First, cotton rag is the pinnacle of framing materials used by museums, galleries and artists for centuries.  Naturally acid-free and lignin free, cotton is a superb archival backing.  However, there are some conservationists that say some photographic prints, especially some older processes like albumen, chromogenic, dye transfer and cyanotype should be framed in a "low alkaline enivironment".  Some say this applies to modern photographic processes and even ink jets prints, often called giclees in the art market.  There are diverse opinions that are beyond my depth of knowledge.  Regardless, buffered or unbuffered, cotton rag is the best mat and back. 4 ply rag, approximately .060 thick, in white.