Plaster of Paris will break down when heated to forging temperatures, so it's no wonder yours didn't last. Read the JABOD threads for good discussions of earthen fills.
A nice enough little video about making frying pans, but there’s a rather unusual “treadle” hammer shown, starting around 1:50.
It’s nicely made, but it seems rather inefficient.
My Mousehole (aka The Undisputed King of Anvils) went through a similar transition when I siliconed it down onto its metal base: it's not just a reduction in volume, but also in pitch. In other words, the higher frequencies that ring so painfully (and that sustain for so long) are muted by the silicone and the magnet, so the high pitches that are most damaging to the hearing are significantly reduced.
Welcome to IFI! If you haven't yet, please READ THIS FIRST!!!
I like your look. It's deliberately primitive, to be sure, but there's a sophistication of line and form nonetheless. "Primitive" doesn't have to equal "crude".