The technique is the same, Robert, I agree. The point is that you can see through the clear glass whereas obviously seeing through black glass is going to be problematic. Not that I've ever seen a period one on black glass, just clear, purple and ruby. But I have seen them reversed quite often - maybe some old-time punters didn't like their wedding rings on the wrong hands, or whatever. I've seen quite a few coated with asphaltum on the collodion side, too - more's the pity - once the asphaltum flakes/breaks down/becomes damaged, salvaging is hard if not impossible. If coated on the clear side, this wouldn't present a problem.