T. E. (Thomas Edward) Thorpe.

A dictionary of applied chemistry online

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The tannic acid may then serve for the fixation
either of metallic mordants or of basic dyestuffs
upon the fibre.

The vat dyes for cotton are now assuming a
very prominent position, since, by their use,
extremely fast colours are obtained.

The so-caUed ingrain dyes are used almost
exclusively on cotton. The term is applied to
designate a class of dyes which are produced in
situ on the fibre by precipitation.

Mercerised cotton is virtually a distinct
textile fibre ; and whilst its dyeing properties are
essentially the same as those of ordinary cotton,
its treatment requires special precautions on
account of its great affinity for the direct



The dyeing of linen. The behaviour of linen
towards the respective classes of dyestuffs is
similar to that of cotton, but it is more difficult
to dye. The inferior dyeing affinity of linen is
probably due to the different physical structure
of the fibre.

The dyeing ol jute. Although a cellulose
derivative, jute differs from cotton and linen,
both in its chemical composition and properties.
As regards dyeing processes, it behaves, to a
certain extent, like tannin-mordanted cotton,
and may be dyed not only with the ordinary
direct, but with the basic colouring matters with-
out the aid of a mordant.

The dyeing Ol leather. Leather resembles
wool in dyeing properties, but the difficulty of
dyeing it in fast coloiu:s is increased by the
necessity of using lukewarm liquors only, since
boiling water destroys ordinary leather. Certain
of the newer methods of producing leather, e.g.
chrome tannage yield a product which ^vill
withstand boiling solutions. It is very im-



portant as regards the permanence of the
leather (when used e.g'. for bookbinding or
furniture covering) that no mineral acid should
be employed in the dyeing process. As a
substitute, formic acid may be recommended.
The acid and basic colouring matters are chiefly
employed, the mordant dyes being used to a
less extent.

The dyeing of feathers. The chief cause of
difficulty in the dyeing of feathers is their
natural water-resisting property. This is due
partially to their ultimate physical structure,
and partially to the natural grease present,
The midrib is specially difficult to dye. Apart
from these special difficulties, feathers behave
in dyeing, Uke wool, and may be dyed with acid,
basic, d^ect, or mordant dyes. Beds, browns,
blues, and other colours are usually dyed with
acid dyes, but the most satisfactory blacks are
obtained by successive and repeated treatments
with logwood extract, and ferrous sulphate with
potassium binoxalate. Dyeing must proceed for
a considerable period at the boil, in order to
ensure penetration.

The dyeing of horn. Horn is dyed in strong
solutions of acid dyes by steeping for a long
period in the cold or slightly warm baths, which
are acidified with acetic or formic acid. Since
it contains sulphur, horn may be dyed brown
by steeping in a solution of sodium plumbite,
which results in the formation of lead sulphide.
Black is also produced by alternate steepings
in solutions of mercuric nitrate and potassium
sulphide.

The dyeing of ivory. Both acid and basic
dyes may be employed in the dyeing of ivory,
the former being used with addition of acetic
or formic acid in hot solutions. Alcoholic solu-
tions of colouring matters may also be em-
ployed. Red is frequently dyed by the cochineal
single-bath process used for wool.

Black is produced either by the logwood and
iron or aniline black process, as used in cotton
dyeing, or by means of silver nitrate.

The dyeing of wood. The method of dyeing
wood in bulk is to boil or steam it to remove the
resins, and then treat it in an apparatus similar
to that employed in creosoting. The wood is
placed in a closed vessel which is exhausted
by a vacuum pump, and when the air has been
sucked out from the pores as completely as
possible, the hot dye Uquor is admitted into
the vessel and put under pressure by a force
pump. The direct and basic dyes may be em-
ployed. Wood is usually dyed black by alter-
nate treatments with logwood extract and
sodium bichromate solution.

The dyeing ol straw. Straw is largely dyed
for use in the manufacture of hats and baskets.
There are two commercial varieties, distinguished
as chip and straw. Chip is readily dyed by means
of the direct cotton dyes or the sulptdde dyes, and
many basic and acid dyes in neutral solution
may also be employed. The process must be
continued for 2 to 3 hours at the boil.

' Straw ' is more difficult to dye than ' chip,'
on account of the highly sihcious character of
the surface. The straw should be steeped for
several hours in warm water before dyeing.
Blacks are usually obtained with logwood aiid
iron, but for oolourd the direct cotton and the
basic dyes are chiefly used.



DYEING.



285



Materials used in Dyeing Opekations.

