high temperature, by which means the process of conversion into steel is completed in a few hours while by the old method it was the work of from fourteen to twenty days.
c9.s3.22
¹ Percy's "Metallurgy," Vol. II, p. 778.
c9.s3.23
As regards the particular kind of wood employed, the selection of *cassia auriculata* (from which catechu is obtained) and of the leaf of *convolvulus* may not be altogether an accident. It is possible that experience had taught the Indian workers the use of these two kinds of wood as affording suitable quantities of carbon and hydrocarbons to the iron so that the conversion of iron into steel could take place more quickly than with other kinds of wood.
c9.s3.24
As regards the proportion of carbon contained in steel, the excess of carbon is burnt away during reheating. In this connection the remarks of Ritter Cecil von Schwarz, who was for some time in charge of the Bengal Iron Works Company, would be interesting. He wrote: "It is well known by every manufacturer of crucible cast-steel how difficult it is sometimes to get the exact degree of hardness to suit certain purposes, especially with reference to steel for cutting the blades, etc. With the ordinary process endeavours are made to reach the required degree of hardness by selecting such raw materials as on an average have the required contents of carbon in order to correspond with the required degree of hardness as far as possible. The natives (of India) reached this degree by introducing into their cast-steel an excess of carbon, by taking this excess gradually away afterwards, by means of the slow tempering process, having it thus completely in their power to attain the exact degree by interrupting this decarbonising process exactly at the proper time in order to cast steel of a quality exactly suitable for the purpose." ²
c9.s3.25
² Quoted in P. C. Ray's History of Hindu Chemistry, Vol. I.
c9.s3.26
We hope we have been able to give a trustworthy account of the process of the manufacture of Indian steel, which was an object of envy of all nations but successfully imitated by none and which supplied the materials of many a true blade of warriors both in the East and the West. It is sad to reflect that an ancient indigenous industry which attracted merchants from Persia, as narrated by Dr. Voysey, barely hundred years ago, is on the point of extinction; but as even the darkest cloud is not without a silver lining, a distinct ray of hope is visible in the not very distant horizon presaging that India will yet regain her lost iron industry under modern scientific conditions together with other attendant industries depending upon iron.
Linked texts (1)
- Śārṅgadharapaddhati — Śārṅgadhara, Sanskrit