Hi,

The actual conductor is copper, same purity as anyone uses (try having "six nines" or such metal assayed, have fun).

There is silverplating on the surface, has to be as the extruded FEP at the temperature it must be extruded would literally burn the copper surface otherwise.

The layer is quite thin, no idea on purity, most probably usual industrial purity, that is the same as for the copper.

Ciao T