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PAINTING MATERIALS

cross lines of wire were placed behind the glass so that the outlines traced there
could be transferred to paper which had a similar linear screen.

The word, 'tracing machine,' or 'tracing apparatus/ has also been applied
to a glass plate so arranged that a mirror beneath it throws the light through the
glass. Over the glass is placed a drawing and over that the thin or tracing paper
on which such drawing is to be followed. Transmission of the light through the
paper clarifies the details and makes the tracing operation a relatively easy one.

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FIGURE 26. Tiles and slants: (a) an ancient container of crystal, found in a late classical
tomb at St Medard-des-Pr£s and containing gold powder mixed with a gum-like sub-
stance (from Berger, I and II, 214, fig. 46); (b] a circular slant, with cup and basin; (c)
a divided slant tile; (d) a tile or slab, with cover; (e) a slant and well tile.

Tube. Both oil paint and water color are now largely put on the market in
collapsible metal tubes with screw caps (figure 2,8). These are made of tin, as
other metals are apt to have a slightly deteriorating effect on pigments or even on
mediums. A variety of sizes among these tubes is now available. Certain pigments,
usually white, are put up in pound tubes and many in half pound. The smallest
tube in ordinary use is 2 inches long by about | inch in diameter. Three- and 4-
inch tubes of the same diameter are common. There is a double 4-inch tube about
f inches in diameter, and a studio tube which is somewhat larger. The smallest
size is used mainly for water colors. The development of collapsible tubes as con-
tainers for color occurred largely during the middle years of the XIX century.
Somewhat earlier than this there had been rigid metal tubes with pistons into
which oil colors were put and which could be refilled at the colorman's. Small
bladders were a common container for commercially sold oil paints in the early
part of that century, also, and these bladders of paint are listed along with col-
lapsible metal tubes in the catalogue of C, Roberson and Company for about
1840. Tubed water colors were slow to displace those put up in cakes or in pans,
but had appeared in the trade by 1850.