By LOUIS SIEBOLD, F.I.C., F.C.S.
During the last ten years there has been an enormous increase in the
production of these preparations, and the time will come when their
application in dyeing and calico printing will become so general as to
completely supersede the employment of the raw materials. The
manufacture of these extracts, to be thoroughly successful, requires
to be so conducted as to secure the perfect exhaustion of the dyewoods
without the slightest destruction or deterioration of the coloring
matters contained in them; and though nothing like perfection has been
reached in the attainment of these objects, it is certain that the
processes of extraction and evaporation now employed by the best
makers are a very great improvement on the older methods. Indeed,
there is no difficulty nowadays in procuring dyewood extracts of high
excellence if the consumer is willing to pay a price for them
corresponding to their quality, and knows how to avail himself of the
aid of chemical skill to control his purchases. Unfortunately,
however, there is so much hankering after cheap articles, and so
little care is taken to ascertain their real quality, that every scope
is afforded to the malpractices of the adulterer. There are many dye
and print works in which large quantities of these extracts are used
without being subjected to trustworthy tests. Moreover, much of the
testing is done by fallacious methods and often by biased hands. So
fallacious, indeed, are some of these tests, that grossly adulterated
extracts are often declared superior to the purer ones, the cause of
this being the application of an insufficient proportion of mordant in
the dyeing or printing trials, and the consequent waste of the excess
of coloring matter in the case of the purer preparation.
Professional analytical chemists have hitherto given but little
attention to these preparations, and the employment of experienced
chemists in works is as yet far from general. The testing of dyewood
extracts in such a manner as to throw full light on their purity, the
quality of raw material from which they are prepared, their exact
commercial value their suitability for special purposes, and the
proportion and nature of any adulterants they may contain, is of
course a difficult and tedious task, and must be left to the expert
who is in possession of authentic specimens prepared by himself of all
the different extracts made from every variety and quality of raw
materials, and who combines a thorough knowledge of experimental
dyeing and printing with a large experience in the chemical
investigation of these preparations. But when the object of the
testing is merely careful comparison of the sample in question with an
original sample or previous deliveries, the case is much simplified,
and comes within the scope of the general chemist or the laboratory
attached to works. A few years ago I recommended carefully conducted
dyeing trials on woolen cloth mordanted with bichromate of potash as
the best and simplest mode adapted to such cases, and my subsequent
experience enables me to confirm that observation to the fullest
extent. Most of these extracts contain the coloring matter in two
states, the developed and the undeveloped, and an oxidizing mordant
such as bichromate of potash causes the latter as well as the former
to enter completely into combination with a metallic base; whereas
many of the other mordants, such as alumina or tin compounds, merely
take up the developed portion of the coloring matter together with
such small and variable proportions of the undeveloped as might
undergo oxidation during the process of dyeing. I would therefore
suggest dyeing trials with alumina, tin, iron, etc., only as
subsidiary tests indicating the suitability of an extract for certain
special purposes, while recommending the trial with bichromate of
potash as the one giving the best information respecting the actual
strength of the extract in relation to the raw material from which it
was obtained, and as giving a fair idea of the money value of the
sample. Cotton dyeing does not, as a general rule, afford a good means
of assaying extracts, as it is generally done under conditions which
do not admit of complete exhaustion of the dye bath, but it might
often with advantage be resorted to as an additional trial throwing
further light on the degree of oxidation or development of the
coloring matter. Printing trials are apt to give fallacious results
unless the proportion of mordant is carefully adjusted to the amount
of coloring matter present, and several trials with different
proportions would be necessary to prevent erroneous conclusions. For
the trials with bichromate of potash on wool I would recommend pieces
of cloth weighing about 150 grains, and the most suitable proportion
of bichromate of potash is 3 per cent. of the weight of the cloth. The
requisite number of pieces (equal to the number of samples to be
tested) should be thoroughly scoured and then heated in the bichromate
solution at or near the boiling point for not less than 1½ hours,
after which they should be well washed and then dyed separately in the
solutions of equal weights of the extracts at the same temperature and
for the same length of time; 15 grains of extract is a suitable
quantity for a first trial under these conditions. These trials can
then be repeated with different relative proportions of extract in
order to ascertain what weight of a sample would give the same depth
of color as 15 grains of the standard example. Many precautions are
required both in the mordanting and dyeing processes in order to
obtain trustworthy results; and though the trials with bichromate of
potash give the most reliable information of any single test, they
should be supplemented by the subsidiary tests already alluded to, and
also by a chemical examination, in order to obtain a knowledge, not
merely of the wood strength, but also of the general nature of the
extract. An adulteration with molasses or glucose can be best
determined by fermentation in comparison with a pure sample. Mineral
adulterants may, of course, be detected by an estimation and analysis
of the ash, after making due allowances for variations due to
differences in different kinds of the same dyewoods. The estimation of
the individual coloring matters in these extracts by means of a
chemical analysis is under all circumstances a task requiring much
experience, especially as the coloring principles are associated in
different qualities of each class of dyewood with different
proportions of other constituents which often give much trouble to the
unpracticed experimenter. Extracts made from logwood roots are now
largely manufactured and often substituted or mixed with the extracts
of real logwood, and have in some instances been palmed of as logwood
extracts of high quality. The correct determination of such
admixtures, like the fixing of anything like the exact commercial
value of dyewood extracts, requires nothing less than a complete
chemical investigation coupled with numerous dyeing trials in
comparison with standard preparations, and should be left to an
expert.
