The photographic paper base must be free of photoactive impurities such as iron and lignins. Acid-free tissue or muslin are often used to shield textiles from harmful lignins. The dry matter content of lignins is the residue after drying at specified conditions. Thus O-demethylated lignins are potentially susceptible to partial depolymerization. Chemically, lignins are cross-linked phenolic polymers. Many natural products in plants, e.g. lignins, are generated via catalysis by caffeoyl-CoA O-methyltransferase. Lignins can be characterized by determination of their purity, molecular structure and thermal properties. For lignins, weight-average molar mass (Mw) and number-average molar mass (Mn) are often determined.