The phenomena of dyeing have been ascribed to very different causes; by
some they were supposed to depend upon mechanical causes, and by others
upon the forces from which chemical effects flow. Hellot, in conformity
with the first mode of explanation, thought that the art of dyeing
consisted essentially in opening the pores in order to admit colouring
matters into them, and to fix them there by cooling, or by means of a
mordant imagined to act like a cement.
Dufay in 1737, Bergmann in 1776, Macquer in 1778, and Berthollet in
1790, had recourse to chemical affinities, to explain the fixation of
the colouring principles upon stuffs, either without an intermedium,
like indigo, walnut peels, annotto; or by the intervention of an acid, a
salifiable base, or a salt, which were called mordants. When bodies
present phenomena which we refer to an attraction uniting particles of
the same nature, whether simple or compound, to form an aggregate, or to
an affinity which unites the particles of different natures to form them
into a chemical compound, these bodies are in apparent contact. This
happens precisely in all the cases of the mutual action of bodies in an
operation of dyeing; if their particles were not in apparent contact,
there would be absolutely no change in their respective condition. When
we see stuffs and metallic oxides in apparent contact, form a mutual
union of greater or less force, we cannot therefore help referring it to
affinity. We do not know how many dyes may be fixed upon the same piece
of cloth; but in the operations of the dye-house sufficiently complex
compounds are formed, since they are always stuffs, composed of three or
four elements, which are combined with at least binary acid or basic
compounds; with simple salts compounded themselves of two immediate
principles at least binary; with double salts composed of two simple
salts; and finally with organic dye-stuffs containing three or four
elements. We may add that different species belonging to one of these
classes, and different species belonging to different classes, may unite
simultaneously with one stuff. The union of stuffs with colouring
matters appears, in general, not to take place in definite proportions;
though there are probably some exceptions.
We may conclude this head by remarking, that, besides the stuff and the
colouring matter, it is not necessary, in dyeing, to distinguish a third
body, under the name of _mordant_; for the idea of mordant does not rest
upon any definite fact; the body to which this name has been given being
essentially only one of the immediate principles of the coloured
combination which we wish to fix upon the stuff.
10. Division. Of dyeing in its relation with caloric, mechanics,
hydraulics, pneumatics, and optics.
Dyeing baths, or coppers, are heated directly by a furnace, or by means
of steam conducted in a pipe from a boiler at a certain distance from
the bath. In the first case, the vessels are almost always made of
copper; only, in special cases, for the scarlet and some delicate silk
dyes, of tin; in the second case, they are of copper, iron, or wood. A
direct fire is more economical than heating steam pipes, where there is
only one or two baths to heat, or where the labours are often suspended.
Madder and indigo vats, when heated by steam, have it either admitted
directly into the liquor, or made to circulate through pipes plunged
into it, or between the copper and an exterior iron or wood case. See
the end of this article.
Every thing else being equal, dyeing with heat presents fewer
difficulties towards obtaining an evenly colour, than dyeing in the
cold; the reason of which may be found in the following facts:--The air
adhering to the surface of stuffs, and that interposed between the
fibres of their constituent yarns, is more easily extricated in a hot
bath than a cold one, and thus allows the dye liquor to penetrate more
easily into their interior: in the second place, the currents which take
place in a hot bath, and which tend incessantly to render its contents
uniform, by renewing continually the strata of liquid in contact with
the stuff, contribute mainly to render the dyeing evenly. In cold
dyeing, it is necessary to stir up the bath from time to time; and when
goods are first put in, they must be carefully dipped, then taken out,
pressed, and wrung, several times in succession till they be uniformly
moistened.
The mechanical relations are to be found in the apparatus employed for
wincing, siring, and pressing the goods, as we have described under
CALICO PRINTING and BANDANNA. The hydraulic relations refer to the
wash-wheels and other similar apparatus, of which an account is given
under the same articles. The optical relations have been already
considered. In the sequel of this article an automatic dyeing vat will
be described.
