It is compounded in the manufacture with many of the above substances, for the purpose of obtaining particular advantages for special uses. Ground cork and other light materials are sometimes mixed with the gum to increase the bulk, and make the articles light. The oxides of metals, their filings and pulverized silicas will give imitations of marble. The fibre of cotton, or the dust of different woods will afford simulations of wood of greater or less gravity, as may be required. The combining of plumbago gives the crayon; oxide of zinc, produces lithographic stone, and so on, and on. Pigments and earths are used for color and cheapness, and to increase the weight of the fabric as in the case of carpeting. Bitumen and rosin are sometimes used to give articles a finish, or high lustre. Oxides of some of the metals are used, but white lead and litharge are commonly preferred. From two to four ounces of either of these metals to the pound of gum cause the articles, and particularly those that are thick or massive, to be readily changed or vulcanized, and more completely, or with greater uniformity. Sulphur is applied through the medium of heat in different ways, according to the nature of the articles or fabrics, and their uses. It is generally mixed in the process of crushing or grinding the gum, in the proportion of half an ounce of sulphur to the pound of gum for the vulcanized elastic goods, and about five or six ounces to the pound of gum for the “vulcanite,” or hard goods. In the former case, about 270° of heat are necessary, and in the latter, 300 to 310°. At other times the sulphur is dusted upon the articles in the form of flour of sulphur before they are placed in the heater or oven. This is commonly done in the manufacture of elastic thread and other articles, which possess no extraneous mixture, in which case the gum is penetrated or impregnated with the sulphur, without its being mixed with the gum in the crude state.