Mountain Blue
Mountain blue (copper blue), paint consisting of finely precipitated copper glaze. Mountain blue is a very fiery light blue, but not very stable and is blackened by hydrogen sulphide. It has now been superseded by ultramarine, the mountain blue of Chessy (Cendres bleues) being replaced by artificial lime blue. See Neuwied blue.
Source: Meyers Großes Konversations-Lexikon, 6. Auflage 1905–1909
Mountain blue, pigment made of finely ground and precipitated lazurite. Artificial mountain blue (mineral blue, English blue, lime blue, copper blue, Brunswick blue, Bremer blue, Casseler blue, Hamburg blue, Neuwied blue), alkali carbonate of copper oxide, made from a solution of copper vitriol and soda, sometimes mixed with gypsum, chalk, barite, etc.
Source: Brockhaus’ Kleines Konversations-Lexikon, 5. Auflage 1911