Chemistry Reference
In-Depth Information
whiting
(composed mainly of calcium carbonate), and
gypsum
(composed of
calcium sulfate) were the most commonly used white pigments of antiquity.
Kaolin
was used only in limited geographic regions where this primary clay
was abundant or easily available. Chalk and gypsum rocks are widely dis-
tributed on the surface of the earth. Before being used as pigments, they are
crushed and ground into fine powders.
Kaolin
, also known as
China clay
, gen-
erally occurs naturally mixed with unwanted contaminants, from which it
is separated before use (see Chapter 4).
Probably one of the first artificially made pigments was
bone white
.
Preparing bone white is usually done by first burning bones, generally in
open fires, until all the organic material in the bones is burned away, and
then crushing the product into a white-grayish, slightly gritty powder.
Almost any sort of bone would suffice for making bone white, although
many ancient recipes particularly recommended the use of specific types of
bone such as from the wings of birds or the legs of domestic animals. White
bone is composed mainly (85-90%) of calcium phosphate mixed with
calcium carbonate (13-9%), minor constituents making up the rest.
Lead white
(composed of highly poisonous basic lead carbonate) is
another white pigment known since ancient times; its use seems to have
started during Greek or Roman times. Lead white has excellent hiding
power, that is, it effectively obliterates underlying imperfections on the
painted surface; it also weathers well, remaining in a very good condition
for long periods of time. These properties are unequaled by either chalk or
gypsum. Although it occurs naturally, artificially made lead white seems
often to have been preferred. Artificial lead white was made by exposing
lead to the corrosive effect of vapors of vinegar (a natural solution of acetic
acid) and carbon dioxide. The places where it was manufactured were
reported in many catalogs and lists of pigments written since ancient times
and on into the nineteenth century. Despite its toxicity, of which they may
have been unaware, the ancient Romans used lead white not only as a white
pigment but also as a cosmetic face powder (Gettens et al. 1967).
Black Pigments
The black pigments of antiquity fall into one of two categories:
mineral blacks
and human-made
vegetable
and
animal blacks
. The most common mineral
blacks are
pyrolusite
and
graphite
.
Pyrolusite
, also known as
manganese black
(composed of the dioxide of manganese) has been widely used not only as
a black pigment but also as a glass decolorizer (see Chapter 3).
Graphite
, also
known as
plumbago
, can be either black or dark gray. It is composed of ele-
mental carbon, as it is one of the two naturally occurring mineral allotropes
