294 PAINTING MATERIALS
Probably many different hard stones were adopted by painters for this pur-
pose, but porphyry seems to have been a favorite since the Middle Ages. Thomp-
son (The Practice of Tempera Painting, pp. 88-89) says that porphyry or granite
are still the best materials for slabs and mullers but that glass is a simple substi-
tute. This or marble has to be resurfaced when it gets smooth and that can be
done with moderately fine emery powder, wetted, and ground between muller
and slab. Sir Aurel Stein (Serindia, I, 772) reports the discovery at Tun Huang in
Chinese Turkestan of a wooden block which he thought had been used as a grind-
ing slab. It was D-shaped in section, and one end and the adjacent side still had
wrappings of linen. On it was thick, black paint. A second similar block was found
in the same site. No mention has appeared in the technical literature of the color-
grinding methods used by ancient Egyptians, but it can be assumed that the hard
mineral pigments, particularly the well known Egyptian blue, must have been
ground on a slab or in a mortar. One indication of such a utensil is seen in the
British Museum. It is a piece of dark stone about seven inches across, rather flat on
the bottom, rounded at the edges, circular in plan, and less than half as thick as its
diameter. The top has a shallow, saucer-shaped depression. This seems to be
Egyptian and is something between a mortar and a grinding slab. Little is known
about other ancient practices of preparing paints, but an anecdote by Pliny
(XXXV, 85) suggests that in the studio of Apelles there were boys who ground
the colors. By the early Middle Ages in Europe, the fairly standard equipment of
the painter's work-room appears to have been a heavy slab mounted on a heavy
block. The MS. of Theophilus speaks of it, as does that of Eraclius (see Laurie,
The Painter's Methods and Materials, pp. 25 and 27). Cennino Cennini mentions
porphyry (C. XLII, Thompson, p. 25) and so does the author of the much later
Brussels MS. in 1635 (Merrifield, II, 770). By the XVII century there are many
representations of the grinding tables or blocks, usually with circular or polyg-
onal, heavy slabs.
The hand-grinding of colors undoubtedly continued long after fairly large-
scale commercial manufacture was developed, and some amount of it is still done
by artists' colormen or by painters themselves. In the early XIX century, a form
of hand-operated color mill was developed, and shortly after that a larger mill
with stones propelled by hand, horse, or steam power. The common modern
mill is operated by a motor and is of rotating cylinders. Water colors, as first
manufactured for sale, were usually in small, hard cakes which, like the Far
Eastern ink stick, had to be ground with water on a small slab in order to bring
the color out into the fluid. Such grinding material and equipment were sold
commonly after the middle of the century and, although little mentioned or
advertised, can still be bought.
Hog Hair, the name formerly applied to the heavier artists* brush, now usually
referred to as bristle (see Brush).