use hot ( not boiling) distilled water to soak the cards in. if the glue or adhesive is natural , the paper backing should loosen at least partially from the cards. I use a soft paint brush sometimes to brush away paper scraps. I have also added several drops of DIF to soaking water to help loosen paste or thick paper.
if the adhesive is a rubber cement or early synthetic glue the water soaking may not work that well. the only way to find out is to experiment
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