A great variety of chemical substances are
used in dyeing processes — acids, alkalis, oxidis-
ing and reducing agents, metallic saJts, and a
wide range of organic bodies. Two groups only
need be here considered, viz. : (o) mordants, and
(5) colouring matters.

Jloidante. The function of a mordant is to
chemically combine with the dyestuff employed,
and fix the latter upon the fibre. It has already
been mentioned that many colouring matters do
not require the aid of mordants, but dye direct
from their solutions. This applies to the acid,
direct, hasic, and vat dyes when used for wool
and silk, and the direct and vat dyes when used
for cotton. On the other hand, the mordant
and acid-mordant dyes (as their name implies)
demand the use of mordants on wool, as do the
mordant and the hasic dyes on cotton.

It is evident that the chemical properties of
the dyestufE will govern the character of the
mordant required for its fixation, and two types
of mordants may thus be distinguished : (1)
the metallic or hasic mordants, used for the
mordant dyes; and (2) the acid mordants, re-
quired in applying the hasic dyes to cotton or
other vegetable fibre.

Basic mordants are employed for fixing on
animal and vegetable fibres those colouring
matters which are of an acid character, i.e.
either true acids or bodies containing hydroxyl-
groups, of which the hydrogen is replaceable by
metals. They are metallic salts, of which the
most important are certain compounds of
chromium, aluminium, iron, and tin.

Wool is mordanted by boiling it with dilute
solutions of these metallic salts, usually with the
addition of certain acids or acid salts, e.g. cream
of tartar, sulphuric acid, formic acid, oxalic acid,
&o. During this operation, the wool assists in
the dissociation of the metallio salt, an insoluble
basic salt being fixed within and upon the fibre,
while a more acid salt remains in solution. The
acids or acid salts, which are added along with
^the metallio salt, possibly assist the dissociation
of the latter by forming more sensitive salts
(tartrates, oxalates, &c.), and may be con-
veniently termed assistants.

As a rule, the mordanting operation precedes
that of dyeing ; not infrequently, however, the
two operations take place simultaneously, and
the dyeing is thus effected by the so-called
' single-bath ' or ' one-dip ' method, as, for
example, in producing cochineal scarlet on wool.
In such cases, the wool is boiled in a solution con-
taining colouring matter, metaUic salt, and as-
sistant, in certain accurately determined relative
proportions. Combination takes place between
the colouring matter and the mordant, but the
pigment which would otherwise be precipitated,
is retained in solution by the liberated acid of
the mordant or the assistant present, and is
only gradually withdrawn from solution by
the filare, which thus becomes dyed.

In the case of the ' acid-mordant ' dye-
stufis, the application of the mordant follows
that of the colouring matter.

Silk is mordanted in the same manner as
wool, by boiling with metaUic salt solutions ; or,
more usually, it is steeped for about 12 hours
in cold solutions of the mordants previously



rendered more or less basic and sensitive. By
subsequently washing with (calcareous) water,
the metallio salt absorbed by the sUk is dis-
sociated, and an insoluble basic salt is pre-
oipitated within the fibre.

Cotton is mordanted with metallio salts by
processes very different from those adopted for
wool and sUk, since it has not the property of
causing the dissociation of the metallic salts when
boiled in their solutions. The methods adopted
also vary according to the form of the material,
i.e. whether it be cotton wool, yam, or cloth.

With calico, it is usual to impregnate the
fabric with solutions of metallio acetates, then
to dry it, and expose for several hours to a moist
warm atmosphere. During this so-called ' age-
ing ' process, dissociation of the salt takes place,
acetic acid escapes, and a more or less basic salt
remains fixed upon the fibre.

Sometimes the ' ageing ' is replaced by a
' steaming process,' in which the fabric is exposed
from J to 1 hour to the action of ■ high or low
pressure steam in a closed chamber. This method
is adopted by the printer of textile fabrics in the
so-called ' steam style,' in which a mixture of
colouring matter and metallio salt (usually
acetate) is printed upon the fabric, which is then
dried and steamed. The metallic salt is decom-
posed, a basic salt is precipitated upon the fibre,
at the same time combining with the colouring
matter and producing the coloured pigment.
This method is analogous to the single-bath
process of the woollen dyer.

Cotton may also be mordanted by impreg-
nating it with a metallio salt solution, drying,
and then passing through a solution of an
alkaline substance to neutralise the acid and
precipitate upon the fibre a basic salt or metaUic
oxide, e.g. ammonia, chalk, sodium carbonate,
&c. Solutions of an alkali salt, the acid of
which forms an insoluble compound with the
base of the mordant, e.g. sodium phosphate,
silicate, arsenate, &c., are also employed. All
substances used in this manner for fixing the
mordant upon the fibre are termed fixing agents.

