Glassine is a very thin and smooth paper that is air and water resistant. It is normally translucent manufactured by supercalendering: after pressing and drying, the paper web is passed through a stack of alternating steel and fiber-covered rolls called a supercalender at the end of the paper machine such that the paper fibres flatten facing in the same direction.
Glassine has traditionally been used for storing photographs and negatives for many decades, and is useful for this as it is possible to view the contents.
Downsides: not specifically archival, as there is usually some sulphur content, although any problems may take many years to become apparent. Also problematic if wetted in contact with photo materials, and may be very difficult to separate from them.
31.5cmx48cm is a special size, made to order, lead-time is 2 weeks
FSC or PEFC certified