Chapter II: Operations on Land (4)

In _wool_-dyeing they are applied along with other Mordant Colours on
chromium mordant for the production of a large variety of compound
shades, browns, drabs, greys, &c., the presence of acetic acid in the
dyebath being advantageous. When alum and tartar mordant is employed,
for the production of reds, it is necessary to add a small amount (4%)
of calcium acetate to the dyebath, in order to neutralize the strong
acidity of the mordanted wool, and to furnish the calcium of the
colour-lake fixed upon the fibre, which is regarded as an
aluminium-calcium compound of the colouring matter.

In _cotton_-dyeing the above colouring matters are chiefly used for
the production of so-called Turkey-red, a colour remarkable for its
brilliancy and its fastness to light and soap. These properties are
due to the preparation of the cotton with oil, in addition to the
ordinary mordanting and dyeing, whereby there is fixed on the fibre a
permanent and stable lake, in which aluminium and calcium are combined
with alizarin and some form of fatty oxy-acid. In the older processes
employed, the preparation of the cotton with oil was effected by
passing the material several times through emulsions of olive oil and
potassium carbonate solution; at a later date, and even now in the
case of cloth, the cotton is first impregnated with hot oil (Steiner's
process), then passed through solutions of alkali carbonate. After the
preparation with oil or oil-emulsions, the cotton is "stoved," i.e.
heated for several hours in special chambers or stoves to a
temperature of about 70 deg. C., during which operation the oil is
decomposed and oxidized and becomes indelibly attached to the fibre.
The oil-prepared cotton is steeped in cold solutions of basic
aluminium sulphate or acetate, washed, dyed with alizarin, and finally
boiled for several hours with soap solution under pressure in order to
brighten the colour. In the more recent and much more expeditious
"sulphated-oil process," castor oil is employed instead of olive oil,
and before use it is submitted to a treatment with sulphuric acid, the
sulphated oil thus obtained being finally more or less neutralized
with alkali. The cotton is impregnated with this sulphated-oil
solution, dried, mordanted with aluminium acetate, dyed, dried,
steamed and soaped. The operation of steaming plays an important part
in brightening and fixing the colour-lake on the fibre. In these and
all other Turkey-red processes, the oil, probably in the form of a
fatty oxy-acid, acts as a fixing agent for the aluminium and enters
into the composition of the red lake, imparting to it both brilliancy
and permanency.

_Alizarin S_ is a sulphonic acid derivative of alizarin, and since it
is much more soluble, it readily yields level colours. Silk is dyed in
a similar manner to wool, the fibre being mordanted by the ordinary
methods and then dyed in a separate bath.

_Diamine Fast Red_ is applied to cotton as a Direct Colour, with the
addition of soda or soap to the dyebath. By treating the dyed colour
with a solution of fluoride of chromium, its fastness to washing is
materially increased. Wool is dyed in a similar manner, sodium
sulphate being added to the dyebath, and the dyed colour treated with
fluoride of chromium or bichromate of potash. On wool, the colour is
so extremely fast to light and to milling that it may well serve as a
substitute for alizarin.

_Alizarin Orange_ is employed in the same manner as alizarin. In
wool-dyeing it is usually applied on chromium mordant for browns and a
variety of compound shades in combination with other Alizarin Colours
and dyewood extracts, less frequently on aluminium mordant.

_Galloflavin_ is used in wool and silk dyeing on chromium mordant as a
substitute for fustic and other yellow dyewoods, to furnish the yellow
part of compound shades.

The alizarin yellows, R and GG, anthracene yellow, diamond flavine,
chrome yellow, diamond yellow, carbazol yellow, chrysamine, &c., are
Direct Colours with mordant-dyeing properties. They also serve as
substitutes for fustic in wool or silk dyeing, and are dyed either on
a chromium mordant, or first in an acid bath and afterwards saddened
with bichromate of potash.

_Coerulein_ is employed in dyeing wool, silk or cotton with aluminium
or chromium mordants, either as a self-colour or for compound shades.
With aluminium mordant the colour is a moderately bright green, more
particularly on silk; with chromium mordant, an olive-green. Coerulein
S is the more soluble bisulphite compound of the ordinary coerulein.
It is applied in the same manner, care being taken, however, to dye
for some time (one hour) at a temperature not exceeding 60 deg. C.
until the bath is nearly exhausted, and then only raising the
temperature to the boiling point. Without this precaution coerulein S
is decomposed, and the ordinary insoluble coerulein is precipitated.
The colours obtained are very fast to light.

_Fast Green_, _Dioxine_ and _Gambine_ are chiefly of use in
calico-printing and in wool-dyeing. With iron mordant they yield
olive-greens, which on wool are extremely fast to light. Cotton is
impregnated with ferrous acetate, dried, aged and fixed with silicate
of soda, then dyed in a neutral bath. Wool is mordanted with ferrous
sulphate and tartar (3% of each) and dyed in a neutral bath.

