THE ACID COLORS
The discovery and introduction into commerce of Mauveine and the
other Basic dyes, focussed the attention of chemists, all over the
world, upon this new and important application of their science.
And it was soon noticed that certain organic bodies, of a decidedly
_acid_ character, had the power of dyeing wool and silk. These early
dyes were so-called “nitro” compounds, formed by the action of strong
nitric acid upon derivatives of coal tar, and in most cases they
gave strong and brilliant, but rather fugitive, shades of yellow.
The most interesting of these, perhaps, was the compound known as
“picric acid,” which at one time was considerably used for dyeing
silk yellow. Now it has been abandoned for that purpose but is
manufactured on an enormous scale for use as an explosive.
These original acid dyes were of little importance. But in the
early seventies chemists began to make use of a reaction—known as
“diazotizing”—for making new organic compounds by the coupling of
aniline or bodies similar to aniline, with all sorts and kinds of
other compounds derived from coal tar. The number of derivatives
of this sort proved enormous, and many of them had more or less
valuable dyeing properties. And in a very short time new dyestuffs
had been discovered, good, bad, and indifferent, numbering not
hundreds, but thousands.
A very few of these so-called “Azo” dyes were of the Basic class,
like Bismarck Brown, mentioned in the last chapter. Others,
discovered ten or fifteen years later, constituted the class of
Direct Cotton colors or Salt colors. But the great bulk of these
colors belonged to the so-called “Acid” class, forming salts with
bases and alkalies, and being liberated from the salts by strong
acids.
The number of Acid Azo colors is very large. In the catalogues of
commercial coal-tar colors there are some two hundred and fifty
of these dyes which have been picked out of the rest as having
sufficient value to be carefully described, and to have been placed
on the market by the great dye houses. Most of these are red and
orange colors, with a few yellows. As a rule they are brilliant and
clear, but, with a few exceptions, not particularly fast to light.
When these were introduced it was soon recognized that they were of
practically no value for cotton and linen. They are as a rule much
more soluble than the Basic dyes of the foregoing chapter, and hence
are occasionally used as stains for wood, rattan, and other vegetable
materials where considerable penetration is needed, without fastness
to washing. But such use is of little importance.
=Properties.=—Acid dyes are almost exclusively employed for dyeing
wool, silk, feathers, and other animal fibres, and for this they
are extremely valuable. The introduction of the Acid Azo colors
so simplified and improved the dyeing of wool and silk, that every
effort was made to increase the range of colors. And when it was
found that the Azo colors were weak on the line of blue, purple,
and green, efforts were made, which after several years proved
successful, to change the various powerful Basic dyes, the Methyl
Violets, Fuchsin or Aniline Red, Aniline Blue, Malachite Green, and
the rest, into Acid dyes, so that they could all be used in the same
dye-baths. This has resulted in a very wide range of colors indeed,
for the Acid Azo colors cover fully all the shades of yellow, orange,
and especially of red, from scarlets of all sorts and kinds to deep
full crimsons. And then the remaining shades are covered by the
acidified or sulphonated Basic colors.
These latter, by the way, though very brilliant and strong and rich,
are no faster to light than the original Basic colors from which
they are derived. Of late years the Acid colors have held their
own, and still monopolize the commercial, as well as the special,
dyeing of wool and silk excepting under unusual circumstances, when
considerable fastness to washing is required.
With these dyes, as in the case of the Basic dyes, the fastness to
washing is of little or no consequence to the craftsman. Nobody
expects to scrub hand-dyed leather; and woollen and silk goods,
unless specially prepared, are not supposed to be turned over to the
tender mercies of the family laundress. However, it may be well to
emphasize here the fact that these dyes are as a rule “stripped”
quite readily by boiling in a neutral soap bath. And when the
craftsman wishes to dye wool or silk fast to washing, he must either
use the Salt dyes, in a boiling bath, or must dye, with special
precautions against tendering, with either the Sulphur or the Vat
Dyes.
With regard to light-fastness, however, the case is different. A
great many hundreds, possibly even thousands, of Acid dyes have been
discovered, and scores of them, covering every shade, can be obtained
in the open market. Most of these are of but little permanence, but a
few products, from each of the great color houses, can be selected,
whose fastness to light is extremely satisfactory. The dyes in the
following list can hardly be considered as fast as the Vat dyes,
previously described, but are probably faster, as a class, than any
other class mentioned in this book. They would rank at the very top
of the second class, and some at least would fairly enter the first
class, being absolutely satisfactory against even the strongest
sunlight.
