Jeff Layman wrote:

One of the wedges on my old lump hammer has come out. Before it came out
completely I had a look and it seemed to be made of wood

Can you get the wedge back in and soak the head in PEG (anti-freeze)?

Not tried it myself, but I gather it soaks in, swells the wood, but
unlike water, once swelled it doesn't shrink back as it dries ...