Here's a tip from developing and printing years ago: Copy first, like so many have written here. Remember that these old papers were not resin coated and the gelatin emulsions were relatively thick. What has happened is that they have dried while stuck to the glass.

Soak them loose with distilled water. Now, the wet prints will have to be dried so that they do not wrinkle. In addition, if the pictures were meant to be glossy, one will need ferrotype tins to dry them on--or an old-fashioned print dryer. Put the prints on the tins emulsion side down and gently roll out all the air bubbles with a rubber roller. Don't ask me why, but prints did not stick forever to ferrotype tins, but tended to peel off as they dried, leaving a glossy surface.

If they are meant to be semi-matte or matte finish, simply dry them, pressed between something that they will not stick to. Most portrait and double-weight papers would have been in this category, but not all.