When the cost of producing slaves became too high to justify slave labourers for the many mines throughout the empire around the second century, a system of indentured servitude was introduced for convicts. In 369 AD, a law was reinstated due to the closure of many deep mines; the emperor Hadrian had previously given the control of mines to private employers, so that workers were hired rather than working out of force. Through the institution of this system profits increased (Shepard 1993). In the case of Noricum, there is archaeological evidence of freemen labour in the metal trade and extraction through graffiti on mine walls. In this province, many men were given Roman citizenship for their efforts contributing to the procurement of metal for the empire. Both privately owned and government run mines were in operation simultaneously (Shepard 1993).