pigments	167
in contracts and commissions for paintings* It was used from Byzantine times
through the XVIII century in European paintings and, though it had more fre-
quent literary mention than azurite, it is found less often in old paintings than
this latter pigment. Diirer used ultramarine, and in his letters to his patron,
Jacob Heller (see W. M. Conway, Literary Remains of Albrecht Diirer [Cam-
bridge: University Press, 1889], pp. 66-69), he complains of its cost. De Wild
lists (p. 18) several Dutch and Flemish paintings, beginning with the Van Eyck
* St Barbara * (1437, ^at Antwerp), on which he identified it. He also found it on a
Dutch painting dated as late as 1810. It disappeared from the painter's palette
soon after that but can still be bought from certain English colormen, and it is
claimed that the ultramarine is purified by methods similar to those described in
the mediaeval recipes.
Ultramarine Green is nearly identical in composition with ultramarine blue
(see Ultramarine Blue, artificial) and is produced in a similar way. It is the palest
in color of the ultramarines and the most sensitive to acids; otherwise, it is quite
similar, physically and chemically, to ultramarine blue. Microscopically, it is
heterogeneous; the bright, transparent particles of ultramarine green are mixed
with a large proportion of ultramarine blue, as well as colorless particles. Though
still listed by a few colormen, it apparently has never been extensively used.
It was first prepared by Kottig in Meissen in 1828, but was not manufactured on a
commercial scale until 1854-1856 (see Trillich, III, 78).
Ultramarine Violet (ultramarine red) is made by mixing soda ultramarine blue
with sal ammoniac and heating for some hours at a temperature of about 150° C.
Ultramarine red is prepared in a similar way, except that dry hydrochloric acid
gas replaces sal ammoniac. These special ultramarines were developed in Germany
mainly in the years between 1870 and 1880 (see Rose, pp. 193-195). Chemically
and physically, ultramarine violet is similar to ultramarine blue and usually con-
tains much of it. Not being affected by alkalis, it is one of the very few violets
that can be used in fresco painting. In oil technique, it is pale and has poor covering
power. It is readily available today in dry powder and in various mediums, in-
cluding oil.
Ultramarine Yellow, a name sometimes misapplied to Strontium Yellow.
Umber (raw umber, burnt umber). Raw umber is a brown earth pigment simi-
lar to the ochres and siennas but contains manganese dioxide as well as hydrous
ferric oxide. The general run of raw umbers has 45 to 55 per cent iron oxide, 8 to
16 per cent manganese dioxide, silica, alumina, etc. Raw umber is rather widely
distributed in nature. One of the best, found on the island of Cyprus, has long
been known as Turkey umber. Others are found in England, France, Germany,
and America. From the crude lump umber, it is prepared as a pigment by the usual
process of grinding and levigation. The best earths have a warm, reddish brown
color with a greenish undertone. Microscopically, the pigment is heterogeneous
in composition and particle size. It contains much goethite, but the grains of that
mineral are finer and darker yellow-brown or more nearly opaque than the