_Acid Alizarin Green_, _Alizarin Cyanine Green_ and _Diamond Green_
all dye wool direct in a bath acidified with acetic or sulphuric acid,
and the dyed colour may be afterwards fixed or saddened with
bichromate of potash, or they may be dyed on chromium-mordanted wool.
The first method is very useful for pale shades, since the colours are
very level or regular.

_Alizarin Blue_ is a dark blue dyestuff which, owing to the fastness
of the colours it yields, has for many years been regarded as a worthy
substitute for indigo in wool-dyeing. It is applied in the same manner
as alizarin, the chromium mordant being alone employed. Alizarin blue
S is the soluble sodium bisulphite compound of alizarin blue; it
corresponds, therefore, to the above-mentioned coerulein S, and in its
application the same precautions as to the temperature of the dyebath
are necessary. The fastness of the dyed colours to light, milling and
acid satisfy the highest requirements.

_Alizarin Cyanine_, _Anthracene Blue_ and _Brilliant Alizarin Blue_
were discovered later than the above-mentioned alizarin blues, and,
owing to their greater solubility and other advantages, they have
largely replaced them as substitutes for indigo. They are dyed on
chromium-mordanted wool, silk or cotton, and yield dark purplish or
greenish blues, according to the particular brand employed. The
fastness of the dyed colours to light, and general durability, are
very satisfactory, but in fastness to milling and acids they are to
some extent inferior to alizarin blue.

_Celestine Blue_ and _Chrome Blue_ dye purplish blue and bright blue
respectively, and are dyed in the ordinary way upon a chromium
mordant. The colours they yield are inferior to the Alizarin Colours
in fastness to light, but on account of their clear shades they are
often used for brightening other colours.

_Brilliant Alizarin Cyanine_, _Alizarin Viridine_ and _Alizarin
Saphirole_ are true Alizarin Colours, and possess the same fastness to
light as other colours of this class. Unlike most of the Alizarin
Colours, they are capable of dyeing wool satisfactorily without the
aid of a metallic mordant--namely, with the addition of sulphuric acid
to the dyebath, in the same manner as the Acid Colours. If necessary,
the dyed colours may be treated with bichromate of potash. The colours
thus produced are very fast to light and very level, hence these
dyestuffs are valuable in the production of the most delicate compound
shades, such as drabs, slates, greys, &c., which are desired to be
fast to light. Alizarin saphirole dyes clear blue, the colour produced
being much more brilliant even than those of brilliant alizarin
cyanine.

_Gallein_, _Gallocyanine_, and especially _Chrome Violet_, dye
somewhat bright purple shades, and are hence frequently employed for
brightening other colours, but they are only moderately fast to light.
They are applied in the usual manner on a chromium mordant.

_Anthracene Brown_ is largely employed in the production of compound
shades. It dyes a dark, somewhat reddish, brown on chromium mordant,
the colour being very even and extremely fast to light.

_Alizarin Black_ is dyed on chromium mordant in the same manner as
alizarin, and is used as a self-colour or in combination with other
Alizarin Colours.

_Diamond Black_ is very useful for dyeing good blacks on wool, fast to
light and acids. The wool is first dyed with the addition of acetic
and finally sulphuric acid. When the dyebath is exhausted, bichromate
of potash (2%) is added, and boiling is continued for half an hour
longer.

The _erio chrome colours_ (black, brown, red, &c.) are applied in wool
dyeing like diamond black.

_Chromotrope_, of which there are several brands, is an Acid Colour
which is applied to wool in an acid bath in the usual manner. The red
or purple colours thus obtained are saddened in the same bath with
bichromate of potash and changed into black, the colouring matter
being oxidized and simultaneously combined with chromium.

MISCELLANEOUS COLOURS.--Under this head there may be arranged a few
dyestuffs which, although capable of inclusion under one or other of
the foregoing groups, it is more convenient to treat of separately.
Indigo, Aniline Black and Catechu, for example, might be placed in the
class of Developed Colours, since they are all developed on the fibre,
and indeed by the same method, namely, by oxidation.

_Indigo_ is one of our most important blue dyestuffs, which has been
employed from the earliest times. Indigo, being insoluble in water,
would be of no use in dyeing if it were not capable of being rendered
soluble. This is effected in two ways, corresponding to which there
are two methods of dyeing with indigo. One method consists in
dissolving the indigo in very strong sulphuric acid, whereby it is
converted into indigotin-disulphonic acid (Indigo Extract), which is
readily soluble in water. This substance belongs to the group of Acid
Colours; hence it is applied to the animal fibres, wool and silk, by
boiling in a solution of the colouring matter slightly acidified with
sulphuric acid. The second and most important method is based on the
fact that under the influence of reducing agents (i.e. substances
capable of yielding nascent hydrogen) indigo blue is changed into
indigo white, which is soluble in alkali, the solution thus obtained
being called a "vat." If textile materials are steeped in a clear
yellow solution of the reduced indigo and then exposed to air, the
indigo white absorbed by the fibre is oxidized and reconverted into
indigo blue within and upon the fibre, which thus becomes dyed blue;
this is the so-called "indigo-vat" method of dyeing. Comparing the two
methods, the "indigo-extract" method is only applicable to the animal
fibres, and although it gives brighter colours, they are fugitive to
light and are decolourized by washing with alkaline solutions; the
"vat method" is applicable to all fibres, and gives somewhat dull
blues, which are very fast to light, washing, &c.

