Rood-screens represent some of England's most significant surviving medieval painting and polychromy. The majority date from the 15th and early 16th centuries and most are of oak. Many retain their decorative polychromy and a large number have paintings of saints on the dado panels. Techniques are varied, with mixed media, glazes and metal leaf, often misleadingly discolored. The Reformation in the mid-16th century, followed by various changes in religious practice and aesthetic taste, resulted in extensive mutilation. This often includes total overpainting, usually taking the form of brown oak graining, sometimes over white lead. Unskilled attempts to remove these coatings have compounded the problems which confront conservators, while the thick overpaint prevents the penetration of consolidants. The problem most frequently encountered during cleaning is discolored oil varnish, often containing undissolved resin particles and foreign matter. Retouching is usually limited to the toning in of fillings and structural repairs. Coatings must be selected having regard to the uncontrolled and unmonitored environment of ecclesiastical buildings in normal use.