us try to discover what aniline is, and how formed. I pointed to benzene
or benzol in the table as a hydrocarbon, C_{6}H_{6}, which forms a
principal colour-producing constituent of coal-tar. If you desire to
produce chemical appetite in benzene, you must rob it of some of its
hydrogen. Thus C_{6}H_{5} is a group that would exist only for a moment,
since it has a great appetite for H, and we may say this appetite would
go the length of at once absorbing either one atom of H (hydrogen) or of
some similar substance or group having a similar appetite. Suppose, now,
I place some benzene, C_{6}H_{6}, in a flask, and add some nitric acid,
which, as we said, is NO_{2}OH. On warming the mixture we may say a
tendency springs up in that OH of the nitric acid to effect union with
an H of the C_{6}H_{6} (benzene) to form HOH (water), when an appetite
is at once left to the remainder, C_{6}H_{5}--on the one hand, and the
NO_{2}--on the other, satisfied by immediate union of these residues to
form a substance C_{6}H_{6}NO_{2}, nitro-benzene or "essence of
mirbane," smelling like bitter almonds. This is the first step in the
formation of aniline.
I think I have told you that if we treat zinc scraps with water and
vitriol, or water with potassium, we can rob that water of its oxygen
and set free the hydrogen. It is, however, a singular fact that if we
liberate a quantity of fresh hydrogen amongst our nitrobenzene
C_{6}H_{5}NO_{2}, that hydrogen tends to combine, or evinces an
ungovernable appetite for the O_{2} of that NO_{2} group, the tendency
being again to form water H_{2}O. This, of course, leaves the residual
C_{6}H_{5}N: group with an appetite, and only the excess of hydrogen
present to satisfy it. Accordingly hydrogen is taken up, and we get
C_{6}H_{5}NH_{2} formed, which is aniline. I told you that ammonia is
NH_{3}, and now in aniline we find an ammonia derivative, one atom of
hydrogen (H) being replaced by the group C_{6}H_{5}. I will now describe
the method of preparation of a small quantity of aniline, in order to
illustrate what I have tried to explain in theory. Benzene from coal-tar
is warmed with nitric acid in a flask. A strong action sets in, and on
adding water, the nitrobenzene settles down as a heavy oil, and the acid
water can be decanted off. After washing by decantation with water once
or twice, and shaking with some powdered marble to neutralise excess of
acid, the nitrobenzene is brought into contact with fresh hydrogen gas
by placing amongst it, instead of zinc, some tin, and instead of
vitriol, some hydrochloric acid (spirits of salt). To show you that
aniline is formed, I will now produce a violet colour with it, which
only aniline will give. This violet colour is produced by adding a very
small quantity of the aniline, together with some bleaching powder, to a
mixture of chalk and water, the chalk being added for the purpose of
destroying acidity. This aniline, C_{6}H_{5}NH_{2}, is a base, and forms
the foundation of all the so-called basic aniline colours. If I have
made myself clear so far, I shall be contented. It only remains to be
said that for making Magenta, pure aniline will not do, what is used
being a mixture of aniline, with an aniline a step higher, prepared from
toluene. If I were to give you the formula of Magenta you would be
astonished at its complexity and size, but I think now you will see that
it is really built up of aniline derivatives. Methyl Violet is a colour
we have already referred to, and its chemical structure is still more
complex, but it also is built up of aniline materials, and so is a basic
aniline colour. Now it is possible for the colour-maker to prepare a
very fine green dye from this beautiful violet (Methyl Violet). In fact
he may convert the violet into the green colour by heating the first
under pressure with a gas called methyl chloride (CH_{3}Cl). Methyl
Violet is constructed of aniline or substituted aniline groups; the
addition of CH_{3}Cl, then, gives us the Methyl Green. But one of the
misfortunes of Methyl Green is that if the fabric dyed with it be boiled
with water, at that temperature (212° F.) the colour is decomposed and
injured, for some of the methyl chloride in the compound is driven off.
