History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s4: Thermo-Mechanical Treatment
History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s4
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Thermo-Mechanical Treatment
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According to Tylecote (1976) the beginning of the craft of cold working of metals might have begun with the discovery of native Gold or Copper and discovery of their deformation when hammered. The lithic people might have hammered these nuggests with stone to break away chips having sharp edge, as in the case of stone tools, and to their surprise they might have discovered that these nuggets could be deformed by hammering. This might have lead to the manufacture of some simple tools and weapons by cold working, and sharpening of the edges by grinding on stoneslab.
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The other possibility is that the initial metallic nuggets might have been brittle due to the presence of oxide and sulphide inclusions or separation of some other phase on cleavage planes such as separation of Ni on Widmannstat planes in meteoric iron. Such nuggets will break into pieces having sharp edges. In such a case the concept of cold working metals may be associated with the casting technology because, according to Tylecote some of the early copper objects (3000 B.C.) have been found to show the structure of annealed and recrystalised metal.
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Fig. 52. Terracota mould disks for coins and their assembly ready for casting
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So far as India is concerned the technology of shaping of metals by cold working can be dated back to the Vedic period if not earlier. The ‘Richas’ in Fig.4 clearly indicates the acquaintance of ancient Indians with the use of steam power.
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This is possible only when they had gained high degree of knowledge about the ductility of metals like copper and iron and the skill to give them the desired shape by hot and cold working. In Vedas there are also descriptions of the development and use of metallic war weapons.
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There is ample evidence that during the period of Indus Valley civilization Indian craftsmen had gained high level of knowledge regarding the effect of cold working of metals and their recovery on annealing and perhaps they also knew the improvements achieved by hot working of metals. Following is a short account of some of the metal forming technology and the skill achieved by the ancient Indians in improving the properties by thermal treatments such as annealing, hardening and tempering of metals.
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(i) Embossing and Engraving - The art of preparing metallic sheets by cold working of Gold, Silver and other non-ferrous alloys and their shaping by embossing to produce various pots and pans as well as jewellery has been probably the second phase of metal working technology. The art of engraving on metals to produce beautiful designs of flora and fauna might have been a continuation of the traditions of wood carving and knowledge of painting. In many cases the wooden panelling and carvings have been inlayed with fine gold and silver wires, pearls, shells and pieces of mirrors etc. to produce master-pieces giving grandeur to the life style of Kings, Queens and wealthy people. Covering the wooden panelling with Gold, Silver, Copper and Brass sheets and carving beautiful designs on them by embossing and engraving have been in practice and artisans must have used very fine and precisely designed tools mostly made of iron and steel.
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The study of the ancient objects of art and socio-cultural pattern indicate that these ancient craft were highly specialised and disciples were trained from the very childhood in their traditional craft. For example while the iron and steel tools were made by blacksmiths the Gold and Silver work was done by ‘Sonars’ and sheet metal work was done by ‘Thatheras’. Thus the skill was passed on from one generation to the other and sometimes lost due to the natural calamity or other reasons.
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(ii) Wire Drawing and Jari Work - Wire drawing craft has been known in India both for the manufacture of Gold and Silver Jari as well as fine wire of bronze, brass, iron and steel used in the string musical instruments like sitar, sarangi, veena etc. The art of jari making and production of ‘Brocade’ and embroidery silk has been known atleast for the past 4000 years. The silk cloth and Brocade of India had been as famous in the world as the Chinese silk. The process of manufacture of jari thread consists of first making a fine wire of Gold, Silver or their alloy and then flattening them to obtain thin strips having highly polished and shining surface. These strips were then wound around silk and cotton thread to produce jari thread.
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The technique of wire drawing using hardened steel dies or die plates having hair like fine holes has been in use for ages and even today the jari manufacturers of Varanasi and Gujarat are skilled in producing their own dies instead of using the modern diamond dies and machines. The wire manufacturers had the idea of work hardening and annealing to soften the material by recrystallization and grain growth.
