Copper powder - 325 mesh

Powdered metals used as fillers result in castings with a realistic metallic finish.
They are used in the same way as filler powder, pt the resin is catalysed BEFORE adding the powder,this is to neutralise the catalyst, which would otherwise have a corrosive effect on the metal.
For metal castings, always use 2ml catalyst per 100g resin. The higher proportion of the metal to resin, the better the finish.
Generally speaking, a 50/50 mix, by volume, is a minimum ratio. For every 100g of bronze powder you will produce approx 20ml in volume.
The finished casting will not look very metallic the surface will be dulled by a thin layer of resin. Removing this surface layer by buffing with wire wool then metal polish produces the realistic metallic sheen.
Different metal powders can be mixed together if required mixing aluminium and a little brass, for example, produces a good simulation of old tarnished silver.
The powders should be mixed before they are added to the resin.
Graphite powder rubbed onto the finished casting will also give an aged effect. A better effect can be obtained by mixing the graphite with the metal powder (1 part graphite to 10 parts metal, by weight) before adding to the resin.

TIP:
To save material you can brush filled Gelcoat onto mould surface, and then cast with resin / filled resin or layed up with fiberglass matt and resin.
Alternatively brush the powder onto the mould first, tip off any excess powder then fill the mould with resin.
When stripped from mould you can polish to reveal the metal powder.