It won't work no matter what you do. Hate to be so absolute but the surface of glass won't "fuse" carbon into it. It will just burn off or wipe off when its cooled. Raku Clay is a totally different kind of material that is low fire and porous so the carbon has a way to work into the surface.
You can still try it...but it won't work
You can always pull super thin stringers of something like intense black and that may give you the same kind of effect.