Removing old very hard putty - what's the best way to do this? It's in a
Critall steel window, and I wouldn't be surprised if it's the original
1930s putty.

I've been using an old wood chisel and a hammer, but it's very very slow
work. Is there a much better way? Not keen on a blow torch, as there's
probably loads of lead paint on it, but would a hot air gun help soften it?

..