The extracts of solutions of native dye-stuffs may be divided into two
classes, in reference to their habitudes with the oxygen of the
atmosphere; such as continue essentially unaltered in the air, and such
as suffer oxidation, and thereby precipitate a determinate colouring
matter. The dyes contained in the watery infusions of the different
vegetable and animal substances which do not belong to the second class,
are feebly attached to their solvents, and quit them readily for any
other bodies that possess an attraction for them. On this principle, a
decoction of cochineal, logwood, brazil wood, or a solution of sulphate
of indigo, by digestion with powdered bone black, lose their colour, in
consequence of the colouring particles combining by a kind of capillary
attraction with the porous carbon, without undergoing any change. The
same thing happens when well-scoured wool is steeped in such coloured
liquids; and the colours which the wool assumes by its attraction for
the dye, is, with regard to most of the above coloured solutions, but
feeble and fugitive, since the dye may be again abstracted by copious
washing with simple water, whose attractive force therefore overcomes
that of the wool. The aid of a high temperature, indeed, is requisite
for the abstraction of the colour from the wool and the bone-black,
probably by enlarging the size of the pores, and increasing the solvent
power of the water.
Those dye-baths, on the contrary, whose colouring matter is of the
nature of extractive or apothème, form a faster combination with stuffs.
Thus the yellow, fawn, and brown dyes, which contain tannin and
extractive, become oxygenated by contact of air, and insoluble in water;
by which means they can impart a durable dye. When wool is impregnated
with decoctions of that kind, its pores get charged by capillarity, and
when the liquid becomes oxygenated, they remain filled with a colour now
become insoluble in water. A similar change to insolubility ensues when
the yellow liquor of the indigo vat gets oxidized in the pores of cotton
and wool, into which it had been introduced in a fluid state. The same
change occurs when protosulphate of iron is converted into persulphate,
with the deposition of an insoluble peroxide in the substance of the
stuff. The change here effected by oxidation can, in other
circumstances, be produced by acids which have the power of
precipitating the dye-stuff in an insoluble state, as happens with
decoction of fustic.
Hence we perceive that the dyeing of fast colours rests upon the
principle, that the colours dissolved in the vat, during their union
with the stuff, should suffer such a change as to become insoluble in
their former menstruum. The more this dye, as altered in its union with
the stuff, can resist other menstrua or agents, the faster it will be.
This is the essential difference between dyeing and painting; or
applying a coat of pigment devoid of any true affinity for the surface.
If we mix a clear infusion of a dye with a small quantity of a solution
of an earthy or metallic salt, both in water, the limpid liquids soon
become turbid, and there gradually subsides sooner or later, according
to the nature of the mixture, a coloured precipitate, consisting of the
altered dye united with a basic or subsalt. In this compound the
colouring matter seems to act the part of an acid, which is saturated by
a small quantity of the basis, or in its acid relationship is feeble, so
that it can also combine with acids, being in reference to them a base.
The decomposition of a salt, as alum, by dyes, is effected principally
through the formation of an insoluble subsalt, with which the colour
combines, while a supersalt remains in the bath, and modifies, by its
solvent reaction, the shade of the dyed stuff. Dyed stuffs may be
considered as composed of the fibrous body intimately associated with
the colouring matter, the oxide, and acid, all three constituting a
compound salt. Many persons have erroneously imagined, that dyed goods
contained none of the acid employed in the dye bath; but they forget
that even potash added to alum does not throw down the pure earthy
basis, but a subsalt; and they should not ascribe to colouring matter a
power of decomposition at all approaching to that of an alkali. Salts,
containing strong acids, saturate a very large quantity of colouring
matter, in proportion to their place in the scale of chemical
equivalents. Mere bases, such as pure alumina, and pure oxide of tin,
have no power of precipitating colouring matter; when they seem to do
so, they always contain some acid.
Such salts, therefore, as have a tendency to pass readily into the basic
state, are peculiarly adapted to act as mordants in dyeing, and to form
coloured lakes. Magnesia affords as fine a white powder as alumina, and
answers equally well to dilute lakes, but its soluble salts cannot be
employed to form lakes, because they do not pass into the basic state.
