DYES AND DYEING - Archaeological Chemistry - page 367

14.2. THE DYEING PROCESS

Most fibrous materials readily absorb water. If textile fabrics such as cotton
or wool, are soaked in water in which a dye is dissolved, part of the dye
leaves the solution, reacts with the fibers that make up a fabric, and imparts
its color to the fibers. The textile fabric is then said to be dyed . The strength
of fixation , as the strength of the bond between a dye and a fiber is usually
termed, is determined by the chemical affinity between them, that is, by the
nature of the chemical bond formed between the dye and the fiber. Dyes that
bond strongly to fibers generally produce fast colors , resistant to fading by
washing or exposure to light. Such dyes are generally known as direct dyes .
Others, which bond only weakly to fibers, usually fade when the dyed fibers
are washed or exposed to light; such dyes are known as fugitive dyes (see text
below) (Padfield and Landi 1966).

14.3. MORDANTS

Many dyes that have no chemical affinity to fibrous substrates can be
attached to such substrates by intermediary (go-between) substances known
as mordants . These are either inorganic or organic substances that react chem-
ically with the fibers as well as with the dyes and thus link the dyes to the
fibers. Mordants are traditionally classified into two main classes, acid and
metallic mordants . The acid mordants are organic substances that contain
tannins (see Textbox 64) as for example, gall nuts and sumac. The metallic
mordants are inorganic substances, mostly mineral oxides and salts that
include metal atoms in their composition. Table 94 lists mordants of both
these types, which have been used since antiquity.
The mordants may be applied to the substrate either before, during, or
after application of the dye. Premordants , as the mordants applied before the
dye are known, seem to have been the most commonly used in antiquity.
Some mordants not only are instrumental in attaching the dye to the fibers
but also alter the shade and even the hue of some dyes; a single dye often
provides a range of hues when used with different mordants. The color or
hue of textiles dyed with madder, weld, and logwood, for example, are
determined by the chemical nature of the mordant used; Table 95 lists hues
obtained when dyeing with the same dye but with different mordants.

14.4. THE NATURE OF DYES
Direct and Mordant Dyes. The ancient dyes (see Fig. 77) may be
classified into three main groups: direct dyes , mordant dyes , and vat dyes (see