Chromium Oxide is, as its name implies, an oxide of chromium; this is an opaque variety. Viridian is also an oxide of chromium, but is transparent. Both these are quite permanent.
Emerald Green is aceto-arsenite of copper, and a somewhat dangerous colour to use; it is darkened by impure air, but this is not so serious as are its effects upon other colours when mixed with them. If used at all, it should be used quite by itself, as if it is mixed with other colours it is sure to have a bad effect on them. It turns some colours black very quickly.
BROWN PIGMENTS
Raw Umber, Burnt Umber, Sepia, Vandyke Brown.
Raw Umber is a natural earth, containing oxide of manganese; it is quite permanent. Burnt Umber is the same colour burnt, by which it becomes darker and richer in colour.
Sepia is generally prepared from the ink of the cuttle-fish, although occasionally a natural earth is substituted. Vandyke Brown is also a natural earth; both these colours are permanent.
BLACK PIGMENTS
Ivory Black, Lamp Black, Indian Ink.
Ivory Black is made from ivory and bone charred to blackness.
Lamp Black is a smoke-black, being a finely divided soot formed by the incomplete combustion of hydrocarbons.