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The starting material for this industry was a rod of gold or silver about 10 mm in diameter and 100 to 200 mm in length. This was first cold worked to produce a thinner section having 3 to 5 mm diameter and then it was annealed and converted into fine wire by drawing through a sequence of die holes having smaller and smaller diameter. The process was accompanied by intermittent annealing in a slow heat produced by burning cow dung cakes. The final thin wire was flattened by rolling it between a set of highly polished rolls (50 to 100 mm diam.) having mirror like surface finish. As the thin wire was flattened by these rolls the shining surface finish was transferred to the Gold and Silver strip which was finally wound around cotton or silk thread. A large collection of ancient jari craft and the fine pieces of ‘Brocade’, woven and embroidered silk pieces are being preserved in Bharat Kala Bhawan Museum of Banaras Hindu University and in other National Museums.
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The manufacture of variety of string musical instruments of Indian origin required wires of various diameters produced from very precisely controlled composition. This industry also desired metals and alloys to be produced free of non-metallic inclusions so that long wire could be drawn without its breaking. The Indian craftsmen were not only skilled smelters but also alloy makers as well as skilled in fluxing technology to produce metal ingots, even of steel, free from even the micro size non-metallic inclusions.
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The manufacture of Fe-C alloy i.e. steel wire required skill of very high level and precise experience about the thermomechanical behaviour of these alloys, their work hardening properties as well as annealing behaviour all learnt by time consuming experience was transferred from Master Craftsman to his chosen disciples.
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(iii) Bidri Work - This craft of producing utensils and decorative objects, jewel boxes etc. decorated with beautiful designs produced by inlaying Gold and Silver wires on the surface of the object has been a speciality of the ‘Bidar’ tribe of Hyderabad. In this process as described by Mahamud (1988) at first a zinc base casting of the object is made using lostwax or any other molding and metal casting technique. The composition of the special alloy consists of 16 parts of copper, 4 parts of lead, 2 parts of tin and 112 parts of zinc. The casting is cleaned and its surface is smoothened and polished and then its surface is bleached by dipping the object in a solution containing Sal Ammonia, Saltpeter, Common Salt, Blue Vitriol and Resin mixed in a definite proportion known to the craftsman. This treatment produced a metal ware with grey black finish.
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After this treatment the surface of the object is engraved with beautiful designs of fauna and flora and inlayed with Gold and Silver wire. Finally the inlayed surface is carefully cleaned and buffed to give a shinning finish.
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(iv) Gilding - With the increasing demand of gold objects and the development of Alchemical Principles Indian Craftsman had learnt about the amalgamation property of Mercury with copper, gold, silver and their alloys and this led to the introduction of a new technique of extraction of the precious metals as well as their coating or plating on the surface of cheaper base metals and non- ferrous alloys. This technique of coating or ‘Gilding’ consisted of cleaning the surface of the base metal and then rubbing the Au-Hg or Ag-Hg amalgam on the surface till all the Hg was evaporated away leaving a coating of Au or Ag or their alloy on the surface. Such coatings were long lasting and maintained a highly shinning golden or silvery lustre on the cheaper base metal objects.
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(v) Gold and Silver Foil Technology - Some times gold and Silver foils were also applied to the surface to obtain a thicker coating. These foils were produced by putting a small piece of the metal in between two layers of Chamoi leather and beating it till a very tin foil of the metal was produced. This thin foil was lifted on paper, to give backing support and stored till applied to the surface. Gold and silver foils were also used to decorate food stuff and pan etc. The Ayurvedic medicinal system mentions the beneficial therapeutic use of these foils and curing some of the diseases.
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(v) Smithy Craft in *Iron and Steel* - From the study of the archae ological iron objects there remains no doubt that ancient blacksmiths had attained very high level of skill in cold and hot working and sheet metal forming of wrought iron and iron-carbon alloys. They were also aware of the better metal workability of Fe-C alloys in hot condition i.e. greater than 900°C, and that two pieces of iron could be force welded in the white hot condition (~1100°C).
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The examination of some of the ancient objects such as the iron cannon shown in *Fig 53* show a high level of skill and understanding of these processes.
