Black pigment. It is a solid residue containing impure carbon produced by slowly heating animal and vegetable substances, such as wood, bone and nutshells, at high temperatures under reducing conditions to remove the volatile constituents. Charcoal was probably discovered as a by-product of wood fires. Since prehistoric times it has been used and intentionally produced as a superior and very efficient fuel. Traces of charcoal have been found in the blackened hearths of caves occupied in Palaeolithic times. It was undoubtedly used for drawing on cave walls because of its convenience. Technical analysis has confirmed the presence of vegetable and wood charcoals in prehistoric cave paintings from, for example, Altamira and Niaux, along with other black pigments such as calcined bones and manganese oxide. Charcoal was probably used for drawing in Classical times; it is mentioned by Pliny the elder and has been found at Pompeii. Charcoal from grape twigs was one of the two standard medieval blacks. Vine-charcoal black (nigrum optimum, ‘best of blacks’) was used in sticks for drawing and ground to powder for painting.