In fact, by stronger heating we may drive off all the methyl chloride
and get the original Methyl Violet back again.
But we have coal-tar colours which are not basic, but rather of the
nature of acid,--a better term would be _phenolic_, or of the nature of
phenol or carbolic acid. Let us see what phenol or carbolic acid is. We
saw that water may be formulated HOH, and that benzene is C_{6}H_{6}.
Well, carbolic acid or phenol is a derivative of water, or a derivative
of benzene, just as you like, and it is formulated C_{6}H_{5}OH. You can
easily prove this by dropping carbolic acid or phenol down a red-hot
tube filled with iron-borings. The oxygen is taken up by the iron to
give oxide of iron, and benzene is obtained, thus: C_{6}H_{5}OH gives O
and C_{6}H_{6}. But there is another hydrocarbon called naphthalene,
C_{10}H_{8}, and this forms not one, but two phenols. As the name of the
hydrocarbon is naphthalene, however, we call these compounds naphthols,
and one is distinguished as alpha- the other as beta-naphthol, both of
them having the formula C_{10}H_{7}OH. But now with respect to the
colours. If we treat phenol with nitric acid under proper conditions, we
get a yellow dye called picric acid, which is trinitro-phenol
C_{6}H_{2}(NO_{2})_{3}OH; you see this is no aniline dye; it is not a
basic colour, for it would saturate, _i.e._ destroy the basicity of
bases. Again, by oxidising phenol with oxalic acid and vitriol, we get a
colour dyeing silk orange, namely, Aurin, HO.C[C_{6}H_{4}(OH)]_{3}. This
is also an acid or phenolic dye, as a glance at its formula will show
you. Its compound atom bristles, so to say, with phenol-residues, as
some of the aniline dyes do with aniline residue-groups.
We come now to a peculiar but immensely important group of colours known
as the azo-dyes, and these can be basic or acid, or of mixed kind. Just
suppose two ammonia groups, NH_{3} and NH_{3}. If we rob those nitrogen
atoms of their hydrogen atoms, we should leave two unsatisfied nitrogen
atoms, atoms with an exceedingly keen appetite represented in terms of
hydrogen atoms as N*** and N***. We might suppose a group, though of two N
atoms partially satisfied by partial union with each other, thus--N:N--.
Now this group forms the nucleus of the azo-colours, and if we satisfy a
nitrogen at one side with an aniline, and at the other with a phenol, or
at both ends with anilines, and so on, we get azo-dyes produced. The
number of coal-tar colours is thus very great, and the variety also.
_Adjective Colours._--As regards the artificial coal-tar adjective
dyestuffs, the principal are Alizarin and Purpurin. These are now almost
entirely prepared from coal-tar anthracene, and madder and garancine are
almost things of the past. Vegetable adjective colours are Brazil wood,
containing the dye-generating principle Brasilin, logwood, containing
Hæmatein, and santal-wood, camwood, and barwood, containing Santalin.
Animal adjective colours are cochineal and lac dye. Then of wood colours
we have further: quercitron, Persian berries, fustic and the tannins or
tannic acids, comprising extracts, barks, fruits, and gallnuts, with
also leaves and twigs, as with sumac. All these colours dye only with
mordants, mostly forming with certain metallic oxides or basic salts,
brightly-coloured compounds on the tissues to which they are applied.
LECTURE XI
DYEING OF WOOL AND FUR; AND OPTICAL PROPERTIES OF COLOURS
You have no doubt a tolerably vivid recollection of the illustrations
given in Lecture I., showing the structure of the fibre of wool and fur.
We saw that the wool fibre, of which fur might be considered a coarser
quality, possesses a peculiar, complex, scaly structure, the joints
reminding one of the appearance of plants of the _Equisetum_ family,
whilst the scaled structure resembles that of the skin of the serpent.
