Kautilya's Arthashastra 2.14.30

2.14.30

śulbaṃ tāraṃ vā garbhaḥ pattrāṇāṃ saṃghātyeṣu kriyate //

In amalgams, a single or double layer (of a superior metal) is made to cover a piece (of base metal). Copper or silver may also be placed between two leaves (of a superior metal). A copper piece (sulbarúpya) may be covered over with gold leaf, the surface and the edges being smoothened; similarly a piece of any base metal may be covered over with double leaf of copper or silver, the surface and the edges being smoothened.