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The forging of the Dhar Iron Pillar has shown a technological process of manufacturing somewhat similar to that used for the manufacture of the iron cannon. Prakash (1989-90) has studied and analysed the technique of manufacture of the Dhar Iron Pillar. *Fig. 54* shows a schematic drawing giving details of the joint of two iron blocks by the use of a tapered pin as envisaged by Prakash. The joint of the two blocks has been reinforced by butt welding of iron plates from all the sides. *Fig. 55* shows a photograph of the top of the Delhi Iron Pillar and this also shows the signs of hot fitting of a square ring on the circular shaft of the pillar. In the centre of the pillar there is a square cavity supposed to be used for fixing
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*Fig. 53. Photograph of an early iron cannon (Tanjor) made by fixing iron pieces inside a number of iron rings* either a sculpture of ‘Garuda’ or the ‘Lion Emblem’ of ‘Ashoka’. Bindal (1988-89) has reported the presence of sponginess at the centre of the Delhi pillar at certain heights. This may be the result of the tapered pin joint in this pillar similar to that proposed in the Dhar pillar. Unfortunately no documented evidence of remains of
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WELD JOINT TOP PLATE PLUG WELD HOLE SQ. SECN. I SQ. SECN. II BOTTOM PLATE
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*Fig. 54. Drawing shows the probable method of joining two iron blocks in Dhar iron pillar*
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A part of the pillar showing the distorted, broken top
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*Fig. 55. Photograph shows the fixing of a massive iron square ring to form the ornamented top of Delhi’s iron pillar* the site of their production has been traced so far and in the absence of these the iron pillars of Delhi, Dhar and beams of Konark temple need a detailed metallurgical study to reveal the exact process of their manufacture. Bonner et al. (1972), have translated the account book for the manufacture of the iron beams of the Sun temple of Konark, but the details of the operations and process of their manufacture mentioned in this document are insufficient to draw a definite conclusion.
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In order to appreciate the knowledge of the ancient smiths it is essential that we make reference to the Fe-C diagram shown in *Fig. 56*. This figure shows the change in structure of five different Fe-C alloys as attained after normal cooling. This diagram also shows various temperature ranges to which the Fe-C alloy should be heated to carry out different thermomechanical treatment like forging, forge welding, annealing and spherodizing etc. The ancient blacksmiths were ignorant about such phase diagram but they had gained knowledge by experience and they understood the effect of carbon on properties of iron as depicted by the classification and proved by the manufacture of many massive iron objects which have no parallel in the world.
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The world famous ‘Wootz’ steel swords were made by forge welding small rods of wrought iron and steel cakes in a very narrow range of temperature and then giving it the traditional ‘watering’ treatment. This must have been possible only by the extensive knowledge about the hot workability of various iron carbon alloys. The cast structure of high carbon hyper eutectoid steel consists of massive platelets of Cementite separating out on Widmanstatten planes and Ledeburite and it is extremely difficult to hot forge it without cracking. Yater (1983,1983-84) and Verhoevens (1987) have discussed the technicalities of forging such steels to produce Damascus swords. The process generally consisted of careful forging of the ingot in cherry red hot (~750°C) condition followed by spherodizing treatment to convert the Cementite plates into spherical massive Cementite. This treatment was followed by cold forging of the sharp edge and low temperature annealing (~800°C). The forged sword blade was cleaned and given proper surface finish before it was hardened.
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Thus the Indian swords and other war implements were manufactured, by forge welding of a number of ‘Wootz’ steel ingots or skill fully sandwiching the high carbon steel pieces between wrought iron plates to obtain high carbon edge with flexible wrought iron blade. In some cases the whole blade was made from wrought iron and then the sharp edge was given carburizing treatment before hardening.
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(vii) Heat Treatment - The ancient blacksmiths had gained very precise knowledge regarding the grain texture of various iron carbon alloys, and they knew that the final hardness achieved on any cutting tools is dependent upon the combined effect of the following five variables :
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(a) the carbon content of the alloy
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(b) the massivity of cross-sectional thickness of the object,
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(c) the final temperature of the object from which it was quenched (900-1000°C),
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(d) The severity of cooling achieved during quenching, and (e) Other factors such as control of the phase transformation by arresting the hardening treatment at an intermediate stage.
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Temperature °F °C 2000- 1100 1850- 1000 1700- 900 1550- 800 1400- 700 1250- 1100- 600 950- 500 0 0,5 1,0 1,5 Upper temperature limit for forging A cm γ + Fe3C Austenite (γ) Normalizing Oil quenching α + γ Oil quenching Water quenching A1 P S Ferrite (α) Water quenching Annealing Recrystallization annealing Stress-relief annealing Ferrite + pearlite Pearlite Cementite Fe3C + pearlite Carbon wt.%
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Fig. 56. Fe-C phase diagram showing microstructure of five typical Fe-C alloys and temperature ranges for common thermo-mechanical treatment
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Prakash (1990) has discussed the effect of these factors on the final hardness obtained and the ancient hardening practice with reference to the modern knowledge of the Time-Temperature and Transformation curves, and hardenability of steels.
