These hearts are made from high-purity Copper, with 25/75 Ag/Cu fused to the surface, then press-formed. The idea is that on colouring in Rokusho, the copper will turn a deep red and the shakudo will go a deep grey.
I also cut the baseplate for the tattoo machine today:
The metal is a shakudo alloy of 3/97 Au/Cu which should go a deep blue-black. The fine silver skull will stay white, polished silver. I read an article today which said that Japanese craftspeople who work with this material aim to colour it "like a wet crow's wing", which I think is rather lovely.
Details of the baseplate will be picked out in sterling and various different shibuichi alloys and there will be stones set on it too.