Cobalt is a chemical element with symbol Co and atomic number 27. Like nickel, cobalt in the Earth’s crust is found only in chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, produced by reductive smelting, is a hard, lustrous, silver-grey metal.
Cobalt-based blue pigments (cobalt blue) have been used since ancient times for jewellery and paints, and to tint glass blue, but the colour was later thought by alchemists to be due to the known metal bismuth. Miners had long used the name kobold ore (German for goblin ore) for some of the blue-pigment producing minerals; they were named because they were poor in known metals, and gave poisonous arsenic-containing fumes upon smelting. In 1735, such ores were found to be reducible to a new metal (the first discovered since ancient times), and this was ultimately named for the kobold.
Cobalt is primarily used as the metal, in the preparation of magnetic, wear-resistant and high-strength alloys. Its compounds cobalt silicate and cobalt(II) aluminate (CoAl2O4, cobalt blue) give a distinctive deep blue color to glass, ceramics, inks, paints and varnishes.
|Cobalt blue tinted glass|
Free cobalt (the native metal) is not found in on Earth, except as recently delivered in meteoric iron (see below). Though the element is of medium abundance, natural compounds of cobalt are numerous. Small amounts of cobalt compounds are found in most rocks, soil, plants, and animals.
Cobalt forms many useful alloys. It is alloyed with iron, nickel, and other metals to form Alnico, an alloy with exceptional magnetic strength. Cobalt, chromium, and tungsten may be alloyed to form Stellite, which is used for high-temperature, high-speed cutting tools and dies. Cobalt is used in magnet steels and stainless steels. It is used in electroplating because of its hardness and resistance to oxidation. Cobalt salts are used to impart permanent brilliant blue colours to glass, pottery, enamels, tiles, and porcelain. Cobalt is used to make Sevre’s and Thenard’s blue. A cobalt chloride solution is used to make a sympathetic ink. Cobalt is essential for nutrition in many animals. Cobalt-60 is an important gamma source, tracer, and radiotherapeutic agent.
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