In some cases of mordanting, the cotton is
impregnated with the fixing agent first {e.g.
tannic acid, sulphated oil, &c.), then dried and
passed tjirough the mordant solution afterwards,
as for example, in Turkey-red dyeing.

Special methods of mordanting cotton are
occasionally adopted, in accordance with the
properties of the metaUic salt employed. Stannic
oxide, for example, may be fixed on cotton by
passing the fabric through a solution of sodium
stannate, and then through dilute sulphuric acid,
and finaUy washing. Alumina is precipitated on
the fibre by the mere exposure to air of cotton
impregnated with sodium aluminate, in which
case atmospheric carbonic acid is the active
fixing agent.

The chromium mordants are of the first
importance, since, with different colouring
matters, they yield a considerable range of
colours which are remarkable for their per-
manence. The most important chromium
mordants are potassium and sodium dichro-
mates, and they are more largely used as
mordants in wool dyeing than any other metaUic
salts. In cotton dyeing they are not used as
mordants, but are frequently employed as oxidis-
ing agents, e.g. to develop the colour in dyeing



286



DYEING.



cateohu brown or aniline black, and are also
used in piodnoing chromate of lead yellow
and orange. Chromium fluoride (CrP3,4HjO)
is also used to a limited extent as » mordant
for wool. Other chromium salts employed as
mordants, chiefly for cotton, are chrome alum
and chromium acetate. Chrome alum is ob-
tained in large quantities as a by-product during
the manufacture of alizarin; chromium acetate
is prepared by mixing together solutions of
chrome alum and lead acetate.

Wool is mordanted with chromium by boiling
for l-lj hours with 3 p.o. potassium (or sodium)
dichromate. In many cases it is beneficial to
add 1 p.o. sulphuric acid (sp.gr. 1'84) or
1-5 p.c. hydrochloric acid (sp.gr. 1 •!()), or
6 p.c. tartaric acid, or 1 to 2 p.c. formic acid,
whereby fuller or brighter colours are ultimately
obtained. When potassium dichromate alone
is employed, partial decomposition of the
salt occurs, accompanied by some reduction,
the wool absorbing some chromic acid and
chromium chromate as well as potassium di-
chromate. When an addition of sjilphurio
acid is made, the wool absorbs only chromic
acid. In both cases the mordanted wool
has a yeUow or bufl colour, and only in the
dye-bath does the teduction to the real mor-
dantihg body, chromic oxide, take place within
the fibre through the interaction of the colouring
matter. When formic or tartaric acid is the
assistant employed, the mordanted wool has a
green colour, reduction of the chromic acid
taking place already in the mordanting bath,
and chromic hydrate being deposited wimin the
wool. If large amounts of potassium dichromate
are employed in mordanting, especially if sul-
phuric acid is also added to the bath, bad results
may be obtained through the colouring matter
being oxidised and destroyed by the excess of
chromic acid present in the wool. This defect
is known as ' over-chroming.' In certain
cases, potassium dichromate may be usefuUy
applied in the same bath with the colouring
matter {e.g. with alizarin), or by an after pro-
cess {e.g. in the case of camwood).

Silk is not usually mordanted with chromium.
It becomes mordanted, however, if boiled with a
solution of potassium dichromate and tartaric
acid after the manner of wool, but not if potas-
sium dichromate alone, or with the addition
of Bulphtiric acid, is employed.

Cotton may be mordanted with chromium
by impregnating it with a more or less concen-
trated solution of chrome alum, drying, and
passing through a boiling solution of sodium
carbonate (100 grams per litre) or caustic soda.
Better results are obtained by the use of
'chromium mordant G. A. I.' or 'G. A. II.'
(M. Ii. & B.), which are chromium chromates.
Other satisfactory mordants for cotton are
chromium bisulphite (Knecht) and sodium
ohromite (Koechlin). A method, in vogue with
the printer is to impregnate (pad) or print the
cotton with a mixture of colouring matter and
chromium acetate, dry and steam (' steam
colours ').

The chief aluminium mordants employed for
wool are aluminium sulphate and alum. The
best results are usually obtained by merely
boiling the wool for 1-lJ hours with a' solution of
8-10 p.o. of the aluminium salt, with the addition



of cream of tartar, in the proportion of 3 mols.
to 1 mol. aluminium sulphate. Aluminium
tartrate also gives excellent results, but it is
cheaper to allow this salt to be formed in the
mordanting bath itself in the manner described.
With some colouring matters, oxalic acid or
potassium binoxalate are to be preferred as
assistants to cream of tartar.

