The archival paper specifications are contained in ISO 11108:1996, Archival Paper--Requirements for Permanence and Durability. Archival paper specifications call for a neutral/alkaline pH and no lignin. Lignin is the material in wood that bonds the cellulose fibers. In the chemical process of papermaking the wet pulping and bleaching process dissolves the lignin. Groundwood pulp, on the other hand, is made by grinding the wood and the lignin remains. Paper with groundwood pulp will be less stable over time; paper with an acidic pH will interact with the lignin and accelerate the yellowing of the paper, especially when exposed to UV light. Most paper made today has alkaline sizing and a neutral or alkaline ph.