242 PAINTING MATERIALS
contains tannin. In European panel paintings, particularly of the Flemish school,
oak was much used (see also Wood).
Olive-Wood (Olea). This genus includes about 20 species widely scattered,
chiefly over the Old World from the basin of the Mediterranean to South Africa.
Among them, the common olive tree (0. europaea) is most important. Although
it grows to no great height, it frequently develops trunks large enough for timber.
The wood, which is hard and-close-grained, is valued by cabinet makers and by
wood-turners; it is yellow or light, greenish brown in hue but is often veined with
a darker tint. It may be polished to give a smooth marble-like surface similar to
that of boxwood. The pores are scarce and obscure. It is known that a few Italian
paintings were made on olive-wood panels.
Panel (see also Wood) is a word generally used to describe all movable or easel
paintings that are not executed on a support of fabric. At times, however, it is
used even more broadly to designate all paintings not done on walls, as, * the
panel painting of the Italian Renaissance.' Technically, panels are understood
usually to be supports of wood.
Paper. The papers used in the fine arts are commonly made from cotton or
linen but the Far East and the classical world have been used to coarser plant
materials like mulberry and papyrus. When any such fibres are separated by mac-
erating the plant or other source into a pulp and then are disposed in a thin,
compact web, dried in a sheet, and pressed flat, the product is some class of paper.
The chief material of all paper is cellulose, the pure plant substance which forms the
cell walls, and this, regardless of its source, is made up of carbon, hydrogen, and
oxygen, present in the same proportions (see also Fibrous Substances). Classes of
paper can be distinguished most conveniently for study according to the sources
of the fibre, though in industry it is a frequent practice to classify papers according
to their particular uses. H. N. Lee has made a convenient arrangement of paper-
making fibres: (i) the cotton group, including cotton, linen, ramie, hemp, and
perhaps paper mulberry; (2) the wood-pulp group including the two distinct
classes—(a) ground wood or mechanical wood-pulp, the entire wood substance
reduced to a fibrous form, and (b} chemical wood-pulp in which intercellular
matter has been considerably dissolved away leaving a much purer cellulose; (3)
the grass group, including esparto and bamboos; and (4) the rope group, a kind
used only for rough paper in localities of the Far East.
Although cellulose remains the same, the characteristics of the different
groups of fibres vary to a great extent. Cotton fibre has a natural full length of
ao to 40 mm.; linen runs usually from 25 to 30 mm. and has a less uniform diam-
eter than cotton. Paper mulberry fibre shows great variation in width and has a
natural full length of about 15 mm. Wood-pulp fibre is much shorter, its natural
full length being about 3 mm. It can be seen that the longer fibre makes a stronger
paper, and it has also been suggested that the fibres of the cotton group contain
a much purer cellulose than those from wood or grass. The latter are more apt to