SilJe is mordanted , 'by boiling with dilute
aluminium sulphate solution, or by steeping 24
hours ia a concentrated solution, and then
washing in water, preferably calcareous.

The methods of mordanting cotton with
aluminium are various. It may be impregnated
with a more or less concentrated solution of
alum or aluminium sulphate, dried, and then
passed for a few minutes into a hot solution of
sodium phosphate, arsenate, or silicate or am-
monium carbonate. Or the cotton may be
impregnated with a cold solution of neutralised
sulphated oil, or of tannic acid, then dried, and
steeped in a solution of aluminium acetate or
basic aluminium sulphate. Another method
consists in impregnating the cotton with a solu-
tion of sodium aluminate, drying and exposing
to the air, or passing through a solution of am-
monium chloride. Normal and basic aluminium
acetates are also employed, the cotton being
impregnated with their solutions, then dried)
and exposed to a moist warm atmosphere
(' ageing '), and finally passed through a hot
solution of, e.g., sodium arsenate. The alumin-
ium acetates employed are prepared by dis-
solving alumina hydrate in acetic acid, or by
decomposing a solution of aluminium sulphate
with calcium or lead acetate (' red hquor ').
As a general rule, the more basic the aluminium
salts are, the more readily do their solutions
precipitate by heating or on dilution with water,
and the larger is the quantity of alumina they
yield to the fibre when used as mordants.

The chief iron mordants employed are ferrous
sulphate and acetate, and ferric sulphate, ace-
tate, and nitrate.

The ferrous acetate employed is made by dis-
solving scrap iron in pyroligneous acid. The
' pyroUgnite of iron ' or ' black iron hquor ' thus
obtained is preferable to that made by the
mutual decomposition of ferrous sulphate and
lead acetate solutions ; it is less liable to oxidise
because of the presence of tarry matter and
reducing agents, e.g. pyrocatechol,; &c. Ferric
sulphate is prepared by heating a mixture of
definite proportions of ferrous sulphate, sul-
phuric acid, and nitric acid. Although the
last-mentioned substance is only used as an
oxidising agent, the final preparation is generally
termed oy dyers ' nitrate of iron.' The so-
called ' pure nitrate of iron ' (ferric nitrate)
is prepared by dissolving scrap iron in nitric
acid.

Wool may be mordanted by boiling in a
solution of 7 p.o. ferric sulphate [Fej(S04)j] and
4 p.c. potassium binoxalate, but iron is now very
little used as a mordant for wool.

Silk may be mordanted by steeping for about
12 hours in a cold solution of pyroUgnite of
iron (sp.gr. 1-026), and afterwards washmg well.
The most usual mordant, employed largely in
black-silk dyeing, is the basic ferric sulphate
Fe4(S04)5(OH)a. The silk is steeped in a some-
what concentrated solution, of this salt (sp.gr.



DYEING.



287



1-25) for about 1 hour, then squeezed and well
washed. The operations are repeated seven or
eight times, after which the silk is boiled in an
old soap-bath and finally washed. In this pro-
cess, the very sensitive basic ferric sulphate ab-
sorbed by the fibre is decomposed during the
several washings, a still more basic and insoluble
salt being precipitated within the fibre.

Cotton is usually mordanted with iron by
first impregnating it with a cold solution of tan-
nin matter {e.g. myrobolans), and' then, with or
without drying, passing into a dilute solution of
ferrous acetate, ferric sulphate, or nitrate (sp.gr.
1-0I-1-O2). The cotton may alternatively he
impregnated with ferrous or ferric acetate, ^ied,
and exposed to a moist warm atmosphere (' age-
ing '), and finally fixed by passing through a hot
solution of, e.g., sodium phosphate.

Tin mordants are now rarely used, but it
required the stannous salts are employed for
umol and the stannic salts for cotton. The chief
stannous mordant is stannous chloride or ' tin
crystals.' It is frequently sold as a somewhat
acid solution under the names of ' single muriate
of tin ' (sp.gr. 1 -3), or ' double muriate of
tin ' (sp.gr. 1-6). The general name ' tin spirits '
is given to a variety of solutions of tin in mixtures
of hydrochloric, sulphuric and nitric acids.

Wool is mordanted by boiUng in a solution
containing 6 p.c. of the weight of wool of stan-
nous chloride, and 6-10 p.c. cream of tartar.
Large proportions of stannous chloride should
be strictly avoided, otherwise the dyed wool feels
harsh and its general properties are impaired.

Silk may be mordanted in the same manner
as wool, or steeped several hours in a more
or less concentrated solution of stannic chloride,
and then well washed.

