After you get the emulsion off, you can mix up a solution of 1/3 rottenstone, 1/3 alcohol and 1/3 distilled water to use as a glass cleaner/polisher and clean the glass with it before re-coating. This is the stuff the wetplate-heads use to clean and prep the glass plates they use for ambrotypes before they coat. There's another "proper" name for rottenstone, but you can get it from fine woodworking supply places - they use it for polishing wood prior to staining.