168	painting materials
goethite of raw sienna and yellow ochre; there are many orange, yellow, and color-
less ones and there is also a small proportion of birefracting material. Umber is
durable; it is compatible with other pigments, and is adaptable to all mediums.
Poor grades, which contain humus matter, are not so stable and are liable to fade
in strong sunlight.
Burnt umber is made by roasting the raw earth at a dull red heat until the
desired shade is obtained. The heating changes the ferric hydrate to ferric oxide
and the product is redder and warmer than raw umber. Microscopically, the
burnt differs little from the raw, except that it is a little redder and more
transparent.
The umbers are unaffected by alkalis and by dilute mineral acids. Because of
their manganese content, they dry well in oil, and have been used as driers for
varnishes. They have high oil absorption, requiring up to 80 per cent to grind, and
for this reason oil films pigmented with them have a tendency to become darker
with age*
The umbers have been available since earliest times, but Thompson says (The
Materials of Medieval Painting, pp. 88-89) that they were not found on the early
mediaeval palette and they did not come into general use in Europe before the
close of the XV century.
Van. Dyke Brown (Cassel earth, Cologne earth) is a name commonly used to
designate a brown organic pigment which is derived from earthy substances simi-
lar to lignite or brown coal. It contains usually over 90 per cent of organic matter,
with a small amount of iron, alumina, silica, etc. Most of the raw Van Dyke
brown has come from Germany, from places near Cassel and Cologne. Harrison
(p. 242) says that the best grades of this pigment are prepared from good, clean
peat deposits which have been well carbonized by slow formation and long weath-
ering. It is said (Weber, p. 115) that it got its name from ' the famous artist who
was partial to the use of brown in his pictures.* It is prepared first by heating to
drive off excess moisture, and then by the usual processes for earth pigments. It
has a warm, reddish brown shade and, since it is partially transparent in oil, it is
used for staining of woods and for glazing in pictures. When seen microscopically,
it is heterogeneous in particle size and composition, and the particles seem more
opaque and less crystalline than .ochres and umbers. When ignited, it burns and
leaves a gray ash, and when heated in an ignition tube, tarry vapors are given off.
It dissolves in dilute sodium hydroxide to give a deep brown solution. Because of
its tarry and bituminous nature, it is a fugitive pigment. It fades on exposure to
strong light and develops a cold, gray tone. In oil, however, it is more permanent
than in water color. The pigment appears to dissolve in oil or varnish and to stain
it; for this reason, it is difficult to isolate and to identify in such mediums.
Little is known about the early use of the lignite colors. Probably they came
into use in the late XVII and early XVIII centuries when brown shadows and
backgrounds became popular.