MORDANTS
Any dye belongs to one of two classes. _Substantive_, giving colouring
directly to the material: and _adjective_, which includes the greater
number of dyes and requires the use of a mordant to bring out the
colour.
There are thus two processes concerned with the dyeing of most
colours; the first is mordanting and the second is the colouring or
actual dyeing. The mordanting prepares the stuff to receive the dye
(_mordere_, _to bite_).
The early French dyers thought that a mordant had the effect of
opening the pores of the fibre, so that the dye could more easily
enter; but according to Hummel, and later dyers, the action of the
mordant is purely chemical; and he gives a definition of a mordant as
"the body, whatever it may be, which is fixed on the fibre in
combination with any given colouring matter." The mordant is first
precipitated on to the fibre and combines with the colouring matter in
the subsequent dye bath. But, whether the action is chemical or merely
physical, the fact remains that all adjective dyes need this
preparation of the fibre before they will fix themselves on it. The
use of a mordant, though not a necessity, is sometimes an advantage
when using substantive dyes.
In early days the leaves and roots of certain plants were used. This
is the case even now in India and other places where primitive dyeing
methods are still carried on. Alum has been known for centuries in
Europe. Iron and tin filings have also been used. Alum and copperas
have been known in the Highlands long ages.
_Mordants_ should not affect the physical characteristics of the
fibres. Sufficient time must be allowed for the mordant to penetrate
the fibre thoroughly. If the mordant is only superficial, the dye will
be uneven: it will fade and will not be as brilliant as it should be.
The brilliancy and fastness of Eastern dyes are probably due to a
great extent to the length of time taken over the various processes of
dyeing. _The longer time that can be given to each process, the more
satisfactory will be the result._
Different mordants give different colours with the same dye stuff. For
example:--Cochineal, if mordanted with alum, will give a crimson
colour; with iron, purple; with tin, scarlet; and with chrome or
copper, purple. Logwood, also, if mordanted with alum, gives a mauve
colour; if mordanted with chrome, it gives a blue. Fustic, weld, and
most of the yellow dyes, give a greeny yellow with alum, but an old
gold colour with chrome; and fawns of various shades with other
mordants.
Silk and wool require very much the same preparation except that in
the case of silk, high temperatures should be avoided. Wool is
generally boiled in a weak solution of whatever mordant is used. With
silk, as a rule, it is better to use a cold solution, or a solution at
a temperature below boiling point. Cotton and linen are more difficult
to dye than wool or silk. Their fibre is not so porous and will not
hold the dye stuff without a more complicated preparation. The usual
method of preparing linen or cotton is to boil it first with some
astringent. The use of astringents in dyeing depends upon the tannic
acid they contain. In combination with ordinary mordants, tannic acid
aids the attraction of the colouring matter to the fibre and adds
brilliancy to the colours. The astringents mostly used are tannic
acid, gall nuts, sumach and myrobalams. Cotton has a natural
attraction for tannic acid, so that when once steeped in its solution
it is not easily removed by washing.
ALUM
This is the most generally used of all the mordants, and has been
known as such from early times in many parts of the world. For most
colours a certain proportion of cream of tartar should be added to the
alum bath as it helps to brighten the ultimate colour. The usual
amount of alum is a quarter of a pound to a pound of wool. As a rule,
less mordant is needed for light colours than for dark. Excess of alum
is apt to make the wool sticky. The usual length of time for boiling
is about an hour. Some dyers give as much as 2-1/2 hours.
_Example of mordanting with alum_--1/4 lb. of Alum and 1 oz. cream of
tartar for every pound of wool. This is dissolved and when the water
is warm the wool is entered. Raise to boiling point and boil for one
hour. The bath is then taken off the fire and allowed to cool over
night. The wool is then wrung out (not washed) and put away in a linen
bag in a cool place for 4 or 5 days, when it is ready for dyeing,
after being thoroughly washed.
IRON
(_Ferrous Sulphate, copperas, green vitriol._)
Iron is one of the oldest mordants known and is largely used in wool
and cotton dyeing. It is almost as important as alum. The temperature
of the mordanting bath must be raised very gradually to boiling point
or the wool will dye unevenly. A general method of dealing with
copperas is to boil the wool first in a decoction of the colouring
matter and then add the mordant to the same bath in a proportion of 5
to 8 per cent of the weight of the wool, and continue boiling for half
an hour or so longer. With some dyes a separate bath is needed, such
as with Camwood or Catechu. Great care is needed in the using of
copperas, as, unless it is thoroughly dissolved and mixed with the
water before the wool is entered, it is apt to stain the wool. It also
hardens wool if used in excess or if boiled too long. A separate bath
should always be kept for dyes or mordants containing iron. The least
trace of it will dull colours and it will spoil the brilliancy of
reds, yellows and oranges.
Copperas is mostly used for the fixing of wool colours (Fustic, etc.)
to produce brown shades; the wool being boiled first in a decoction of
the dye for about 1 hour, and then for 1/2 an hour with the addition
of 5 to 8 per cent of copperas. If used for darkening colours,
copperas is added to the bath after the dyeing, and the boiling
continued for 15 to 20 mins.
TIN
(_Stannous chloride, tin crystals, tin salts, muriate of tin._)
Tin is not so useful as a mordant in itself, but as a modifying agent
with other mordants. It must always be used with great care, as it
tends to harden the wool, making it harsh and brittle. Its general
effect is to give brighter, clearer and faster colours than the other
mordants. When used as a mordant before dyeing, the wool is entered
into the _cold_ mordant bath, containing 4 per cent of stannous
chloride and 2 per cent oxalic acid; the temperature is gradually
raised to boiling, and kept at this temperature for 1 hour. It is
sometimes added to the dye bath towards the end of dyeing, to
intensify and brighten the colour. It is also used with cochineal for
scarlet on wool in the one bath method.
CHROME
(_Potassium dichromate. Bichromate of Potash._)
Chrome is a modern mordant, unknown to the dyer of fifty years ago. It
is excellent for wool and is easy to use and very effective in its
action. Its great advantage is that it leaves the wool soft to the
touch, whereas the other mordants are apt to harden the wool. The wool
should be boiled for 1 to 1-1/2 hours with bichromate of potash in the
proportion of 2 to 4 per cent of the wool. It is then washed well and
immediately dyed. Wool mordanted with chrome should not be exposed to
light, but should be kept well covered with the liquid while being
mordanted, else it is liable to dye unevenly. An excess of chrome
impairs the colour, 3 per cent of chrome is a safe quantity to use for
ordinary dyeing. It should be dissolved in the bath while the water is
heating. The wool is entered and the bath gradually raised to the
boiling point, and boiled for 3/4 of an hour.
COPPER
(_Copper Sulphate, Verdigris, Blue Vitriol, Blue Copperas,
Bluestone._)
Copper is rarely used as a mordant. It is usually applied as a
saddening agent, that is, the wool is dyed first, and the mordant
applied afterwards to fix the colour. With _cream of tartar_ it is
used sometimes as an ordinary mordant before dyeing, but the colours
so produced have no advantage over colours mordanted by easier
methods.