Removing putty from old windows & glass replacement
There is a tool specifically made to heat up old window putty. It looks like a soldering iron, with a 90 degree point on the end. It works great.
I have restored countless old colonial windows in the Concord, Mass. area using our window iron. Some of that putty was approaching one hundred years old, or older, and was hard as a rock. Take your time, let the heat do the work, and remove the softened putty with a stiff putty knife.