_Cotton_ is dyed by means of the "_lime and copperas vat_," the "_zinc
powder vat_," or the "_hydrosulphite vat_." In the first-mentioned vat
the ingredients are quicklime, ferrous sulphate and finely ground
indigo; the lime decomposes the ferrous sulphate and precipitates
ferrous hydrate; this quickly reduces the indigo to indigo white,
which dissolves in the excess of lime present. The ingredients of the
zinc powder vat are zinc powder, lime and indigo; in the presence of
the lime and indigo the zinc takes up oxygen from the water,
liberating the hydrogen necessary to reduce the indigo, as in the
previous vat. The constituents of the hydrosulphite vat are
hydrosulphite of soda, lime and indigo. The requisite hydrosulphite of
soda is prepared by allowing zinc powder (13 lb.) to act upon a cold
concentrated solution of bisulphite of soda (17 gallons of sp. gr.
1.225), taking care to avoid, as much as possible, access of air and
any heating of the mixture, to prevent decomposition. The solution
thus obtained is thoroughly neutralized by the addition of lime; and
after settling, the clear liquor is used for the vat, along with
indigo and lime. Here again the hydrosulphite takes up oxygen from the
water and liberates the necessary hydrogen. It is found convenient to
prepare, in the first instance, a very concentrated standard of
reduced indigo, and to add as much of this to the dye-vat as may be
required, along with lime and a little hyposulphite of soda. The
advantages of this vat are that it is easily prepared and that there
is very little sediment; moreover, it can be employed in dyeing wool,
as well as cotton, and it is now very generally in use. The vat
usually employed for dyeing _wool_ is the so-called "woad vat," which
differs from the foregoing in that the hydrogen necessary to reduce
the indigo and bring it into solution is furnished, not by the action
of chemical agents, but by means of fermentation. The ingredients of
the woad vat are indigo, woad, bran, madder and lime. The woad here
employed is prepared by grinding the leaves of the woad plant (_Isatis
tinctoria_) to a paste, which is allowed to ferment and then partially
dried. It serves as the ferment to excite lactic and butyric
fermentation with the aid of the bran and madder, the necessary
hydrogen being thus evolved. Excessive fermentation is avoided by
making timely additions of lime; sluggish fermentation is accelerated
by additions of bran and slightly raising the temperature. When the
reduction and complete solution of the indigo is effected, the vat is
allowed to settle, and the woollen material is immersed and moved
about in the clear liquor for half an hour to two hours, according to
the shade required, then squeezed and exposed to the air in order to
develop the blue colour on the fibre.

_Thioindigo red_ is an artificial colouring matter belonging to the
indigo series and comes into the market in the form of a paste. It is
used in dyeing in exactly the same way as indigo, yielding shades
which range from a somewhat dull pink to a full claret shade of red.
The colours obtained are remarkable for their fastness.

_Indanthrene_. This colouring matter, which is also sold as a paste,
is an anthracene derivative, being formed by the action of caustic
potash on [beta]-amidoanthraquinone. It is reduced by hydrosulphite of
soda yielding a blue vat, in which cotton and other vegetable fibres
are dyed in the same way as in the indigo vat. Since a fair amount of
caustic soda is necessary for the setting of the vat, the dyestuff is
not suitable for animal fibres. Indanthrene yields on cotton reddish
shades of blue which are extremely fast to all external influences; in
fact the colour is so fast that when once fixed on cotton it cannot be
removed again from the fibre by any known means.

Other vat colours belonging to this series, which are similarly
applied, are flavanthrene (yellow), viridanthrene (green),
fuscanthrene (grey-brown), violanthrene (dull violet) and melanthrene
(grey to black). The _algol colours_ resemble the indanthrene colours
in their properties and application.

