The mines at Modum in Buskerud county north-west of Oslo were one of Europe’s principal sources of cobalt ores, the source of a widely-used blue colouring matter used in paint and in the manufacture of glass and ceramics. Cobalt ores were discovered at Modum by Ode Witloch in 1772, and a blue colouring works was established by royal decree in 1776. The works was managed by Germans for many years and from 1822 it was in in German ownership. The cobalt ore was mixed with potassium carbonate and quartz and poured into water in a molten state which caused it to form droplets which were then ground by quartzite millstones to make colouring. Arsenic and sulphur compounds were removed from the ore in calcining kilns and some compounds were distilled and sold. There were about 500 employees at the works in 1840, but production ceased in 1855 after the discovery of ultramarine which was used instead of cobalt compounds in paint, although mining continued until 1898. A trust was formed in the 1960s to conserve the remains of the works and a museum was opened in 1968. Part of the old cobalt mine was re-opened for guided tours in 1993, and a small open air museum has been created on the site. The buildings house an art gallery as well as an industrial museum.