MADDER AND ITS COLOURING MATTERS.
The root of the madder (_Rubia tinctoria_) contains a red colouring
matter which is distinguished over all other colours of vegetable
origin by great fastness, on account of which it occupies a most
important position in dyeing and colour making. The use of madder in
dyeing has been much restricted by the discovery of artificial methods
of making alizarine, the most important colouring matter of madder.
Thus the following account of madder and its products is chiefly of
historical interest only.
Madder is cultivated in many countries. The roots, which vary from the
thickness of a quill to that of a finger, are cleansed from adherent
earth, carefully sorted according to size, and dried. In commerce a
distinction is made between stripped and unstripped madder. The cortex
of the root contains but little colouring matter and is generally
removed by mill-stones moving at some distance apart. The stripped
roots are finely ground, and are then known commercially as madder.
Merchants distinguish a large number of varieties, which are designated
according to the place of production. In addition to ground madder
there are a number of madder preparations which contain the colouring
matter in concentrated form, and are consequently used in smaller
quantity. The most important of these preparations are garancin,
garanceux, and the madder extracts.
”Refined madder” is made from the natural substance by subjecting it
to a process of fermentation, by which the substances accompanying
the colouring matters are largely decomposed, and the product becomes
relatively richer in colouring matter.
”Flowers of madder,” which have double the colouring power of madder,
are made by treatment with very dilute sulphuric acid. Madder is
stirred with five or six times its quantity of water to which about 1
per cent. of sulphuric acid has been added. After leaving the mixture
in a fairly warm place for five to six days alcoholic fermentation
takes place, and many substances which would be injurious in dyeing
are decomposed. The residue becomes in consequence richer in colouring
matter. When the fermentation is finished the residue is filtered
off, subjected to powerful hydraulic pressure, and the mass broken up
and very thoroughly dried at a temperature of 60° to 70° C. If it is
not thoroughly dried it soon becomes mouldy. This method of treating
madder, due to Julians, is simple, and has the advantage that from the
alcoholic liquid spirit may be obtained. In some respects this process
is more rational than the one immediately following: certain substances
which are destroyed in the manufacture of garancin are made useful, and
also the process requires little outlay.
=Garancin= is obtained from madder by treatment with sulphuric acid,
which decomposes the glucoside, and also sets free the colouring
matter united to lime and magnesia, which would otherwise be lost. In
addition, the warm acid radically attacks the nitrogenous compounds,
the greater portion of which is destroyed. Thus the residue contains
relatively more colouring matter, and the removal of substances which
accompany the colouring matter considerably facilitates the dyeing
process.
Although the preparation of garancin was recommended in 1828, it was
much later before the prejudice which prevented its general employment
was completely overcome, and garancin recognised as a very valuable
material for the dyer. The simplest process for preparing garancin is
as follows: Madder is several times washed with water, then pressed,
and the residue mixed with dilute sulphuric acid in a lead-lined
vessel. To 100 parts of madder 50 parts of sulphuric acid and 50
parts of water are taken, the mixture is heated by steam to 100° C.,
and maintained at this temperature for half an hour. By the action
of sulphuric acid of this strength considerable charring occurs, and
in particular the cell walls of the madder are attacked, so that in
the succeeding process the colouring matter is readily dissolved; in
consequence of the charring, the garancin acquires a deep brown to
black colour. When the action of the acid is finished, it is drawn off
and the residue washed with water until it is free from acid, when it
is dried. The process here described is the original method; it has
been much modified, hydrochloric acid, zinc chloride, alkalis, soap
solution, etc., being used in the manufacture of garancin. The colour
maker will rarely require to make garancin himself, so that it is
sufficient for our purpose to indicate what is understood by garancin,
and in what manner it is made.
=Garanceux.=—Madder which has been once used in dyeing still retains
some quantity of colouring matter; this is utilised by treating it by
the same process which is used to obtain garancin from fresh madder.
The treatment with sulphuric acid almost entirely destroys the cellular
structure, so that the residual colouring matter is made accessible to
solvents, and consequently the garanceux may be again used for dyeing.
=Madder Extract.=—The fixation of the colouring matter of madder upon
the fibre is attended with many difficulties; attempts were made to
present it in the purest form possible, or in such a condition that
the operation of dyeing was simplified. The commercial madder extracts
consist either of liquid extracts of madder, or of the more or less
pure colouring matter itself. Among the numerous solid or liquid madder
extracts, without doubt the most important is the crude alizarin (the
principal colouring matter of madder), made by the process discovered
almost simultaneously by Rochleder and Pernod. This is produced
by extracting either madder or garancin with hot water containing
sulphuric acid; about 5 grammes of acid are mixed with 1 kilogramme
of water and the madder boiled with this dilute acid in a lead-lined
vessel. The liquid separated from the solid residue becomes turbid on
cooling, and yellow flocks separate which consist of impure alizarin;
no further purification is required for practical purposes. Crude
alizarin is also obtained by treating madder with superheated steam.
These preparations are important, not only to the dyer and calico
printer, but also to the colour maker; from them the madder pigments
can be made in a very simple manner.
=The Constituents of Madder.=—Madder has been most thoroughly
investigated: its colouring matter, alizarin, is now made artificially.
In addition to woody fibre, madder contains sugar, mucilage, resin, a
glucoside (_i.e._, a substance which can be decomposed into a sugar and
another substance), and, in particular, two colouring matters known as
alizarin and purpurin.
Alizarin occurs ready formed in madder, and is also produced by the
decomposition of the madder glucoside into alizarin and sugar. To
obtain pure alizarin finely ground madder is extracted with boiling
water; on the addition of sulphuric acid to the decoction alizarin
separates together with other substances. The moist precipitate is
boiled with a solution of aluminium chloride, and, after filtering,
hydrochloric acid is added to the filtrate, when deep red flocks
separate, consisting of a mixture of alizarin and purpurin. The
colouring matters are further purified by dissolving in alcohol and
adding freshly precipitated alumina, which unites with them; the
alumina compounds are then boiled with strong soda solution, in which
the purpurin dissolves. The residue consists of aluminium alizarinate
mixed with resin, the latter is extracted with ether or benzene, and
the residue then decomposed with hydrochloric acid, when alizarin is
set free and is purified by recrystallisation.
Alizarin has the formula C₁₄H₆O₂(OH)₂. It forms fine red crystals
which are very little soluble in cold but more easily in hot water;
they dissolve readily in strong alcohol to a yellow solution. Alizarin
dissolves in alkaline liquids, the solution is dichroic; it appears
dark purple by transmitted light and pure blue by reflected light. When
the alkaline solution of alizarin is brought in contact with freshly
precipitated alumina the colouring matter is completely thrown down and
a beautiful red lake is formed.
The second madder colouring matter is purpurin. This is more soluble in
water than alizarin, it also dissolves in alum solutions; its alkaline
solutions are not dichroic. The formula of purpurin is C₁₄H₅O₂(OH)₃.
In addition to these two colouring matters, madder contains a third,
known as rubiacin, which also forms red compounds. It appears to be
fixed upon fabrics dyed with madder in company with the other colouring
matters.
Dyeing and printing with the colouring matters of madder are among the
most difficult processes in dyeing; a long series of operations is
required to fix the colour permanently on the fabric. The beautiful
deep red colour known as Turkey red is produced by madder; it is
distinguished from other vegetable colours by its great fastness. By
means of madder various other shades besides pure red may be produced.
This very valuable substance is largely used in dyeing and calico
printing to produce permanent colours.