Glassine envelopes are not archival quality and should not be used for long-term storage. You should avoid normal glassine. Biaxially-oriented polypropylene (BOPP) can be identified chiefly by its superior clarity and "crinkly" or crisp feel. BOPP is growing in popularity among manufacturers of archival products because of the ability to make a nice presentation sleeve as well as being safe for long-term storage.