Now you may easily understand that a structure like this, if it is to be
completely and uniformly permeated by a dye liquor or any other aqueous
solution, must have those scales not only well opened, but well
cleansed, because if choked with greasy or other foreign matter
impervious to or resisting water, there can be no chance of the
mordanting or dye liquids penetrating uniformly; the resulting dye must
be of a patchy nature. All wool, in its natural state, contains a
certain amount of a peculiar compound almost like a potash soap, a kind
of soft soap, but it also contains besides, a kind of fatty substance
united with lime, and of a more insoluble nature than the first. This
natural greasy matter is termed "yolk" or "suint"; and it ought never to
be thrown away, as it sometimes is by the wool-scourers in this country,
for it contains a substance resembling a fat named _cholesterin_ or
_cholesterol_, which is of great therapeutical value. Water alone will
wash out a considerable amount of this greasy matter, forming a kind of
lather with it, but not all. As is almost invariably the case, after
death, the matters and secretions which in life favour the growth and
development of the parts, then commence to do the opposite. It is as if
the timepiece not merely comes to a standstill, but commences to run
backwards. This natural grease, if it be allowed to stand in contact
with the wool for some time after shearing, instead of nourishing and
preserving the fibres as it does on the living animal, commences to
ferment, and injures them by making them hard and brittle. We see, then,
the importance of "scouring" wool for the removal of "yolk," as it is
called, dirt, oil, etc. If this important operation were omitted, or
incompletely carried out, each fibre would be more or less covered or
varnished with greasy matter, resisting the absorption and fixing of
mordant and dye. As scouring agents, ammonia, carbonate of ammonia,
carbonate of soda completely free from caustic, and potash or soda
soaps, especially palm-oil soaps, which need not be made with bleached
palm oil, but which must be quite free from free alkali, may be used. In
making these palm-oil soaps it is better to err on the side of a little
excess of free oil or fat, but if more than 1 per cent. of free fat be
present, lathering qualities are then interfered with. Oleic acid soaps
are excellent, but are rather expensive for wool; they are generally
used for silks. Either as a skin soap or a soap for scouring wools, I
should prefer one containing about 1/2 per cent. of free fatty matter,
of course perfectly equally distributed, and not due to irregular
saponification. On the average the soap solution for scouring wool may
contain about 6-1/2 oz. of soap to the gallon of water. In order to
increase the cleansing powers of the soap solution, some ammonia may be
added to it. However, if soap is used for wool-scouring, one thing must
be borne in mind, namely, that the water used must not be hard, for if
insoluble lime and magnesia soaps are formed and precipitated on the
fibre, the scouring will have removed one evil, but replaced it by
another. The principal scouring material used is one of the various
forms of commercial carbonate of soda, either alone or in conjunction
with soap. Whatever be the form or name under which the carbonate of
soda is sold, it must be free from hydrate of soda, _i.e._ caustic soda,
or, as it is also termed, "causticity." By using this carbonate of soda
you may dispense with soap, and so be able, even with a hard or
calcareous water, to do your wool-scouring without anything like the ill
effects that follow the use of soap and calcareous water. The carbonate
of soda solutions ought not to exceed the specific gravity of 1° to 2°
Twaddell (1-1/2 to 3 oz. avoird. per gallon of water). The safest plan
is to work with as considerable a degree of dilution and as low a
temperature as are consistent with fetching the dirt and grease off. The
scouring of loose wool, as we may now readily discern, divides itself
into three stages: 1st, the stage in which those "yolk" or "suint"
constituents soluble in water, are removed by steeping and washing in
water. This operation is generally carried out by the wool-grower
himself, for he desires to sell wool, and not wool plus "yolk" or
"suint," and thus he saves himself considerable cost in transport. The
water used in this process should not be at a higher temperature than
113° F., and the apparatus ought to be provided with an agitator; 2nd,
the cleansing or scouring proper, with a weak alkaline solution; 3rd,
the rinsing or final washing in water.
