Storage materials and storage conditions are key elements in the preservation of archival material. Materials should be stored in archival-quality containers constructed of permanent paper; this paper is often mistakenly called acid-free paper. To be truly archival-quality, the paper should not be acid-free or neutral, but low lignin, with pH of 7.5-9.5 and an alkaline reserve of 2%. Photographs should be stored in chemically neutral plastic enclosures made of either polyester, polypropylene, or polyethylene (brand names include DuPont Mylar D and ICI Melinex #516).