Cotton is mordanted with stannic oxide by
impregnating the cotton with cold tannin solution
(e.^. sumach) for 3 hours, then squeezing, and,
with or usually without drying, steeping 1
hour in a dilute solution of stannic chloride
(sp.gr. 1-02), and washing. Cotton thus mor-
danted was much used formerly for fixing
mordant dyes — peachwood, logwood, &o., in
which case the stannic oxide constituted the
mordant, the tannic acid acting as fixing agent.
Now that the basic coal-tar dyes are so much
employed in dyeing cotton, although the same
method of mordanting is employed, the rdles of
the tin salt and tannic acid are reversed, for it
is really the tannic acid which is the essential
mordant, while the stannic chloride serves only
as the fixing agent.

Acid moidants are employed for fixing basic
colouring matters on cotton. They are not used
for this purpose in wool and silk dyeing, since
these fibres themselves act as acid mordants,
and are dyed directly without the aid of any
external mordant.

Tannic acid is the mordant par excellence
for fixing the basic coal-tar colours on cotton.
This fibre absorbs tannic acid from its cold solu-
tion, and if, when thus prepared, it is immersed
in a solution of a basic colouring matter, the
tannic acid combines with the colour base, a
coloured lake is formed within the substance of
the fibre, and the cotton becomes dyed. During
the dyeing process, the acid originally in com-
bination with the colour base is displaced, and
flince in the fr^P state it interferes with the com-



plete formation of the colour lake, it is found
beneficial to add a small proportion of sodium
carbonate or acetate to the dye-bath. Since the
tannates of colour bases are somewhat soluble, in
excess of tannic acid or in alkaline solutions, it
is necessary in practice to fix the tannic acid
on the fibre, previous to dyeing, in the form of
an insoluble metallic tannate. This is done by
passing the tannic acid prepared cotton through
a hot or cold solution of tartar emetic or stannic
chloride ; tannate of antimony or tin is thus
fixed on the fibre, and these salts have as great
an attraction for the colour base as free tannic
acid has. Cotton thus mordanted and dyed is
probably permeated with a very insoluble double
tannate of antimony or tin and colour base, a
triple compound which is insoluble in alkaline
solutions, and therefore fast to washing with
soap, &c.

The tannin mordants, chiefly used by the
dyer, are gall nuts and sumach, for light colours ;
and myrobolans, divi-divi, quebracho, and
chestnut extract, for dark colours.

As already pointed out, tannic acid is some-
times used as a fixing agent for tin, iron, or
aluminium mordants, and since its iron com-
pound is of a bluish-black colour, it sometimes
serves as a useful colouring matter.

Tannic acid is not used as a mordant for
wool, but for silk it is largely employed, partly
for adding weight to the silk, and partly for the
purpose of dyeing it black in conjunction with
iron mordants. Its utility as a weighting agent
is owing to the fact that under suitable eon-
ditions silk absorbs as much as 25 p.c. of its own
weight of tannic acid from a hot solution,
whereby the fibre gains not only in weight, but
also in volume.

Fatty acids are used as mordants very much
in the same way as tannic acid for fixing
the basic coal-tar colours on cotton. They are,
however, not attracted by cotton from their
solutions, but they form insoluble lakes with
the colour bases. The method of procedure
is to impregnate the cotton with a more or less
concentrated solution of their alkali salts, e.g.
soap ; after drjring, the fabric is passed through, or
preferably steeped in, a cold solution of aluminium
sulphate, and washed. Although the fatty acid
is thus fixed on the fibre as an insoluble alu-
minium soap, it is still capable of combining with
the colour base when the cotton is passed into
a solution of a basic colouring matter. Colours
dyed by this method are brighter than, but not
so fast to washing as those fixed by means of
tannic acid.

Instead of ordinary soap, a more con-
venient fatty-acid mordant to employ is the
so-called ' sulphated oil.' This is prepared by
mixing two parts by weight of castor oil with
about one part of sulphuric acid (sp.gr. 1 -84), and
allowing the mixture to stand for 24 hours. It
is then washed with a strong solution of common
salt to free it from excess of acid, and is finally
neutralised with ammonia or caustic soda. The
product of the action of the acid upon the oil
is somewhat complex, but its practical utility
depends upon the fact that it behaves like a
fatty acid and gives an extremely soluble soap
with alkalis. Ordinary castor-oil soap pro-
duced by boiling the oil with caustic soda is
also very soluble, and may be used as a mordant.



288



DYEING.



Sulphated oil is frequently called ' Turkey-
red oil,' ' alizarin oil,' and ' soluble oil,'



Online LibraryT. E. (Thomas Edward) ThorpeA dictionary of applied chemistry → online text (page 68 of 183)