What is "ultramarine ashes"

Ultramarine \Ul`tra*ma*rine"\, n. [Cf. Sp. ultramarino. So called because the lapis lazuli was originally brought from beyond the sea, -- from Asia.] (Chem.) A blue pigment formerly obtained by powdering lapis lazuli, but now produced in large quantities by fusing together silica, alumina, soda, and sulphur, thus forming a glass, colored blue by the sodium polysulphides made in the fusion. Also used adjectively.

Green ultramarine, a green pigment obtained as a first product in the manufacture of ultramarine, into which it is changed by subsequent treatment.

Ultramarine ash or Ultramarine ashes (Paint.), a pigment which is the residuum of lapis lazuli after the ultramarine has been extracted. It was used by the old masters as a middle or neutral tint for flesh, skies, and draperies, being of a purer and tenderer gray that produced by the mixture of more positive colors.
--Fairholt.

n. (plural of ultramarine ash English)