Thus far we have proceeded along the same lines as the woollen
manufacturer, but now we must deviate from that course, for he requires
softness and delicacy for special purposes, for spinning and weaving,
etc.; but the felt manufacturer, and especially the manufacturer of felt
for felt hats, requires to sacrifice some of this softness and delicacy
in favour of greater felting powers, which can only be obtained by
raising the scales of the fibres by means of a suitable process, such
as treatment with acids. This process is one which is by no means
unfavourable to the dyeing capacities of the wool; on the whole it is
decidedly favourable.
So far everything in the treatment of the wool has been perfectly
favourable for the subsequent operations of the felt-hat dyer, but now I
come to a process which I consider I should be perfectly unwarranted in
passing over before proceeding to the dyeing processes. In fact, were it
not for this "proofing process" (see Lecture VII.) the dyeing of felt
hats would be as simple and easy of attainment as the ordinary dyeing of
whole-wool fabrics. Instead of this, however, I consider the hat
manufacturer, as regards his dyeing processes as applied to the stiffer
classes of felt hats, has difficulties to contend with fully comparable
with those which present themselves to the dyer of mixed cotton and
woollen or Bradford goods. You have heard that the purpose of the
wool-scourer is to remove the dirt, grease, and so-called yolk, filling
the pores and varnishing the fibres. Now the effect of the work of the
felt or felt-hat proofer is to undo nearly all this for the sake of
rendering the felt waterproof and stiff. The material used, also, is
even more impervious and resisting to the action of aqueous solutions of
dyes and mordants than the raw wool would be. In short, it is impossible
to mordant and to dye shellac by any process that will dye wool. To give
you an idea of what it is necessary to do in order to colour or dye
shellac, take the case of coloured sealing-wax, which is mainly composed
of shellac, four parts, and Venice turpentine, one part. To make red
sealing-wax this mixture is melted, and three parts of vermilion, an
insoluble metallic pigment, are stirred in. If black sealing-wax is
required, lamp-black or ivory-black is stirred in. The fused material is
then cast in moulds, from which the sticks are removed on cooling. That
is how shellac may be coloured as sealing-wax, but it is a totally
different method from that by which wool is dyed. The difficulty then is
this--in proofing, your hat-forms are rendered impervious to the dye
solutions of your dye-baths, all except a thin superficial layer, which
then has to be rubbed down, polished, and finished. Thus in a short
time, since the bulk of that superficially dyed wool or fur on the top
of every hat is but small, and has been much reduced by polishing and
rubbing, you soon hear of an appearance of bareness--I was going to say
threadbareness--making itself manifest. This is simply because the
colour or dye only penetrates a very little way down into the substance
of the felt, until, in fact, it meets the proofing, which, being as it
ought to be, a waterproofing, cannot be dyed. It cannot be dyed either
by English or German methods; neither logwood black nor coal-tar blacks
can make any really good impression on it. Cases have often been
described to me illustrating the difficulty in preventing hats which
have been dyed black with logwood, and which are at first a handsome
deep black, becoming rather too soon of a rusty or brownish shade. Now
my belief is that two causes may be found for this deterioration. One is
the unscientific method adopted in many works of using the same bath
practically for about a month together without complete renewal. During
this time a large quantity of a muddy precipitate accumulates, rich in
hydrated oxide of iron or basic iron salts of an insoluble kind. This
mud amounts to no less than 25 per cent. of the weight of the copperas
used. From time to time carbonate of ammonia is added to the bath, as it
is said to throw up "dirt." The stuff or "dirt," chiefly an ochre-like
mass stained black with the dye, and rich in iron and carbonate of iron,
is skimmed off, and fresh verdigris and copperas added with another lot
of hat-forms. No doubt on adding fresh copperas further precipitation of
iron will take place, and so this ochre-like precipitate will
accumulate, and will eventually come upon the hats like a kind of thin
