A crayon is a stick of chalk, wax, or a combination of wax and clay used for drawing or marking. Pigments are added to these materials for color. Chalk crayons, often called pastels, have long been used by artists because they produce softer tones than either oils or watercolors. They are made by mixing chalk, coloring agents, and water-soluble gum into a paste that is then dried. Wax crayons, developed in the latter part of the 19th century, are used in industry as well as in the home and at schools. Wax crayons produced in the United States are usually made of nontoxic materials and encased in paper tubes. Among the newer types of crayon is the "wash-off" crayon, the composition of which is a trade secret. However, its components are thought to include a soap base. A special crayon, made of soap, wax, resins, and lampblack, is used for drawing on stones or plates for reproduction in lithography. Crayons are manufactured in various shapes, such as round, hexagonal, or square, and in different sizes. The most common shape, however, is cylindrical.