This illustration is calculated to throw much light upon dyeing
processes in general.
The colour of the lake depends very much upon the nature of the acid,
and the basis of the precipitating salt. If it be white, like alumina
and oxide of tin, the lake will have, more or less, the colour of the
dye, but brightened by the reflection of white light from the basis;
while the difference of the acid occasions a difference in the hue. The
coloured bases impart more or less of their colour to the lakes, not
merely in virtue of their own tints, but of their chemical action upon
the dye.
Upon these principles a crimson precipitate is obtained from infusions
of cochineal by alum and salt of tin, which becomes scarlet by the
addition of tartar; by acetate of lead, a violet blue precipitate is
obtained, which is durable in the air; by muriate of lime, a pink brown
precipitate falls, which soon becomes black, and at last dirty green; by
the solution of a ferruginous salt, the precipitates are dark violet,
and black; and, in like manner, all other salts with earthy or metallic
bases, afford diversities of shade with cochineal. If this dye stuff be
dissolved in weak water of ammonia, and be precipitated with acetate of
lead, a green lake is obtained, which, after some time, will become
green on the surface by contact of air, but violet and blue beneath.
Hence it appears, that the shade of colour of a lake depends upon the
degree of oxidation or change of the colour caused by the acid of the
precipitating salt, upon the degree of oxidation or colour of the oxide
which enters into union with the dye, and upon its quantity in reference
to that of the colouring principle.
Such lakes are the difficultly soluble salts which constitute the dyeing
materials of stuffs. Their particles, however, for the purposes of
dyeing, must exist in a state of extremely fine division in the bath
liquor, in order that they may penetrate along with it into the minute
pores of textile fibres, and fill the cavities observed by means of the
microscope in the filaments of wool, silk, cotton, and flax. I have
examined these stuffs with an achromatic microscope, and find that when
they are properly dyed with fast colours, the interior of their tubular
texture is filled, or lined at least, with colouring matter. When the
bath contains the colouring particles, so finely divided that they can
pass through filtering paper, it is capable of dyeing; but if the
infusion mixed with its mordant be flocculent and ready to subside, it
is unfit for the purpose. In the latter case, the ingredients of the dye
have already become aggregated into compounds too coherent and too gross
for entering into combination with fibrous stuffs. Extractive matter and
tannin are particularly liable to a change of this kind, by the
prolonged action of heat in the bath. Hence also an alkaline solution of
a colouring matter, affords no useful dye bath, when mixed with the
solution of a salt having an earthy or metallic basis.
These circumstances, which are of frequent occurrence in the dye-house,
render it necessary always to have the laky matter in a somewhat soluble
condition, and to effect its precipitation within the pores of the
stuffs, by previously impregnating them with the saline solutions by the
aid of heat, which facilitates their introduction.
When a mordant is applied to any stuff, the portion of it remaining upon
the surface of the fibres should be removed; since, by its combination
with the colouring matter, it would be apt to form an external crust of
mere pigment, which would block up the pores, obstruct the entrance of
the dye into the interior, and also exhaust to no purpose the dyeing
power of the bath. For this reason the stuffs, after the application of
the mordant, are drained, squeezed, washed, and sometimes (particularly
with cotton and linen, in calico printing), even hard dried in a hot
stove.
The saline mordants, moreover, should not in general possess the
crystallizing property in any considerable degree, as this opposes their
affinity of composition for the cloth. On this account the deliquescent
acetates of iron and alumina are more ready to aid the dyeing of cotton
than copperas and alum.
Alum is the great mordant employed in wool dyeing. It is frequently
dissolved in water, holding tartar equal to one fourth the weight of the
alum in solution; by which addition its tendency to crystallize is
diminished, and the resulting colour is brightened. The alum and tartar
combine with the stuff without suffering any change, and are decomposed
only by the action of the colouring matters in the dye bath. The alum
operates solely in virtue of its sulphuric acid, and earthy basis; the
sulphate of potash present in that salt being rather injurious. Hence,
if a sulphate of alumina free from iron could be readily obtained, it
would prove a preferable mordant to alum. It is also probable, for the
reason above assigned, that soda alum, a salt much less apt to
crystallize than potash or ammonia alum, would suit the dyer very well.
