The retouching stand (the mirrored thingy) was used with the glass plate set into the middle frame and the mirror faced toward a window. The upper portion shaded out room light. In effect it was a pre-electric light-box. The plate was then augmented with graphite to increase density- allowing more sharply edged highlights, etc.
I've used a method proposed by Jim Galli to successfully clean old, uncoated lens glass. I take the glass and immerse it in a bath of warm water with a good swish of dishwashing liquid. Leave it in there for 15-20 minutes, take it out, rinse and gently dry with a very clean cotton or linen cloth. It beats lens fluid and rubbing every time.