There are a number of oils and other products that can loosen and remove the compounds of window putty from the glass of the window itself. Goo Gone or Easy Off oven cleaner, which is made for really tough grime, will work on window putty, but it takes a bit of time, as you have to apply it, let it sit in the sun for about fifteen to twenty minutes and then scrape it, with a chisel or with a razor blade.
Do this process over and over again, until the putty has been removed. Make sure your razor blade, chisel or scraper are sharp.
Hammer and chisel
The old-fashioned way of removng putty from the window pane entails simply taking a small chisel, jamming it into the putty and tapping a hammer against the middle of the back of the chisel as you run it along the windowpane. If struck correctly, the putty should come right out with ease.
However, there are downsides to this method. If you don't hit the chisel correctly, the putty won't come up as easily.
Also, hitting the hammer too firmly can damage the window sill and even break the window pane. So use caution if removing putty in this fashion.
An inventive way of removing putty from window panes consists of taking a high-powered heat gun and moving it slowly along the window pane to soften and loosen the putty from the window frame itself. The putty will warm and can then easily be scraped away with a putty knife or a chisel.
If you can obtain a heat gun, this may be the most efficient method for removing putty. However, heating up the putty also heats up the glass significantly and can cause it to crack, so be sure you use care not to hold the heat gun to close to the glass itself.