In order to counteract the tendency of common alum to crystallize, and
to promote its tendency to pass into a basic salt, one eighth part of
its weight of potash is added to its solution, or the equivalent in
chalk or soda.
We shall conclude this account of the general principles of dyeing, with
Mr. Delaval’s observations on the nature of dyes, and a list of the
different substances used in dyeing, in reference to the colours
produced by them.
Sir Isaac Newton supposed coloured matters to reflect the rays of light;
some bodies reflecting the more, others the less, refrangible rays most
copiously; and this he conceived to be the true, and the only reason of
their colours. Mr. Delaval, however, proved in the 2d vol. of the
“Memoirs of the Philosophical and Literary Society of Manchester,” that,
“in transparent coloured substances, the colouring substance does not
reflect any light; and that when, by intercepting the light which was
transmitted, it is hindered from passing through substances, they do not
vary from their former colour to any other colour, but become entirely
black;” and he instances a considerable number of coloured liquors, none
of them endued with reflective powers, which, when seen by transmitted
light, appeared severally in their true colours; but all of them, when
seen by incident light, appeared black; which is also the case of black
cherries, black currants, black berries, &c., the juices of which
appeared red when spread on a white ground, or otherwise viewed by
transmitted instead of incident light; and he concludes, that bleached
linen, &c. “when dyed or painted with vegetable colours, do not differ
in their manner of acting on the rays of light, from natural vegetable
bodies; both yielding their colours by transmitting through the
transparent coloured matter, the light which is reflected from the white
ground:” it being apparent, from different experiments, “that no
reflecting power resides in any of their components, except in their
white matter only,” and that “transparent coloured substances, placed in
situations by which transmission of light through them is intercepted,
exhibit no colour, but become entirely black.”
The art of dyeing, therefore, (according to Mr. Delaval) “consists
principally in covering white substances, from which light is strongly
reflected, with transparent coloured media, which, according to their
several colours, transmit more or less copiously the rays reflected from
the white,” since “the transparent media themselves reflect no light;
and it is evident that if they yielded their colours by reflecting,
instead of transmitting the rays, the whiteness or colour of the ground
on which they are applied, would not in anywise alter or affect the
colours which they exhibit.”
But when any opaque basis is interposed, the reflection is doubtless
made by it, rather than by the substance of the dyed wool, silk, &c.,
and more especially when such basis consists of the white earth of alum,
or the white oxide of tin; which, by their strong reflective powers,
greatly augment the lustre of colours. There are, moreover, some opaque
colouring matters, particularly the acetous, and other solutions of
iron, used to stain linen, cotton, &c., which must necessarily
themselves reflect, instead of transmitting the light by which their
colours are made perceptible.
The compound or mixed colours, are such as result from the combination
of two differently coloured dye stuffs, or from dyeing stuffs with one
colour, and then with another. The simple colours of the dyer, are red,
yellow, blue, and black, with which, when skilfully blended, he can
produce every variety of tint. Perhaps the dun or fawn colour might be
added to the above, as it is directly obtained from a great many
vegetable substances.
1. Red with yellow, produces orange; a colour, which upon wool, is given
usually with the spent scarlet bath. To this shade may be referred flame
colour, pomegranate, capuchin, prawn, jonquil, _cassis_, chamois, _café
au lait_, aurora, marigold, orange peel, _mordorés_, cinnamon, gold, &c.
Snuff, chesnut, musk, and other shades are produced by substituting
walnut peels or sumach for bright yellow. If a little blue be added to
orange, an olive is obtained. The only direct orange dyes are annotto,
and subchromate of lead; see SILK and WOOL Dyeing.
2. Red with blue produces purple, violet, lilac, pigeon’s neck, mallow,
peach-blossom, _bleu de roi_, lint-blossom, amaranth.