Refractories are inorganic, nonmetallic, porous, and heterogeneous. These tools are mostly used for the machining of nonmetallic and nonferrous materials. Nonmetallic conductors, such as conductive plastic, are sometimes used. Many nonmetals have less stable allotropes, with either nonmetallic or metallic properties. Some authors count metalloids as nonmetals with weakly nonmetallic properties. This process uses thermal energy to remove material from metallic or nonmetallic surfaces. The reactive nonmetals vary in their nonmetallic character. Being a metalloid, most of its chemistry is nonmetallic in nature. A nonmetallic element has low electrical conductivity; it acts as an insulator. Most of the chemistry of boron is nonmetallic in nature. Aluminium can form anionic aluminates, such behaviour being considered nonmetallic in character. The largest commodities were coal, chemicals, farm products, nonmetallic minerals and intermodal. Valinomycin acts as a nonmetallic isoforming agent in potassium selective electrodes. Collectively, the highest values of these properties are found among oxygen and the nonmetallic halogens. The chemistry of arsenic is predominately nonmetallic.