A series of skeins, dyed all colors of the rainbow, including many
delicate light shades, with a red, yellow, and blue dye of those
mentioned below, withstood an exposure test which quite ruined a
similar set of skeins dyed with the very best natural dyestuffs. And
a large hand-woven rug, made of wool dyed light shades with the same
dyes, was placed for two weeks on a roof in New York, half of it
being covered with boards and the rest exposed to the direct action
of the July sunlight, and at the end of this time it was impossible
to notice any difference in shade.
The colors in the following list are to be used, principally, for
wool. They will all dye silk, leather, and feathers, but in the
chapters dealing with those materials some additional dyes may be
mentioned, which are specially suited for them.
_List of Selected Dyes._—
Badische— Palatine Scarlet A, 3 R
Palatine Light Yellow, R
Tartrazine (yellow)
Wool Fast Blue, B L
Cassella— Brilliant Cochineal, R R
Acid Yellow, A T, conc.
Tetracyanol, S F
Elberfeld—Azo Crimson, S
Fast Red, A
Fast Yellow, 3 G
Alizarine Blue, S A P
Cashmere Black, 3 B N
Kalle— Biebrich Acid Red, 2 B
Wool Yellow, T A
Nero cyanine Blue, B
Nero cyanine Black, D
Metz— Fast Acid Red, M
Fast Acid Orange, G
Fast Acid Yellow, 3 G
Fast Acid Blue, B B
DYEING DIRECTIONS
The Acid dyes, like the Basic, are used in an acid bath, but for a
different reason. With the Basic dyes acetic acid or some other weak
acid is added, for the purpose of readily dissolving the color. In
the case of the Acid dyes, however, the dyestuffs are almost always
put on the market in the form of the potassium or ammonium salts of
the color acid. And the presence of some acid is always necessary,
to liberate the color acid, and allow it to combine with the basic
principles existing in the animal fibres.
_For Wool._—The goods, well washed and soaked, are warmed gently in a
bath containing, besides the dyestuff dissolved in plenty of water,
a little sulphuric acid and a good deal of Glauber’s salt. Both acid
and salt should be free from iron, or the shade will be dulled.
The amount of acid to be used may vary between considerable limits
without affecting the results. If too much is present, there is
danger of injuring the feel and the lustre of the fibre. If there
is not enough acid in the bath, the color will wash right out of
the wool, as soon as it is rinsed. In general it is well to start
with about one tablespoonful of dilute (30%) sulphuric acid for each
gallon of dye-liquor and about twice that amount of Glauber’s salt.
It is hard to tell just what is the function of the Glauber’s salt.
It seems, however, to open up the pores of the wool in some way, and
to make it dye more evenly and deeply. The bath is gently heated,
with constant stirring of the goods, until the right shade is
produced, or, if it is desired to exhaust the bath and so waste no
color, until near the boiling point.
The goods when taken out of the dye-bath must be washed very
thoroughly, to remove the last trace of acid, which otherwise on
drying would ruin the wool.
It must be remembered that these Acid dyes hardly affect cotton
in the least, and so the goods dyed in this way must be free from
vegetable fibres, if level dyeings are to be obtained.
In dyeing wool skeins commercially it is, of course, of the utmost
importance to have the colors perfectly level and uniform. This
uniformity is obtained easily enough, when using these Acid dyes, by
having the wool thoroughly wet before placing it in the dye-bath;
by having it well loosened out and well stirred so that the color
will penetrate evenly every part of the material; and, finally,
by starting the bath at a moderate temperature, and heating it
gradually, until the proper shade is obtained.
For handicraft dyeing the student is strongly advised to practise
shaded and irregular effects, the so-called Rainbow dyeing, with wool
in skeins, just as, in previous lessons, with raffia and with cotton.
By using coarse heavy yarns, very beautiful two and three color
effects can be produced, which, when used for embroidery or weaving,
will prove most interesting.
Great care must always be taken, in wool dyeing, to preserve the
lustre and the soft effect of the wool, and to avoid felting. This
can best be done by using moderate amounts of acid, by dyeing at
moderate temperature and never raising the dye-bath quite to the
boil; and finally, by handling the goods as little as possible in
the acid dye-bath, consistent of course with exposing every portion
equally to the action of the dyestuff. Cotton skeins can be worked
and rubbed, and pulled, and thrown up and down in the hot dye-bath,
without fear of injuring them. But wool should be handled carefully,
and worked in the dye-pot quietly and gently, just sufficiently
to accomplish two results. First, the wool at the bottom of the
pot should be raised by a lifting and turning motion and replaced
by fresh material; and second, when the wool is lowered back into
the liquor it should be loosened, so as to allow the dye-liquor to
penetrate the mass.