How to Replace a Broken Glass Pane in a Wood-Frame Window
Use the heat gun to warm the old putty; and then scrape it away with a putty knife.
If the putty doesn’t lift off easily, apply more heat and try again. Be patient —the putty around really old windows is as hard as concrete, but it will soften.
Warning: Don’t chisel out the old putty — you might wreck the window. The putty will soften with enough heat and time.