Store your duplicate stamps in semi-transparent glassine envelopes, similar to what is used by the U.S. Post Office for stamps purchased at the their counters.
“Glassines’ are also recommended for the storage of negatives and plates from electron microscopes, and temporary storage of seeds and etimology specimens. Glassine Envelopes are made from semi-transparent #30 glassine. pH neutral archival quality. Open on long edge, with ungummed flap. Strong seams, using anti-tarnish adhesive. envelopes feature seams on two side. Labels adhere to glassine, or printed direct on the envelope.
Sizes #3 through #6 available below: SPECIAL Quantity Price Breaks Now Available for 10 Packs (1,000 Glassines)