I read once about using glycerin to soak hammer heads in. They said it would swell the wood, but wouldn't shrink as it dried. I never tried it, in fact, I don't even know what it is or where to find it. Personally, I use steel and wood wedges to keep wooden tool handles tight. I have a splitting maul with a fiberglass handle, and it got dangerously loose. Bought an epoxy kit at Rural King to repair the head/handle fit, and it's been good for a couple years now.