In 1879, the piece of metal was made. It was officially chosen to be the kilogram in 1889. It was made of 90% platinum and 10% iridium. Those metals were chosen because they do not rust or corrode like most metals. It is stored in a vault at the BIPM in Sèvres, France. From 1795 to 1799, the unit of mass was not called "kilogram" but was called "grave". 