The presence in dyewood extracts of coloring matters in various stages
of development has hitherto militated against their use in place of
the raw materials by many dyers and printers who are still employing
inherited and antiquated processes in which the whole of the coloring
matter is not rendered available. It is often asserted by these that
even the best of extracts fail to give anything like the results
attained by the use of well-prepared woods, and that, indeed, their
application proves a complete failure. Such failure, however, is
simply due to the want of chemical knowledge on the part of the dyers,
for there is no real difficulty in making any good and pure extract
serve all the purposes for which the woods were used. It is to be
hoped that in this branch of industry, as well as in many others, the
employment of chemists will become more general than at present, and
not be restricted, as is often the case, to young men without
experience and without the trained intellect so essential to success
in chemical investigations. High class chemical skill is of course
available to the manufacturer, but the man of science who brings
matured knowledge and valuable brain work into the business required
social as well as pecuniary recognition, and the sooner and more
fuller this fact is appreciated the better it will be for the
maintenance and progress of our industries.
With regard to the astringent extracts, such as sumac, myrabolam,
divi, valonia, quebracho, oak, etc., it is the aim of the
manufacturer, whenever such extracts are intended for the purposes of
dyeing and printing, to obtain the tannin in a form in which it is
best calculated to fix itself upon the fiber. The case is somewhat
different when the same extracts are required for tanning. For this
purpose it is necessary that the extract shall have considerable
permeating power, and that the tannin contained in it shall readily
yield leather of the desired texture, color, and permanency. Extracts
specially suited for this purpose are by no means always the most
suitable for the dyer, and _vice versa_.
A brief description of the processes by which the astringent extracts
may be tested with particular reference to their fitness for definite
purposes concluded the paper.
With regard to the question as to whether experimental dyeing with
bichromate of potash should be employed as a test even in works where
all the dyeing was done with other mordants, he was decidedly of
opinion that it should always be resorted to as one of the tests,
inasmuch as it was the only simple and expeditious method giving a
fair idea of the actual wood strength and money value of the extract.
The test should, in such cases, be supplemented by dyeing trials with
the mordants used at the works, and, if necessary, also by a chemical
analysis. Printing trials were not necessarily bad tests, since
oxidizing was usually added in these where it was necessary, and any
undeveloped coloring matter would thus be oxidized during the steaming
process: but, as he had stated before, it was essentially necessary in
such cases to have a fair idea of the amount of actual coloring matter
in the extract and to adjust the proportion of mordant accordingly.
Such trials should therefore be preceded by carefully conducted dyeing
trials with bichromate of potash. Mr. Thomson had raised the question
whether it would not be well for the manufacturer to prepare these
extracts in such a manner that they would contain all the coloring
matter in one condition only, in order to insure greater uniformity in
their quality and mode of application. This would, no doubt, be a
desirable step to take if the owners of dye and print works were more
in the habit of availing themselves of the service of competent
chemists experienced in this branch, for then they would be able to
make any extract do its full work irrespective of the state of
development of the coloring matter. Such, however, was not the case,
and it was a very common thing for the consumer of dyewood extracts to
require the manufacturer to prepare them specially for him so as to
suit his own dyeing recipes, or in other words to give exactly the
same shades, weight for weight, by his own method of dyeing as the
article he was in the habit of using. The manufacturer was thus often
compelled to make many different qualities of the same extract to suit
different customers. For the same reason adulterated articles were
often preferred to the pure ones. There was, perhaps, no branch of
industry in which chemical skill of a high order could be applied with
greater advantage than in dyeing, and nowhere was this fact less
recognized. Some of the processes of dyeing were exceedingly wasteful
and stood in much need of improvement. He (Mr. Siebold) knew a large
works in which a ton of logwood extract was used daily for black
dyeing only, and he might safely assert that of this enormous quantity
only a very small proportion would be fixed on the fiber, while by far
the greater proportion was utterly wasted. Such a waste could only be
prevented by a searching investigation of its causes by trained skill.
Mr. Thomson had further alluded to the color obtained with logwood or
logwood extract and wool mordanted with bichromate of potash, and
seemed to be under the impression that the color thus obtained was not
black, but blue. This was undoubtedly the case in dyeing trials
performed as tests, as these were conducted purposely with a very
small proportion of coloring matter in order to admit of a better
comparison of the resulting depth of shades. But with larger
proportions of logwood the color obtained was a fine bluish-black, and
with the addition of a small proportion of fustic or quercitron bark
to the logwood a jet black was readily produced. With regard to Mr.
Watson Smith's observation as to fractional dyeing, he (Mr. Siebold)
did not regard this method as a suitable trial for ascertaining the
strength of an extract, but he admitted it was occasionally very
valuable for detecting an admixture of extracts of other dyewoods,
such as quercitron bark extract in logwood extract. It was also a good
method of ascertaining the speed of dyeing and hence the relative
proportion of fully developed coloring matter of an extract.--_Jour.
Soc. Chem. Industry._
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