_Aniline Black_ differs from other dyes in that it is not sold as a
ready-made dyestuff, but is produced _in situ_ upon the fibre by the
oxidation of aniline. It is chiefly used for cotton, also for silk and
cotton-silk union fabrics, but seldom or not at all for wool. Properly
applied, this colour is one of the most permanent to light and other
influences with which we are acquainted. One method of dyeing cotton
is to work the material for about two hours in a cold solution
containing aniline (10 parts), hydrochloric acid (20 parts),
bichromate of potash (20 parts), sulphuric acid (20 parts), and
ferrous sulphate (10 parts). The ferrous sulphate here employed is
oxidized by the chromic acid to a ferric salt, which serves as a
carrier of oxygen to the aniline. This method of dyeing is easily
carried out, and it gives a good black; but since much of the
colouring matter is precipitated on the fibre superficially as well as
in the bath itself, the colour has the defect of rubbing off. Another
method is to impregnate the cotton with a solution containing aniline
hydrochloride (35 parts), neutralized with addition of a little
aniline oil, sodium chlorate (10 parts), ammonium chloride (10 parts).
Another mixture is 1.8 part aniline salt, 12 parts potassium
ferrocyanide, 200 parts water, 3.5 parts potassium chlorate dissolved
in water. After squeezing, the material is passed through a special
oxidation chamber, the air of which is heated to about 50 deg. C. and
also supplied with moisture. This oxidizing or ageing is continuous,
the material passing into the chamber at one end in a colourless
condition, and after about 20 minutes passing out again with the black
fully developed, a final treatment with hot chromic acid solution and
soaping being necessary to complete the process. In this method,
employing the first-mentioned solution, chlorate of copper is formed,
and this being a very unstable compound, readily decomposes, and the
aniline is oxidized by the liberated chlor-oxygen compounds. The
presence in the mixture of a metallic salt is very important in aiding
the development of the black, and for this purpose salts of vanadium,
cerium and copper have proved to be specially useful. The chemistry of
aniline black is still incomplete, but it would appear that there are
several oxidation products of aniline. The first product is so-called
emeraldine, a dark green substance of the nature of a salt, which by
treatment with alkali yields a dark blue base called azurine. The
further oxidation of emeraldine yields nigraniline, also a dark green
salt, but the free base of which has a violet black colour. The latter
becomes greenish under the influence of acids, especially sulphuric
acid, and this explains the defect known as "greening" which is
developed in ordinary aniline blacks during exposure to air. By a
supplementary oxidation with chromic acid such a black is rendered
ungreenable, the nigraniline being probably changed into the more
stable chromate of nigraniline.

_Catechu_ is a valuable brown dyestuff, obtained from various species
of _Acacia_, _Areca_ and _Uncaria_ growing in India. The wood, leaves
and fruit of these plants are extracted with boiling water; the
decoction is then evaporated to dryness or to a pasty consistency.
Catechu is largely used by the cotton dyer for the production of
brown, drab and similar colours. It is seldom employed for wool.
Cotton is usually dyed by boiling it for about one hour in a decoction
of catechu (100%) containing copper sulphate (5%). After squeezing,
the material is boiled for about fifteen minutes in a solution of
bichromate of potash (1/4 oz. per gal.), then washed and dried. By
repeating the operations two or three times deeper shades are
obtained. During the boiling with catechu the cotton attracts the
active principles catechin and catechu-tannic acid, but it thus
acquires only a pale brown colour; in the bichromate of potash,
however, these are oxidized to form insoluble japonic acid, which
permeates the fibre, and a deep brown colour is thus developed.
Catechu browns are fast to a variety of influences, e.g. washing,
alkalis, acids, &c., but less so to light. Catechu has been recently
much employed, in conjunction with copper sulphate, for dyeing the
so-called khaki-brown on woollen material for military clothing. On
silk, catechu is much used for weighting purposes in dyeing black.

MINERAL COLOURS.--Those include Chrome Yellow, Iron Buff, Prussian
Blue and Manganese Brown.

_Chrome Yellow_ is only useful in cotton-dyeing as a self-colour, or
for conversion into chrome orange, or, in conjunction with indigo, for
the production of fast green colours. The cotton is first impregnated
with a solution of lead acetate or nitrate, squeezed, and then passed
through a solution of sodium sulphate or lime water to fix the lead on
the fibre as sulphate or oxide of lead. The material is then passed
through a solution of bichromate of potash. The colour is changed to a
rich orange by a short, rapid passage through boiling milk of lime,
and at once washing with water, a basic chromate of lead being thus
produced. The colour is fast to light, but has the defect of being
blackened by sulphuretted hydrogen.

_Iron Buff_ is produced by impregnating the cotton with a solution of
ferrous sulphate, squeezing, passing into sodium hydrate or carbonate
solution, and finally exposing to air, or passing through a dilute
solution of bleaching powder. The colour obtained, which is virtually
oxide of iron, or iron-rust, is fast to light and washing, but is
readily removed by acids.

_Prussian Blue_ is applicable to wool, cotton and silk, but since the
introduction of coal-tar blues its employment has been very much
restricted. The colour is obtained on cotton by first dyeing an iron
buff, according to the method just described, and then passing the
dyed cotton into an acidified solution of potassium ferrocyanide, when
the blue is at once developed. A similar method is employed for silk.
Wool is dyed by heating it in a solution containing potassium
ferricyanide and sulphuric acid. The colour is developed gradually as
the temperature rises; it may be rendered brighter by the addition of
stannous chloride. On wool and silk Prussian blue is very fast to
light, but alkalis turn it brown (ferric oxide).

