being known by different names, such as Antwerp blue, Berlin blue, Harlem blue and Chinese blue. Indigo is produced by steeping certain plants in water and allowing them to ferment. It is a transparent color, works well in oil or water, but is not durable, especially when mixed with white lead.
Ultramarine was originally made by grinding the valuable mineral lapis lazuli. Genuine ultramarine so made is very expensive, but artificial French or German ultramarines are made of better color, and cheaply, by fusing and washing and reheating a mixture of soda, silica, alum and sulphur. This blue is chiefly used for coloring wall papers.
Cobalt blue is an oxide of cobalt made by roasting cobalt ore. It makes a beautiful color and works well in water or oil.
Smalt, Saxon blue and royal blue are colored by oxides of cobalt.
There are a few other blues, such as celestial or Brunswick blue, damp blue and verditer, that are chemical compounds, compounds of alum, copper, lime and other substances.
Brunswick blue is essentially a mixture of Prussian blue and barytes. It is prepared by thoroughly mixing barytes with water, adding a solution of copperas, then a solution of red or yellow prussiate of potash, stirring constantly so as to ensure the thorough incorporation of the barytes with the blue. After filtering, washing and drying, the blue is ready for use.
As a pigment it is quite permanent and resists exposure to the air, light and most of the other influences which act on pigments. It has the curious property of fading a little on exposure to light and of recovering its original intensity of color in the absence of light.
Prussian blue can be mixed with nearly all other pigments without being affected or changed by them or affecting them in any way.
