When I was a kid we used hot water to defrost the windscreen regularly. A couple of hundred times over multiple winters. Temperatures around -20. No cracks or anything like that ever occurred.
When was that, out of curiosity? I seem to remember that the structure of the windshields (at least in Europe) change in the 70's from ones that would break with a crack, to the ones that would shatter in plenty of small pieces (due to the internal tension, and at least two layers)
I saw that in the 90s but people were definitely doing it in the 70s as well. I am not aware of any stories of cracks or shattered windscreens because of it, though I am sure that if you do that with boiling water and not-so-frozen windscreen often enough, you would get that result :)
