Not all card is created equal
Paper used to be made from cotton – pure cellulose, no lignin, no acid. Now, of course, it is made mainly from timber and timber is rich not only in cellulose (wood pulp) but also in lignin (found in the cell wall of the plant). It is the presence of lignin that makes commercial paper and boards so deterimental to works of art. Used as a backing or matting board in the framing process, such materials – be it paper, cardboard, MDF (medium density fibreboard) or, as was frequently used during the war years, strips of timber – are responsible over time for the hazy brown or yellow stains found on artworks or the visible mat boards (MDF also contains formaldehyde which may cause further damage). Inserting a 1-ply barrier paper between the art work and an acidic backing such as MDF is no guarantee of protection. The result, as seen here, is acid-burn, caused as the lignin breaks down and the acid migrates from the card or board to the artwork. If the work you want framed is easily replaceable and not of great value to you, standard boards will do the job for the short term, but to protect your art work for the long term it should be encapsulated in fully archival, acid-free boards before it is put into the frame.