It is done the same on clear glass as it is on black glass. Only the black glass needs no backing. Now I have never seen a clear ambrotype that was backed on the collodion side with black cloth. That would reverse the image. Quinn is backing it on opposite side of the collodion. An ambrotype is just an underexposed negative. Normally when done on clear glass the back side is coated with asphaltum or a more modern method is to spray it with glossy black lacquer spray paint.