Chemistry Reference
In-Depth Information
Europe. To extract the dye, the juice of unripe berries is pressed out and then
dried. The coloring matter in the dye is a mixture of three organic com-
pounds:
rhamnazin
,
rhamnetin
, and
quercetin
.
14.6. IDENTIFICATION AND CHARACTERIZATION
OF ANCIENT DYES AND MORDANTS
Only occasionally is a quantity of preserved dye discovered in an archaeo-
logical excavation. Most of the ancient dyes are identified when still attached
to, or after being removed (by extraction) from dyed yarns or textile fabrics.
The extraction usually entails using chemical techniques for detaching the
dye from the substrate and is generally based on differences in the solubil-
ity of the dye and the substrate in liquid organic solvents such as ether,
acetone, or benzene. When dissolved in suitable solvents, dyes form solu-
tions of characteristic color that can be well defined by their characteristic
optical properties. Following extraction, therefore, dyes are identified by
examining the spectral absorption of their solutions, using either visible,
infrared, or ultraviolet spectroscopy (Taylor 1990; Hofenk de Graff and
Roelofs 1978).
The main disadvantage of most techniques that require, the removal of
a dye from its substrate, before identification, is that they are destructive.
Since archaeological dyed fibers are often irreplaceable, destroying some of
them for analytical purposes may not be possible, and the use of nonde-
structive analytical techniques for identification of the fibers and dyes is
usually preferred. Techniques such as
Fourier transform infrared
(FTIR) and
Raman spectroscopy
overcome this disadvantage, allowing the charact-
erization of dyes attached to a substrate. Moreover, using nondestructive
techniques also make it possible to identify extremely small amounts of dyes,
much below those required when extraction of a sample is necessary before
characterization is made possible (Gillard et al. 1994; Guineau 1989).
Other techniques that have also been widely used to identify and char-
acterize ancient dyes are those included under the generic name
chromato-
graphic techniques
.
Thin
-
layer chromatography
in particular has been found
most sensitive for detecting and identifying extremely small dye samples
(Walton and Taylor 1991; Masschelein-Kleiner and Heylen 1968). A combi-
nation of chromatographic and spectroscopic techniques has often been used
to separate, identify, and characterize ancient dyes. In a study combining
these two techniques, for example, a large variety of dyes used on textiles
dating to between the fourth century B.C.E. and the seventh century C.E. were
identified (Koren 1996).
