164 PAINTING MATERIALS
a bright red heat from 12 to 18 hours, the product is cooled, ground, and lixiviated
to remove soluble salts, dried and again ground until the proper color and degree
of fineness are obtained. With a small amount of sulphur the color is dark blue,
but with a high percentage of sulphur it is dark blue with a reddish tinge. Soda
ultramarine also contains a high percentage of silica and is sometimes called acid-
resisting because it is stable in the presence of alum solutions (important where
ultramarine is used in the paper trades). ' Sulphate ultramarine/ which has a
greenish tinge (see Ultramarine Green) and little covering power, is made by
using sodium sulphate (Glauber's salt) in place of soda ash. By washing and roast-
ing with additional sulphur, it may be changed to blue ultramarine (see Beam,
pp. 80-85, and Rose, pp. 173-202). Variations in the process give blue, red, and
violet ultramarines in various shades and hues. The chemical synthetic product
does not differ from natural ultramarine in composition or chemical properties
and it is much purer. Ultramarine is essentially a sodium aluminum silicate which
also contains a certain amount of sulphur. F. M. Jaeger says (Optical Activity and
High Temperature Measurements^ Part ///, Constitution and Structure of Ultra-
marine [New York: McGraw-Hill Book Co., 1930], pp. 403-441) that it has no
fixed formula and the ratios of the various constituents can change within limits.
There appears, however, to be a fixed component in ultramarine with the formula,
NagAl6Si6O24, which may take on sodium and sulphur atoms to give ultramarines
with formulas ranging from NagAleSisC^Si to Na10Al6Si6O24S2. The cause of the
color and differences of color in ultramarines is still more or less a mystery. It ap-
pears to be associated with the sulphur present or a combination of sodium and
sulphur. When the pigment is decomposed by acids, sulphur and hydrogen sul-
phide are released and the color is immediately discharged.
Artificial ultramarine, in contrast with natural ultramarine, is finely divided
and homogeneous. The particles, which are rounded, are about the same size as
Dutch process white lead. The small, individual particles are quite opaque to
transmitted light; they are isotropic, and the refractive index is low (n = 1.50).
The color by reflected light is claimed (see De Wild, p. 20) not to be the pure blue
of natural ultramarine but usually to have a purplish tinge which makes it less
desirable from the artist's point of view. This blue is stable under all conditions,
except in the presence of acids. It is readily decomposed, even by dilute acetic
acid, with decoloration of the pigment and evolution of hydrogen sulphide. It is
permanent to light and is unaffected by high temperatures. Since it is unaffected
by alkalis, it is stable in fresco, Impure ultramarines may contain free sulphur
and, hence, cause darkening when mixed with lead and copper pigments. Artificial
ultramarine in rich oil films occasionally appears to decolorize and to become gray
with age, This phenomenon is sometimes calledc ultramarine sickness * and seems
to be caused by an acid condition in the film. Weber points out (p. 113) that it
never occurs when white pigments are mixed with the film; this is probably be-
cause some whites, like those of lead and zinc, can readily neutralize acid.