PAINTING MATERIALS
the end of the XVIII century, only bistre and India ink were used for washes
(see Meder, p. 70). The comparatively late use of sepia makes possible a distinc-
ticn between late XVIII century sepia additions to earlier bistre drawings.
Sienna (raw sienna, burnt sienna). Raw sienna is a special kind of yellow ochre
which gets its name from the well known Tuscan city near which one of the best
grades of it has long been found; it is still produced there and is shipped from
Leghorn. Good commercial grades are also found in the Hartz Mountains, Ger-
many, and in America. Like ochre, sienna is hydrated ferric oxide (goethite,
Fe2O3-H2O) with alumina and silica, but it has a little deeper tint than yellow
ochre, is a little warmer, and is considerably more transparent. A good sienna
should contain at least 50 per cent of iron oxide (Fe2O3); some of the best contain
70 per cent or over. It generally has a small amount of manganese dioxide (0.6
to 1.5 per cent). Raw sienna is prepared for commerce by processes similar to
those used with the ochres, and the physical and chemical properties are like those
of the other hydrous iron oxides. Microscopically, the pigment is quite hetero-
geneous; it is a mixture of transparent, colorless, yellow and brown-red particles,
along with opaque brown particles and a few scattered pink ones. The transparent
grains are highly birefracting, but the brownish material is quite isotropic. This
latter material is a darkened variety of goethite (see Ochre), occurring in fairly
large spherules.
Burnt sienna is prepared by calcining raw sienna; in the process, the raw sienna
undergoes a considerable change in hue and depth of color. In going from the ferric
hydrate of the raw earth to ferric oxide, it turns to a warm, reddish brown.
Microscopically, it becomes more even in color and the grains are reddish brown
by transmitted light. Merwin says (p. 578) that it shows no evidence of crystal-
Unity, is not birefracting, and the grains have variable, moderate refractive index.
By artists, sienna has been used as a glaze because of its transparency. For
the same reason, both raw and burnt sienna are used in wood finishing for stains
and for graining work. In the microscopic and chemical examination of paintings
the siennas are usually not reported under that name but are grouped under the
ochres or native iron oxide pigments. Often distinction among earth colors is
difficult, because the differences are of degree and not of kind. The siennas have
been available in all periods of European painting and have been used in all
processes.
Silex (see Silica).
Silica (quartz, silex) is silicon dioxide which occurs in clear, crystalline form
as quartz or rock crystal. It is common in other less pure forms as quartzite,
sandstone, sand, and in crystalline grains or masses in granite (see Ladoo, pp. 521-
526). It is widely distributed, being one of the most abundant constituents of the
earth's crust. Finely ground and sieved quartz (silex), with oil or varnish, serves as
a primer for filling the grain of wood before staining and varnishing. Tripoli (not
to be confused with tripolite which is the same as diatomaceous earth) is an amor-