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MORDANT DYE
Mordant dyes are those dyes which require
a mordant in their application and which upon combination with
the mordant deposit insoluble color on the substrate, e.g.,
dyes with metal chelating groups. Like the name suggests these
dyes require a mordant. Mordants are substances of organic or
inorganic origin which combine with the coloring matter and
are used to fix the same in the production of the color. For
the purpose of this class, such materials as oils and sulfonated
oils, soaps, fats and higher acids, are not generally considered
as mordants, but as coming within the scope of “assistants”
in dyeing. The mordant substances include such acids as tannic
acid, sumac, gall nuts, bark extracts, oleic and stearic acids,
and Turkey red oil; and metallic substances such as various
combinations or soluble salts of chromium, aluminum, iron, copper,
and tin. The latter, the metallic mordants, are more used than
the acid mordants. Mordant improves the fastness of the dye
on the fibre such as water, light and perspiration fastness.
The choice of mordant is very important as different mordants
can change the final colour significantly. Most natural dyes
are mordant dyes and there is therefore a large literature base
describing dyeing techniques.
Dyes are generally defined along the lines of
being coloured, aromatic compounds that can ionise. Although
this definition infers that ionic interaction with oppositely
charged tissue constituents is the norm, there are exceptions.
Some dyes require the presence of a metal to properly develop
their colour or staining selectivity. These are termed mordant
dyes. The Colour Index uses this as a classification and naming
system. Each dye is named according to the pattern: mordant
+ base colour + number
These dyes are thereby specifically identified
as dyes of the stated colour, and whose primary staining mechanism
requires the presence of metal atoms. Note that this is a
functional and colour classification. It contains no chemical
information neither does it imply that dyes with similar names
but unique numbers are in any way related. It should also
be noted that the classification refers to the primary mechanism
of staining. Other mechanisms may also be possible.
The most commonly used mordant dyes have hydroxyl
and carboxyl groups and are negatively charged, i.e. anionic.
It is convenient to view these as a specialised subgroup of
acid dyes. Some other mordant dyes may possess amino groups,
and are cationic overall. Despite this, they must still have
hydroxyl or carboxyl groups, since lake formation requires
it. Mordant dyes can usually stain by ionic interaction in
the same way as other ionisable dyes. The colour is often
pale, sometimes so pale that the results have no value.
It is often noted that when a mordant dye forms
a lake with a metal, there is a strong colour change. This
is because metals have low energy atoms. The incorporation
of these low energy atoms into the delocalised electron system
of the dye causes a bathochromic shift in the absorption.
It is this delocalised electron system which is fundamentally
responsible for colour in dyes. Since different metal atoms
have differing energy levels, the colour of the lakes may
also differ.
The most commonly used mordant dye is undoubtedly
hematein (natural black 1), whose status as a natural product
supercedes its mode of dyeing, apparently. Others are eriochrome
cyanine R (mordant blue 3) and celestine blue B (mordant blue
14), both used as substitutes for alum hematoxylin but with
a ferric salt as the mordant. Alizarin red S (mordant red
3) is valuable for the demonstration of calcium, particularly
in embryo skeletons.
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