Argh! What an idiot! :blushing: I was convinced Carrara's glass shaders were a bit strange, but all along it was my stupid model causing all the trouble. As I mentioned earlier I used a lathed vertex object with 64 points for the circle, which looks fine but produces very strange effects when applying a glass shader to it. Anyway, I thought I'd read about how to make a spline object and then created the same object as before except spline instead. I now realise that they can be perfectly circular unlike vertex objects.

This is my new model with Carrara's clear glass preset (unaltered) applied to it. The refraction is the same or very similar to that of my real glass, so problem solved there. I now have a black ring around the base and no highlight(s). I'm not sure why it's done that. Any ideas?

PS: Sorry about the shaders thing.