This is my first collodion I did on BLACK GLASS, today. Maybe not very spectacular photo in terms of subject and light (very very late afternoon, 1min10s exposure), but I had to try how it works....
I've transformed timber holder to take 4mm thick black glass (this is the minimum thickness you can buy). The guy from the glazing company didn't want to sell me the plates. He said he can't really cut the glass so precisely (for the holder I've shown to him). I ordered 3same size plates, at my 'own risk' and one was too big and 2 too small :).
So I did a small metal frame, that goes inside the holder and it solved the problem! Now I can order slightly smaller plates and will still have a reserve of few millimetres.
I absolutely love this material and I think there is no better one for the ambrotypes!