The previous two articles in this series dealt with the chemical practices in India in ancient and medieval times. Metallurgy and metal works are closely related to the knowledge of chemistry. In the present article, the focus will be on the metallurgical aspects. Metal works will be taken up in the next article.
Archeological studies reveal that iron working in India may have begun as early as 1800 BCE, and that by the early 13th century BCE, iron smelting was definitely practiced on a large scale. The Rigveda (earlier than 15th century BCE) refers to ayas, which probably means bronze or copper rather than iron. The later Vedic texts mention krishnayas, which refers to iron.
Wootz steel is primarily iron with a high proportion of carbon (1-1.9%). Wootz is the English rendering of ukku, a Kannada word for steel. The basic process consisted in first making wrought iron. This was packed with wood chips or charcoal in closed crucibles that were heated, causing the iron to absorb appreciable amounts of carbon; the crucibles were cooled and solidified ingots of wootz steel remained. This process was followed in several places in Karnataka and many other parts of South India.
A Roman historian, Quintius Curtius, recorded that among the gifts that Alexander received from Porus of Takshashila (in 326 BCE), there was a large quantity of wootz steel. Edrizi, a 12th century Arab wrote: “The Hindus excel in the manufacture of iron, and in the preparations of those ingredients along with which it is fused to obtain that kind of soft iron which is usually styled Indian steel (Hindiah). They also have workshops wherein are forged the most famous sabres in the world. … It is not possible to find anything to surpass the edge that you get from Indian steel (al-hadid al-Hindi)”. Wootz steel was widely exported throughout West Asia and promoted the production of the famous Damascus sword, admired by the Arabs and the Europeans.
In Dharamapal’s book on Indian Science and Technology in the Eighteenth Century, there is a chapter on ‘The Mode of Manufacturing Iron in Central India’, which is a detailed account of Major James Franklin of the Bengal army, written in 1829. He first discusses the mode of extraction of the ores in the iron mines in the region around Jabalpur. He then describes the construction of the smelting furnaces. Fired charcoal that is used for smelting and the ore are alternatively fed into the furnace through the chimney. Bellows are used to stoke the fire. After smelting, the metal proceeds further into refineries. Franklin observes that on average, 40% of the ore was converted into crude iron by weight. Every 100 sers of crude iron yielded 63 sers of malleable iron. Franklin reports that he handed over the iron to a Captain Presgrave of the Sagar mint near the location of the furnaces. Presgrave wrought it up into bars and rods for an iron bridge on which he was then employed. He remarks that “the bar iron was of excellent quality, possessing all the desirable properties of malleability, ductility at different temperatures and tenacity for all of which I think it cannot be by the best Swedish iron.” The next chapter in Dharampal’s book is on the observations (in 1842) of Captain Campbell on the manufacture of bar iron in southern India, where he gives details of the superior properties of the product.
Carbon dating has revealed that some zinc mines at Zawar in Rajasthan were operational as early as the 4th or 2rd century BCE. The zinc distillation technique was developed around 900 CE at Zawar. Zinc was distilled in retorts maintained at a temperature of 1,150-12,000°C. From the debris of spent retorts, it is estimated that one lakh tons of zinc was produced in Zawar during the 13th to 18th century. This signifies an outstanding level of industrial production in the medieval period.
There was a tradition of copper production as well in India. Copper ore was pounded, made into balls with cow dung, roasted and then smelted in a closed furnace and refined in an open charcoal fire, as shown in the picture.
M S Sriram
Theoretical Physicist & President, Prof. K.V. Sarma Research Foundation
(sriram.physics@gmail.com)