its a bit hard to shade it like glass when you dont have a background, cause glass is all about reflections and refraction. and with no background or scene theres no refraction and hardly any reflections other then the lightsource it self. if would definitly look better with a background.
this is something I made sometime ago (I hate to reference to it all the time, but please, bare with me just this once
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as you can see, for theglass, I had to put in a background just so I could show my point. otherwise it would just be a simple soap bobble