Most items that require conservation framing are also best protected when they are mounted and matted on boards that additionally contain a reserve of calcium carbonate. This is an alkaline substance that guards against acids encountered in handling, in the environment or in the art itself.
Un-Buffered Museum Mat Board
There are some items, though, that "prefer" a low-alkaline environment. This prevents degradation due to chemical reactions between the buffering agent and the materials in the art. These items consist either partially or entirely of protein-based fibers such as silk, wool or leather. Other, not-so-obvious, offenders are older photographs with protein-based emulsions and casings. These are harder for framers to identify, so it is best to use a non-buffered board on any photograph that is suspect.
Some of the types of photographs needing a non-alkaline environment, the years of their popularity, and brief explanations:
These recommendations and definitions were taken from the following resources:
Framing Photography; Allan R. Lamb
Needlework Framing; Vivian Kistler
Preserving Textiles; H. Mailand and D. Alig
The Framer's Book of materials and Techniques; Paul Frederick
Customer Care Representative for Crescent Cardboard
and Certified Picture Framer; Deanne Row