text: There are many references to ayas in the early Indian texts.
  The Atharvaveda and the Shatapatha Brahmana refer to kṛṣṇa-ayas (), which could be iron (but possibly also iron ore and iron items not made of smelted iron). There is also some controversy if the term śyāma-ayas (), refers to iron or not. In later texts the term refers to iron. In earlier texts, it could possibly also refer to darker-than-copper bronze, an alloy of copper and tin. Copper can also become black by heating it. Oxidation with the use of sulphides can produce the same effect.
  The Yajurveda seems to know iron. In the Taittiriya Samhita are references to ayas and at least one reference to smiths. The Satapatha Brahmana 6.1.3.5 refers to the smelting of metallic ore. In the Manu Smriti (6.71), the following analogy is found: "For as the impurities of metallic ores, melted in the blast (of a furnace), are consumed, even so the taints of the organs are destroyed through the suppression of the breath." Metal was also used in agriculture, and the Buddhist text Suttanipata has the following analogy: "for as a ploughshare that has got hot during the day when thrown into the water splashes, hisses and smokes in volumes..."
  In the Charaka Samhita an analogy occurs that probably refers to the lost wax technique. The Silpasastras (the Manasara, the Manasollasa (Abhilashitartha Chintamani) and the Uttarabhaga of Silparatna) describe the lost wax technique in detail.
  The Silappadikaram says that copper-smiths were in Puhar and in Madura. According to the History of the Han Dynasty by Ban Gu, Kashmir and "Tien-chu" were rich in metals.
  An influential Indian metallurgist and alchemist was Nagarjuna (born 931). He wrote the treatise Rasaratnakara that deals with preparations of rasa (mercury) compounds. It gives a survey of the status of metallurgy and alchemy in the land. Extraction of metals such as silver, gold, tin and copper from their ores and their purification were also mentioned in the treatise. The Rasa Ratnasamuccaya describes the extraction and use of copper.