This has happened to me every time that I use the glass attribute in ART. I always get this black shadow thing in the middle of the object, I have tested this on different computers, different lighting setups and just about everything that I know and I can't get rid of it.
I don't know if this is a bug but who ever figures it out I will give them $1,000,000*
Here are some pictures of what I am talking about
I also included an anim8or file with my bug
Thanks for helping
* Please note that the writer of this post was only joking about the money and that it does not exist, but the writer will give you mad respect if you are able to help instead which has a street value priced at $1,000,001. Sorry for the inconvenience 