black mud. Now the effect of this will be that the dyestuff, partly in
the fibre as a proper dye, and not a little on the fibre as if
"smudged" on or painted on, will, on exposure to the weather, moisture,
air, and so on, gradually oxidise, the great preponderance of iron on
the fibre changing to a kind of iron-rust, corroding the fibres in the
process, and thus at once accounting for the change to the ugly brownish
shade, and to the rubbing off and rapid wearing away of the already too
thin superficial coating of dyed felt fibre. In the final spells of
dyeing in the dye-beck already referred to, tolerably thick with black
precipitate or mud, the application of black to the hat-forms begins, I
fear, to assume at length a too close analogy to another blacking
process closely associated with a pair of brushes and the time-honoured
name of Day & Martin. With that logwood black fibre, anyone could argue
as to a considerable proportion of the dye rubbing, wearing, or washing
off. Thus, then, we have the second cause of the deterioration of the
black, for the colour could not go into the fibre, and so it was chiefly
laid or plastered on. You can also see that a logwood black hat dyer may
well make the boast, and with considerable appearance of truth, that for
the purposes of the English hat manufacturers, logwood black dyeing is
the most appropriate, _i.e._ for the dyeing of highly proofed and stiff
goods, but as to the permanent character of the black colour on those
stiff hats, there you have quite another question. I firmly believe that
in order to get the best results either with logwood black or "aniline
blacks," it is absolutely necessary to have in possession a more
scientific and manageable process of proofing. Such a process is that
invented by F.W. Cheetham (see Lecture VII. p. 66).
In the dyeing of wool and felt with coal-tar colours, it is in many
cases sufficient to add the solution of the colouring matters to the
cold or tepid water of the dye-bath, and, after introducing the woollen
material, to raise the temperature of the bath. The bath is generally
heated to the boiling-point, and kept there for some time. A large
number of these coal-tar colours show a tendency of going so rapidly
and greedily on to the fibre that it is necessary to find means to
restrain them. This is done by adding a certain amount of Glauber's
salts (sulphate of soda), in the solution of which coal-tar colours are
not so soluble as in water alone, and so go more slowly, deliberately,
and thus evenly upon the fibre. It is usually also best to dye in a bath
slightly acid with sulphuric acid, or to add some bisulphate of soda.
There is another point that needs good heed taking to, namely, in using
different coal-tar colours to produce some mixed effect, or give some
special shade, the colours to be so mixed must possess compatibility
under like circumstances. For example, if you want a violet of a very
blue shade, and you take Methyl Violet and dissolve it in water and then
add Aniline Blue also in solution, you find that precipitation of the
colour takes place in flocks. A colouring matter which requires, as some
do, to be applied in an acid bath, ought not to be applied
simultaneously with one that dyes best in a neutral bath. Numerous
descriptions of methods of using coal-tar dyestuffs in hat-dyeing are
available in different volumes of the _Journal of the Society of
Chemical Industry_, and also tables for the detection of such dyestuffs
on the fibre.
Now I will mention a process for dyeing felt a deep dead black with a
coal-tar black dye which alone would not give a deep pure black, but one
with a bluish-purple shade. To neutralise this purple effect, a small
quantity of a yellow dyestuff and a trifle of indigotin are added. A
deep black is thus produced, faster to light than logwood black it is
stated, and one that goes on the fibre with the greatest ease. But I
have referred to the use of small quantities of differently coloured
dyes for the purpose of neutralising or destroying certain shades in the
predominating colour. Now I am conscious that this matter is one that is
wrapped in complete mystery, and far from the true ken of many of our
dyers; but the rational treatment of such questions possesses such vast
advantages, and pre-supposes a certain knowledge of the theory of
colour, of application and advantage so equally important, that I am
persuaded I should not close this course wisely without saying a few
words on that subject, namely, the optical properties of colours.