After an image is formed, photographic papers must be "fixed" so they are no longer sensitive to light. Fixing is a chemical process where unexposed silver halide is rendered water soluble and removed from the paper. In the 19th Century as today, this process was generally carried out through immersion in a solution of sodium thiosulfate known a "hypo." Comprehensive looks at the various formulas used for fixing and the recommendations for removing hypo can be found in the work of Towler and van Monckhoven. As hypo contains sulfur (an element known to have a deleterious effect on silver-based photographs), fixing was followed by prolonged rinses in clean water to remove as much hypo as possible. Early on, researchers such as Matthew Carey Lea linked fading and staining of albumen prints to imperfect fixing and washing. A more contemporary look at this problem is Image Deterioration in Albumen Prints by Reilly, Severson and McCabe where is the link between staining and fading to due poor washing is measured.