Glass Squeak is a Sound Library made by Benoit Tigeot and Lauriane Capaldi. Here it's the example of, what can be done with processing on the library (awesome demo by Jean-Edouard Miclot http://jedsound.com/blog/ )
We work almost exclusively on cartoons. Too often the characters are knocking on windows and gliding along the windows. They have lacked of SFX for this type of action that's why we had the idea to record this type of sound.
We recorded on a glass in the studio; squeaks, slide, wail with sponges, cardboard plate, polystyrene plate, small block of polystyrene and finally fingers.
Recorded sounds are very musical and can be used in many other actions that they originally planned.