Only developed plates have a varnished emulsion unused plates don't. If they had the photographer wouldn't have been able to develop the plate.Even used plates were often not varnished. Beneath the emulsion layer you have a varnish layer don't remove it under any circumstances. The pigments that were used are water soluble. The Varnish that protects the starch/color is a mix between Dammar resin, ethyl acetate, nitrocellulose and castor oil.

Dominik