_Manganese brown_ or _bronze_ is applied in wool, silk and cotton
dyeing. The animal fibres are readily dyed by boiling with a solution
of potassium permanganate, which, being at first absorbed by the
fibre, is readily reduced to insoluble brown manganic hydrate. Since
caustic potash is generated from the permanganate and is liable to act
detrimentally on the fibre, it is advisable to add some magnesium
sulphate to the permanganate bath in order to counteract this effect.
Imitation furs are dyed in this manner on wool-plush, the tips or
other parts of the fibres being bleached by the application of
sulphurous acid. Cotton is dyed by first impregnating it with a
solution of manganous chloride, then dyeing and passing into a hot
solution of caustic soda. There is thus precipitated on the fibre
manganous hydrate, which by a short passage into a cold dilute
solution of bleaching powder is oxidized and converted into the brown
manganic hydrate. This manganese bronze or brown colour is very
susceptible to, and readily bleached by, reducing agents; hence when
exposed to the action of an atmosphere in which gas is freely burnt,
the colour is liable to be discharged, especially where the fabric is
most exposed. In other respects manganese bronze is a very fast
colour.

_Dyeing on a large Scale._--It is not possible to give here more than a bare outline of the methods which are used on the large scale for dyeing textile fibres, yarns and fabrics. In principle, dyeing is effected by allowing an aqueous[1] solution of the dye-stuff, with or without additions (alkalis, acids, salts, &c.), to act, usually at an elevated temperature, on the material to be dyed. During the process it is necessary, in order to ensure the uniform distribution of the dyestuff in the material, that the latter should either be moved more or less continuously in the dye liquor or that the dye liquor should be circulated through the material. The former mode of operation is in general use for hank, warp and piece dyeing, but for textile fibres in the loose condition or in the form of "slubbing," "sliver" or "cops" (see SPINNING) the latter method has, in consequence of the introduction of improved machinery, come more and more into vogue within recent years.

_Loose Material._--Cotton and wool are frequently dyed in the loose state, i.e. before being subjected to any mechanical treatment. The simplest method of effecting this is to treat the material in open vessels (boilers) which can be heated either by means of steam or direct fire. Since, however, a certain amount of felting or matting of the fibres cannot be avoided, it is frequently found to be more advantageous to effect these treatments in specially constructed apparatus in which the dye liquors are circulated through the material.

_Yarn._--Yarn may be dyed either in the hank, in the warp or in the cop, i.e. in the form in which the yarn leaves the spinning frame. The dyeing in the _hank_ is carried out in rectangular dye-vats constructed of wood or stone like that shown in fig. 1, in which the hanks are suspended from smooth wooden poles or rods resting on the sides, and are thus immersed almost entirely in the dye liquor. The heating of the vat is effected either by means of live steam, i.e. by blowing steam into the dye solution from a perforated pipe which runs along the bottom of the vat, or by means of a steam coil similarly situated. In order to expose the hanks as uniformly as possible to the action of the dye liquor, they are turned by hand at regular intervals until the operation is finished. Washing off is effected in the same or in a similar vessel, after which excess of water is removed by wringing by hand, through squeezing rollers or, what is generally preferred, in a hydro-extractor (centrifugal machine). The drying of the dyed and washed yarn is generally effected by suspending it on poles in steam-heated drying chambers. Yarn in the _warp_ is dyed in vats or "boxes" like that shown in fig. 2, through which it is caused to pass continuously. The warps to be dyed pass slowly up and down over the loose rollers in the first box B, then through squeezing rollers S into the next, and the same thing occurs in the second (also third and fourth in a four-box machine) box A, whence they are delivered through a second pair of squeezing rollers S1 into the wagon W. The boxes may contain the same or different liquors, according to the nature of the dyestuff employed. Washing is done in the same machine, while drying is effected on a cylinder drying machine like that shown in figs. 8 and 9 of BLEACHING. Latterly, machines have been introduced for dyeing warps on the beam, the dye liquor being caused to circulate through the material, and the system appears to be meeting with considerable success. Large quantities of yarn, especially cotton, are now dyed in the cop. When the dyed yarn is to be used as weft the main advantage of this method is at once apparent, inasmuch as the labour, time and waste of material incurred by reeling into hanks and then winding back into the compact form so as to fit into the shuttle are avoided. On the other hand the number of fast dyestuffs suitable for cop dyeing is very limited. In the original cop-dyeing machine constructed by Graemiger a thin tapering perforated metallic tube is inserted in the hollow of each cop. The cops are then attached to a perforated disk (which constitutes the lid of a chamber or box) by inserting the protruding ends of the tubes into the perforations. The chamber is now immersed in the dye-bath and the hot liquor is drawn through the cops by means of a centrifugal pump and returned continuously to the dye-bath. This principle, which is known as the skewer or spindle system, is the one on which most modern cop-dyeing machines are based. In the so-called "compact" system of cop dyeing the cops are packed as closely as possible in a box, the top and bottom (or the two opposite sides) of which are perforated, the interstices between the cops being filled up with loose cotton, ground cork or sand. The dye liquor is then drawn by suction or forced by pressure through the box, thus permeating and dyeing the cops.

