Hello Dave, thank you so much for your input The changes i did: 1. unticked "skip internal reflections" at the mental ray material 2. changed from glass physical to glass thick preset 3. under advanced refraction I ticked "max distance" with 5 and color with light blue 4. I slightly turned my HDR-image a little bit, so I am getting now sky-refelctions at the glass feet The glass I think looks more realistic now, I still have to work on the caustics.