Thank you, Richard and Thomas. Today I will test with the glass. I am a little sceptical, the reflections may be a real headache. A polarizer might be the solution, I will lose 1,5 stops I think. I have no idea what a polarizer does to saturation and tones. I remember seeing color shots where a polarizer has been used and the sky is extremely blue and other colors like "candy" but what happens with a b/w photo. No idea, so far.
Another day at work, another nights in the darkroom. so it goes...