_Pieces._--Plain shades are usually dyed in the piece, this being the most economical and at the same time the most expeditious means of obtaining the desired effect. The dyeing of piece goods may be effected by running them through the dye liquor either at full breadth or in rope form. The machine in most common use for the first method is the Lancashire "jigger," which is simple in principle and is shown in section in fig. 3. It consists essentially of a dye-vessel constructed of wood or cast iron and containing loose guide rollers, r and r, at the top and bottom. By coupling up the roller B with the driving gear the pieces which are batched on A are drawn through the dye liquor and rolled on to B. A band brake (not shown in the figure) applied to the axis of A gives the pieces the required amount of tension in passing through the dye-bath. As soon as the whole of the pieces have passed through in this way from A to B, the machine is reversed, and roller A draws them back again through the bath in a similar way on to roller A. This alternating process goes on until the dyeing is finished, when the goods are washed off, squeezed and dried. The jigger is especially useful in cotton piece dyeing, one great advantage being that it is suited for what is known as a "short bath," i.e. a bath containing a minimum amount of dye liquor, this being of great importance in the application of dyestuffs which do not exhaust well, like the direct colours and the sulphide colours. The padding machine is similar in principle to the jigger, the pieces running over loose guide rollers through the mordant or dye solution contained in a trough of suitable shape and size, but on leaving the machine they pass through a pair of squeezing rollers which uniformly express the excess of liquor and cause it to be returned to the bath. The padding machine is used more for preparing (mordanting, &c.) than for dyeing.

For the dyeing of pieces in rope form a so-called "dye-beck" is used, which is a machine of larger dimensions than the jigger. Across the dye-bath is attached a winch W (see fig. 4), by means of which the pieces, sewn together at the ends so as to form an endless band, are caused to circulate through the machine, being drawn up on the front side of the machine and allowed to drop back into the dye liquor on the other. This form of machine is particularly suited for the mordanting and dyeing of heavy goods. Washing off may be done in the same machine.

The drying of piece goods is done on steam-heated cylinders like those used for the drying of bleached goods (see BLEACHING).

The operations which precede dyeing vary according to the material to be dyed and the effects which it is desired to produce. Loose wool, woollen and worsted yarn and piece goods of the same material are almost invariably scoured (see BLEACHING) before dyeing in order to remove the oily or greasy impurities which would otherwise interfere with the penetration of the dye solution. Silk is subjected to the process of discharging or boiling off (see BLEACHING) in order to remove the silk gum or sericine. Cotton which is to be dyed in dark shades does not require any preparatory treatment, but for light or very bright shades it is bleached before dyeing. Wool and silk are seldom bleached before dyeing. Cotton, wool and union (cotton warp and worsted weft) fabrics are frequently singed (see BLEACHING) before dyeing. Worsted yarn, especially two-fold yarn, is very liable to curl and become entangled when scoured, and in order to avoid this it is necessary to stretch and "set" it. To this end it is stretched tight on a specially constructed frame, placed in boiling water, and then cooled. Similarly, union fabrics are liable to "cockle" when wetted, and although this defect may be put right in finishing, spots of water or raindrops will give an uneven appearance of a permanent character to the goods. To avoid this, the pieces are subjected previous to dyeing to the so-called "crabbing" process, in which they are drawn under great tension through boiling water and wound on to perforated hollow cylinders. Steam is then blown through the goods and they are allowed to cool.

Theory of dyeing.

With respect to the question of colour, we meet with two kinds of substances in nature, those which possess colour and those which do not. Why this difference? The physicist says the former are bodies which reflect all the coloured rays of the spectrum composing white light--if opaque, they appear white; if transparent, they are colourless. The latter are bodies which absorb some of the spectrum rays only, reflecting the remainder, and these together produce the impression of colour. A black substance is one which absorbs all the spectrum rays. The fundamental reason, however, of this difference of action on the part of substances towards light remains still unknown. All substances which possess colour are not necessarily dyestuffs, and the question may be again asked, Why? It is a remarkable circumstance that most of the dyestuffs at present employed occur among the so-called aromatic or benzene compounds derived from coal-tar, and a careful study of these has furnished a general explanation of the point in question, which briefly is, that the dyeing property of a substance depends upon its chemical constitution. Speaking generally, those colouring matters which have the simplest constitution are yellow, and as the molecular weight increases their colour passes into orange, red, violet and blue. In recent years chemists have begun to regard the constitution of nearly all dyestuffs as similar to that of Quinone, and some even believe that all coloured organic compounds have a quinonoid structure. According to O.N. Witt, a colourless hydrocarbon, e.g. benzene, becomes coloured by the introduction of one or more special groups of atoms, which he terms the colour-bearing or _chromophorous groups_, e.g. NO2, -- N:N --, &c. Benzene, for example, is colourless, whereas nitro-benzene and azo-benzene are yellow. Such compounds containing chromophorous groups are termed chromogens, because, although not dyestuffs themselves, they are capable of generating such by the further introduction of salt-forming atomic groups, e.g. OH, NH2. These Witt terms _auxochromous_ groups. In this way the chromogen _tri-nitro-benzene_, C6H3(NO2)3, becomes the dyestuff _tri-nitro-phenol_ (picric acid), C6H2(NO2)3(OH), and the chromogen _azo-benzene_, C6H5.N : N.C6H5, is changed into the dyestuff _amido-azo-benzene_ (Fast Yellow), C6H5.N : N.C6H4(NH2). These two dyestuffs are typical of a large number which possess either an acid or a basic character according as they contain hydroxyl (OH) or amido (NH2) groups, and correspond to the Acid Colours and Basic Colours to which reference has already been made. Other important atomic groups which frequently occur, in addition to the above, are the carboxyl (COOH) and the sulphonic acid (HSO3) groups; these either increase the solubility of the colouring matter or assist in causing it to be attracted by the fibre, &c. In many cases the free colour-acid or free colour-base has little colour, this being only developed in the salt. The free base rosaniline, for example, is colourless, whereas the salt magenta (rosaniline hydrochloride) has a deep crimson colour in solution. The free acid Alizarin is orange, while its alumina-salt is bright red. It may be here stated that the scientific classification of colouring matters into Nitro-colours, Azo-colours, &c., already alluded to, is based on their chemical constitution, or the chromophorous groups they contain, whereas the classification according to their mode of application is dependent upon the character and arrangement of the auxochromous groups. The question of the mordant-dyeing property of certain colouring matters containing (OH) and (COOH) groups has already been explained under the head of _Artificial Mordant Colours_.

