Possibly the most decisive evidence of change in the economic life of northern India after the Muslim expansion is numismatic. Some evidence of a mild economic revival from the tenth century is provided by the reappearance, in limited quantities, of a gold coinage based on older models of a 40 ratī standard of which the earliest examples appear to be of the Chedis or Kalachuris of Madhya Pradesh followed by dynasties of northern-central India, the Chandelas of Mahoba, and, most numerous of all, the Gahadavālas of Kanawj in the twelfth century. After the Muslim conquest of the Gangetic plain, the earliest gold issues in the name of Mu'izz al-Din Muhammad bin Sām are of this weight and type, with a conventionalized usage of the goddess Lakshmi on the obverse and the name of the ruler in Nagari characters on the reverse. Clearly they have no Muslim ideological content and must represent the survival of pre-existent arrangement with Indian moneyers to provide a coinage in the name of the ruler of the day. Monetary assay being a skill of goldsmiths and sarrāfs or money-changers, who down to the present day have mostly been members of the Hindu sonār caste, we may assume that the implementation of the monetary policy of Muslim sultans was mainly entrusted to these experts. The mint-master at Delhi during the reign of the Khaljī Sultan Qutb al-Din Mubārak, Thakkura Pherū, was a Hindu or Jain, and left a valuable treatise written in Apabhramśa regarding the exchange rate and precious-metal content of coins brought to the royal mint for exchange, the coin-types dating in some cases from a century before the time at which he was writing.