“Because acid free mat board is only temporarily acid free, it is not considered archival and should not be used for conservation purposes. If you want to mat original, one-of-a-kind works of art, limited edition prints, or any type of valuable paper memorabilia, you should not use an acid free mat board. Instead, you should use an archival, museum quality rag board that is naturally acid free. Mat board comes into direct contact with the artwork, so if you want to preserve and protect your artwork, you should choose a mat board such as Crescent Ragmat or Bainbridge Alpharag.” Source