The peculiar property characteristic of dyestuffs, as distinguished from mere colouring matters, namely, that of being readily attracted by the textile fibres, notably the animal fibres, appears then to be due to their more or less marked acid or basic character. Intimately connected with this is the fact that these fibres also exhibit partly basic and partly acid characters, due to the presence of carboxyl and amido groups. The behaviour of magenta is typical of the Basic Colours. As already indicated, rosaniline, the base of magenta, is colourless, and only becomes coloured by its union with an acid, and yet wool and silk can be as readily dyed with the colourless rosaniline (base) as with the magenta (salt). The explanation is that the base rosaniline has united with the fibre, which here plays the part of an acid, to form a coloured salt. It has also been proved that in dyeing the animal fibres with magenta (rosaniline hydrochloride), the fibre unites with the rosaniline only, and liberates the hydrochloric acid. Further, magenta will not dye cotton unless the fibre is previously prepared, e.g. with the mordant tannic acid, with which the base rosaniline unites to form an insoluble salt. In dyeing wool it is the fibre itself which acts as the mordant. In the case of the Acid Colours the explanation is similar. In many of these the free colour-acid has quite a different colour from that of the alkali-salt, and yet on dyeing wool or silk with the free colour-acid, the fibre exhibits the colour of the alkali-salt and not of the colour-acid. In this case the fibre evidently plays the part of a base. Another fact in favour of the view that the union between fibre and colouring matter is of a chemical nature, is that by altering the chemical constitution of the fibre its dyeing properties are also altered; oxycellulose and nitrocellulose, for example, have a greater attraction for Basic Colours than cellulose. Such facts and considerations as these have helped to establish the view that in the case of dyeing animal fibres with many colouring matters the operation is a _chemical_ process, and not merely a mechanical absorption of the dyestuff. A similar explanation does not suffice, however, in the case of dyeing cotton with the Direct Colours. These are attracted by cotton from their solutions as alkali salts, apparently without decomposition. The affinity existing between the fibre and colouring matter is somewhat feeble, for the latter can be removed from the dyed fibre by merely boiling with water. The depth of colour obtained in dyeing varies with the concentration of the colour solution, or with the amount of some neutral salt, e.g. sodium chloride, added as an assistant to the dye-bath; moreover, the dye-bath is not exhausted. The colouring matter is submitted to the action of two forces, the solvent power of the water and the affinity of the fibre, and divides itself between the fibre and the water. After dyeing for some time, a state of equilibrium is attained in which the colouring matter is divided between the fibre and the water in a given ratio, and prolonged dyeing does not intensify the dyed colour.

Some investigators hold the view that in some cases the fibres exert a purely physical attraction towards colouring matters, and that the latter are held in an unchanged state by the fibre. The phenomenon is regarded as one of purely mechanical surface-attraction, and is compared with that exercised by animal charcoal when employed in decolourizing a solution of some colouring matter. Some consider such direct dyeing as mere diffusion of the colouring matter into the fibre, and others that the colouring matter is in a state of "solid solution" in the fibre, similar to the solution of a metallic oxide in coloured glass. According to this latter view, the cause of the dyeing of textile fibres is similar to the attraction or solvent action exerted by ether when it withdraws colouring matter from an aqueous solution by agitation. Latterly the view has been advanced that dyeing is due to precipitation of the colloid dyestuffs by the colloid substance of the fibre.

In the case of colours which are dyed on mordants, the question is merely transferred to the nature of the attraction which exists between the fibre and the mordant, for it has been conclusively established that the union between the colouring matter and the mordant is essentially chemical in character.

