Problems with Refraction

[tuami]

hello all, can anyone help me, what is the reason that these artifacts occur in the glass material and how do I get these dark areas away? scene is attached
thanks

r21 latest daily

[Beanzvision]

Raise them off the ground ever so slightly. They are most likely intersecting with the ground plane. ;) Maybe add some caustics for the hell of it? :D

[tuami]

I knew it was something simple but I did not get it, thanks.
As long as the text (Slow) is next to the caustic enable switch, I usually try  to avoid using caustics :)