Umber is a hydrated iron oxide that also contains significant manganese, calcia, alumina and silica. When umber is calcined (referred to as "burnt" because it is done to lower temperatures), the colour intensifies. The material is used to colour many products (e.g. for construction), in ceramics it is used to darken the fired colour of clay bodies or engobes.
Burnt umber can be any of a variety of natural and synthetic iron oxides (there are many manufacturers and choices, this is because the colouring market for cement, for example, is so large and widespread). A variety of impurities are common. Some companies condition the colour of their umbers by adding various colours of iron oxide