From our present knowledge it will be seen that we are unable to give a final answer to the question of whether the dyeing process is to be regarded as a chemical or a mechanical process. There are arguments and facts which favour both views; but in the case of wool and silk dyeing, the prevailing opinion in most cases is in favour of the chemical theory, whereas in cotton-dyeing, the mechanical theory is widely accepted. Probably no single theory can explain satisfactorily the fundamental cause of attraction in all cases of dyeing, and further investigation is needed to answer fully this very difficult and abstruse question.

Conclusion.

The poisonous nature or otherwise of the coal-tar dyes has been frequently discussed, and the popular opinion, no doubt dating from the time when magenta and its derivatives were contaminated with arsenic, seems to be that they are for the most part really poisonous, and ought to be avoided for colouring materials worn next the skin, for articles of food, &c. It is satisfactory to know that most of the colours are not poisonous, but some few are--namely, Picric acid, Victoria Orange, Aurantia, Coralline, Metanil Yellow, Orange II. and Safranine. Many coal-tar colours have, indeed, been recommended as antiseptics or as medicinal remedies, e.g. Methyl Violet, Auramine and Methylene Blue, because of their special physiological action. In histology and bacteriology many coal-tar colours have rendered excellent service in staining microscopic preparations, and have enabled the investigator to detect differences of structure, &c., previously unsuspected. In photography many of the more fugitive colouring matters, e.g. Cyanine, Eosine, Quinoline Red, &c., are employed in the manufacture of ortho-chromatic plates, by means of which the colours of natural objects can be photographed in the same degrees of light and shade as they appear to the eye--blue, for example, appearing a darker grey, yellow, a lighter grey, in the printed photograph.

Since the year 1856, in which the first coal-tar colour, mauve, was discovered, the art of dyeing has made enormous advances, mainly in consequence of the continued introduction of coal-tar colours having the most varied properties and suitable for nearly every requirement. The old idea that the vegetable dyestuffs are superior in fastness to light is gradually being given up, and, if one may judge from the past, it seems evident that in the future there will come a time when all our dyestuffs will be prepared by artificial means.

AUTHORITIES.--Macquer, Hellot and le Pileur d'Apligny, _The Art of
Dyeing Wool, Silk and Cotton_ (London, 1789); Bancroft, _Philosophy of
Permanent Colours_ (2 vols., London, 1813); Berthollet-Ure, _Elements
of the Art of Dyeing_ (2 vols., London, 1824); Chevreul, _Recherches
chimiques sur la teinture_ (Paris, 1835-1861); O'Neill, _The Chemistry
of Calico Printing, Dyeing and Bleaching_ (Manchester, 1860);
_Dictionary of Calico Printing and Dyeing_ (London, 1862);
Schutzenberger, _Traite des matieres colorantes_ (2 vols., Paris,
1867); Bolley, _Die Spinnfasern und die im Pflanzen- und Thierkorper
vorkommenden Farbstoffe_ (1867); Crookes, _A Practical Handbook of
Dyeing and Calico-Printing_ (London, 1874); Jarmain, _Wool-Dyeing_
(1876); O'Neill, _Textile Colourist_ (4 vols., Manchester, 1876);
Calvert, _Dyeing and Calico Printing_ (Manchester, 1876); Moyret,
_Traite de la teinture des soies_ (Lyon, 1877); O'Neill, _The Practice
and Principles of Calico Printing, Bleaching and Dyeing_ (Manchester,
1878); Girardin, _Matieres textiles et matieres tinctoriales_ (Paris,
1880); Hummel, _The Dyeing of Textile Fabrics_ (London, 1885);
Sansone, _Dyeing_ (Manchester, 1888); Witt, _Chemische Technologie der
Gespinnstfasern_ (Brunswick, 1888); Benedikt and Knecht, _The
Chemistry of the Coal-Tar Colours_ (London, 1889); Hurst, _Silk
Dyeing, Printing and Finishing_ (London, 1892); Noelting and Lehne,
_Anilinschwarz_ (Berlin, 1892); Knecht, Rawson and Loewenthal, _Manual
of Dyeing_ (London, 1908); Steinbeck, _Bleichen und Farben der Seide
und Halbseide_ (Berlin, 1895); Gardner, _Wool-Dyeing_ (Manchester,
1896); Rawson, Gardner and Laycock, _A Dictionary of Dyes, Mordants,
&c._ (London, 1901); Gros-Renaud, _Les Mordants en teinture et en
impression_ (Paris, 1898); Georgievics, _The Chemical Technology of
Textile Fabrics_ (London, 1902); Paterson, _The Science of Colour
Mixing_ (London, 1900); Paterson, _Colour Matching on Textiles_
(London, 1901); Beech, _The Dyeing of Cotton Fabrics_ (London, 1901);
Beech, _The Dyeing of Woollen Fabrics_ (London, 1902); _The Journal of
the Society of Dyers and Colourists_ (Bradford, 1885-1908) and the
publications of the colour manufacturers. (J. J. H.; E. K.)

FOOTNOTE:

[1] The term "dry dyeing," which is carried out only to a very
limited extent, relates to the dyeing of fabrics with the dyestuff
dissolved in liquids other than water, e.g. benzene, alcohol, &c.

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