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The classical Indian heat-treatment given to the steel swords was to heat the blade to red hot condition and then insert the blade from the cutting edge side into a green trunk of Plantain tree and allow it to cool there. During this process the organic fluid and water present in the cells of the trunk created a cooling rate fast enough to transform the Austenite of the edge into Martensite and in the latter period the dried wood helped in self tempering by acting as heat insulator. This typical heat treatment was called ‘watering treatment’ and it produced a typical flowery pattern on the sword blade as shown in Fig. 34. Recently Rakshit et al. (1991) have experimented on this technique of hardening steels and were successful in developing edge hardness of Rc 45 to 47 with decreasing hardness towards the thicker section of the blade. Archaeological evidence of hardened steel has been found by Ghosh and Chattopadhyaya (1982) dating back to 810 B.C. at Barudih in Bihar.
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(viii) Difference between the Damascusing and Pattern Welded Structures -The original structure of the Damascused or ‘watered’ sword as shown in Fig 34 has been admired both by the ancient as well as modern metallurgists and every effort has been done to duplicate it. One of the approaches carried out all over Europe is pattern welding of steel pieces of different carbon contents in a large number of different ways to generate a structure similar to that of Damascus sword but so far it has not led to any success. One of the main reasons is the non-availability of Damascus swords for making detailed study of its structure and properties using modern tools and techniques.
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As Paisowski (1974) and others have mentioned Belaiew (1921) had the privilege of studying four such blades, and later on others have been quoting his results because even those four swords have been lost. According to the available description the watering pattern on the Damascus sword was developed by etching the surface with a solution containing vitriol, vinegar and many other chemicals, known to the sword manufacturers. After polishing and etching the surface of the sword it was washed, polished and smeared with oil. The study of the watering pattern has shown a structure of dark etching Cementite or Pearlite phase present in a matrix of Ferrite. The heat treatment of pattern welded swords have no doubt produced a variety of etched black and white pattern but they have failed to duplicate the original Damascusing pattern developed on swords made from ‘Wootz’ steel. Recently an International Conference ‘The Craft of the Blacksmiths’ had been organised at Belfast in 1989 and the proceedings edited by Scott and Cleere (1989). At this Conference the archaeometallurgical studies and the experiments conducted on pattern welded steel swords and their heat- treatment has been discussed in great detail.
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Tools and Furnaces - The recent discoveries of stone age sites in Indian subcontinent have proved beyond doubt the antiquity of Indian civilization. During the stone age itself the humanoids had learnt the use of simple tools like Axe, Hammer, Chisel, Knife prepared from stones and bones.
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The stone axe and other stone age tools found at Jalapur and Soan in Pakistan by Rendell and Dennell (1987) are probably the oldest in India dating back to Sivalik series (17 million year old). With the discovery of native metals these were replaced by tool made of copper, bronze and iron and the era of metal extraction technology was initiated as the ancient Indians gained knowledge of the naturally occurring metallic minerals and the reducing as well as smelting power of charcoal fire. Some of the iron tools used for mining have been already reported. These tools seem to be very light weight for shaft sinking and excavation of tunnels and galleries through hard stone beds, but these might have been used for digging out the ore minerals from reefs and streak or soft minerals like, Galena, Limonite and Cuprite etc. For breaking the rock bed either the fire setting method or some heavier tools must have been used.
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So far as extraction of metals is concerned design of a large variety of pit and shaft furnaces have been already described in the text. In *Rasa Ratna Samuccaya* very precise description of furnaces for various purposes has been given. These furnaces were precisely designed and constructed for specific operation. For example, for making ornaments from silver by Philgree work or for soldering pieces of gold or Silver, jewellers have been using an oil lamp and a tapered brass blow pipe. The desired high temperature at the precise location was developed by blowing air through the blow pipe and diverting the intense flame at the precise spot. It shows that even in the early Vedic period Goldsmiths were aware of the effect of Oxygen blowing on the flame intensity.
