Sam

Not sure I understand what you are doing but the usual method that
optics people use to hide interfaces is oil/glue of the same refractive
index as the glass. Doesn't take much if you are using float glass,
since it's already nearly perfectly flat. Mineralogists use oil to
measure refractive indexes so you might ask around in that part of the web.

Water works pretty well for a lot of applications, like the old
disappearing quarter illusion where the glass circle sticks to the
bottom of the glass by surface tension and is virtually invisible. I'd
try it first, since it's cheap and readily available. If you're not
doing close up work, you can be a lot less picky. A couple feet of
distance will hide a lot of mismatches.

A few distracting parallels will help hid things too. The eye tends to
ignore repeating patterns (probably part of our rodent ancestors
filtering out the leaves to see the predators), so you can hide a join
in plain sight if it's part of a pattern.

Sounds like an interesting project.

Jim