Assuming they're made from the same material as other glassines, I'd say yes. I bought a collection recently that had a lot of plate blocks in USPS envelopes, which had to be at least 20 years old (since that's about how long the collector had been deceased). They don't show any signs of discoloration at all and the stamps were in excellent condition.
According to my pH test pen used on a USPS glassine, they are slightly acidic. Generally safe for storage for years but will eventually break down. Most glassines are safe to use until just before they start to turn brown. There are archivally safe glassines on the market but I am not familiar with them.
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Posted 04/25/2015 3:50 pm