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Figs 57 and 58 show some of the common metal tools used by smiths for metal working obtained from the excavation site of Taxila dating back to 3rd century B.C. to 5th century A.D. (Marshall, Vol.III, 1951).
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Iron smelters had specialised in producing a variety of refractory crucibles for metal smelting and refining which could easily sustain a temperature of 1500°C
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Fig. 57. Some of the smithy tools found at Taxila (3rd century B.C. to 5th century A.D.)
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[Figure: PLATE 75 Tongs, scissors, pliers, etc. (Marshall, Taxila, Vol. III, 1951)]
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Fig. 58. Some other smithy tools found at the same site
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without any significant deformation. These crucibles were used only once and then discarded. Recent mineralogical study of these crucibles have shown the presence of graphitized Silica and Mullite minerals in these crucibles. These minerals were generally developed by the mixture of clay with rice husk and iron oxide etc. and giving a precisely controlled firing treatment. The smelters and smiths had mastered the technique of controlling temperature and atmosphere of the furnaces by the study of colour and control of the air supply.
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Metal smiths and Blacksmiths had great skill in manufacturing various objects of non-ferrous metal and they had gained knowledge about the work hardening, recrystallization and grain growth during annealing etc., and they designed furnaces suited for the specific job. While, for annealing copper and brass and other low melting metals only cow dung cake was used as fuel in a ‘Ardha Gajaputa’ or ‘Gajaputa’ furnace (1000 cakes), charcoal was used to achieve higher temperature required for annealing of iron. The smithy furnace and the workshop was very simple in the overall out look.
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This photograph has been taken by the author during his trip to Bastar in 1981 and it shows the use of a stone anvil for hot forging small iron objects. Mahamud has described the design of a reverberatory type furnace which was being used for a long time in Assam for annealing and probably malleablizing of Fe-C alloys. Fig.59 shows a drawing of this furnace which is very similar to the modern reverberatory furnace where the flame is diverted back from the roof to strike the charge and heat it.
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For manufacturing large objects like cast bronze cannons or iron forging like Delhi’s Iron Pillar the furnaces were constructed at the site itself using locally available raw material and as described by Bonner et al. (1972) expert blacksmiths and workers were invited from distant places to perform the job. Bonner et al. have also described a variety of tools and techniques used in the construction of the Sun temple of Konark. One very specialised technique described by them is the use of Fig. 59. Sketch of a reverberatory furnace described by Mahmud. It is very similar in construction to the present day furnace
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lead bath for manufacturing the iron beams, although from their description the specific purpose of the use of molten lead mixed with Borax is not very clear. Nagarjuna (Biswas 1981) is considered to be the father of Indian Alchemical Science, and according to Bose et al. (1971) and Filliozat (1951) his work is dated to 1st century A.D. and not 10th century A.D. as estimated by Alberuni. *Rasa Ratna Samuccaya* (8-12 century A.D.) has described variety of crucibles (Musa), Yantra and furnaces for the manufacture of Ayurvedic medicines.
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It also describes in total 51 kinds of metallurgical tools (upkaranas), 36 kinds of equipment (yantras), 17 types of crucibles and 9 types of furnaces for giving Puṭas such as Mahāgajapuṭa, Gajapuṭa, Ardhagajapuṭa, Kukuṭapuṭa, Kapoṭapuṭa etc.
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Fig. 31 shows a sketch of Aṅgār Koṣṭhi capable of achieving temperature of the order of 1500°C. the dimension of this furnace are very similar to those of modern crucible furnace. It also mention the detailed design of the Rāsā Sala or Alchemical laboratory.
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From the description of the process of metal extraction and metal working it is very clear that the metal smelters and smiths of India had gained very advanced knowledge regarding the thermo- mechanical behaviour of Au, Ag, Cu, Pb, As, Zn, Hg and Fe and their various alloys. They had also the skill to produce a variety of objects of intricate design and gained experience in soldering, forge welding, rivetting, casting and thermo- mechanical treatment as well as metal finishing technology and they used very precisely designed tools and furnaces for these purposes.
Linked texts (3)
- Rasaratnasamuccaya — Vāgbhaṭa (alchemist), Sanskrit
- Ṛgvedasaṃhitā — Sanskrit
- Barudih