block by the powder pressed around. This operation, called "tamping", is usually performed using electrolytic copper powder, possibly with silver coating for some high performance applications. After this process, the brush is still very fragile and in professional jargon called a 'green brush'. Firing of green brushes Next follows heat treatment of the "green brushes" under artificial atmosphere (usually hydrogen and nitrogen). Temperatures range up to 1200 °C. This process is called sintering or baking. During sintering, the binders either burn off or carbonize and form a crystalline structure between the carbon, copper and other additives. Baking is followed by graphitization (heat treatment). The heat treatment is transformed by a temperature curve exactly defined for each material mixture. Besides the mixture composition, the used temperature curve is the second big “secret” of each brush manufacturer. After the heat treatment, the brush structure is modified in a way which makes copying of the brush nearly impossible for competing companies. Secondary operations Sintering causes the brushes to shrink and to bend. They must be ground to net shape. Some companies use additional treatments in order to make the brush more durable by methods such as impregnation of the running surface by special oils, resins and grease. Manufacturing of carbon brushes requires an in-depth knowledge of materials and experience in mixture compositions. Very small changes in brush contents by just a few percent of components by weight can significantly change the properties of brushes on their applications. There are just a handful of brush developing companies in the world, which are mostly specialized on certain types of brushes. Carbon brushes are one of the least costly parts in an electric motor. On the other hand, they usually are the key part which delivers the durability (“life-time”) and performance to the motor they are used in. Their production requires very high attention to quality control and production process control throughout all steps of the production process. Liquid metal brushes From time to time the use of liquid metals to make contacts is researched. Drawbacks to this approach include the need to contain the liquid metal (as it is usually toxic or corrosive) and power losses from induction and turbulence. See also Commutator (electric) § Brush construction Slip ring References Electrical power connectors Electric motors Scottish inventions