Mark - sorry if it was a bit cryptic - I was referring to RobertP's post - an ambrotype on clear glass can be viewed from the other side. Darkly coloured glass (eg deep purple) was used as an alternative to a dark backing, like black cloth or velvet, or - in some cases - as a black coating on the collodion itself.
BTW, Robert Szabo's forum goes back quite a way, even before the current forum that is hosted. Most of the big names you hear about such as George Berkhofer, John Hurlock and Mark Ostermann have posted on it along with with others who have probably forgotten more about WP than most know such as Ray Morgenweck and Bob Szabo himself.
PS: as an aside I once read something about Billy the Kid being lefthanded, then somebody pointed out that the photo of him holding his weapon in his left hand was obviously reversed due to the photographic method at the time. Might be apocryphal, but interesting.