Carbon as graphite is the softest of natural substances. Natural graphite may be crystalline flake graphite that forms planes of cyclic, with atoms of carbon arranged in a honeycomb lattice, amorphous, and lump graphite. Natural graphite, mined and then refined, may contain up to 98% of carbon. It is opaque, black, and conducts electricity well so is used, for example in arc lamp electrodes. Natural and synthetic graphite is a good dry lubricant: despite being in the solid phase it reduces friction between surfaces that are sliding against each other. Graphite, which was earlier called black lead or plumbago has been used in steelmaking, brake linings, as a recyclable anode in batteries, as a lubricant in air compressors, railway track joints, ball bearings, for lining molds for cannonballs, and also in food industry. Graphite has been used for decorating pottery since the 4th millennium B.C. Synthetic graphite serves as a matrix in nuclear reactors. Carbon nanotubes reinforce plastics and thus many commercial articles.