Now that you mention it I have never ever read or heard about anyone casting copper

All sorts of other stuff yes

You don't say which hammer, but for example a 1&3/4" copper head insert is £6 +vat. Why bother?

I tried this experiment, and found (thanks to a similar question here) that the cast head of the hammer is crimped around the copper insert to hold it in place. Being cast, it's not malleable, so the crimp is a one time event, and the edges of the socket are liable to split if you try and ease them out enough to allow a new insert to be (ahem) inserted.

So I cast lead inserts in situ.

Here's a link to the escapade in question:

Lead faced hammer

I would push a bar of the correct diameter in some sand and cast as normal. Copper tends to react with oxides so casting into any pipe needs to be done with the pipe very clean, and don't stand over the pipe when casting. Once poured, cover the top of the copper with coal dust, otherwise it "boils". Sand mix:

3% water

10% bentonite (from beauty shop-face mask)

5%coal dust

5%starch (cake flour)

rem sand

With such a simple mould you'll probably get away with 3% water and 10% starch (rem sand)

I'm surprised copper isn't cast more often, a number of the older models had copper chimney stacks…