TESTING OF THE COLOUR OF DYED FABRICS.
It is frequently desirable that dyers should be able to ascertain with
some degree of accuracy what dyes have been used to dye any particular
samples of dyed cloth that has been offered to them to match. In these
days of the thousand and one different dyes that are known it is by no
means an easy thing to do; and when, as is most often the case, two or
three dye-stuffs have been used in the production of a shade, the
difficulty is materially increased.
The only available method is to try the effect of various acid and
alkaline reagents on the sample, noting whether any change of colour
occurs, and judging accordingly. It would be a good thing for dyers to
accustom themselves to test the dyeings they do, and so accumulate a
fund of practical experience which will stand them in good stead
whenever they have occasion to examine a dyed pattern of unknown origin.
The limits of this book does not permit of there being given a series of
elaborate tables showing the action of various chemical reagents on
fabrics dyed with various colours; and such, indeed, serve very little
purpose, for it is most difficult to describe the minor differences
which often serve to distinguish one colour from another. Instead of
doing so, we will point out in some detail the methods of carrying out
the various tests, and advise all dyers to carry these out for
themselves on samples dyed with known colours, and when they have an
unknown colour to test to make tests comparatively with known colours
that they think are likely to have been used in the production of the
dyed fabric they are testing.
One very common method is to spot the fabric, that is, to put a drop of
the reagent on it, usually with the end of the stopper of the reagent
bottle, and to observe the colour changes, if any, which ensue. This is
a very useful test and should not be omitted, and it is often employed
in the testing of indigo dyed goods with nitric acid, those of logwood
with hydrochloric acid, alizarine with caustic soda, and many others. It
is simple and easy to carry out, and only takes a few minutes.
To make a complete series of tests of dyed fabrics there should be
provided the following reagents:--
1. Strong sulphuric acid, as bought.
2. Dilute sulphuric acid, being the strong acid diluted with twenty
times its volume of water.
3. Concentrated hydrochloric acid.
4. Dilute hydrochloric acid, 1 acid to 20 water.
5. Concentrated nitric acid.
6. Dilute nitric acid, 1 acid to 20 water.
7. Acetic acid.
8. Caustic soda solution, 5 grams in 100 c.c. water.
9. Ammonia (strong).
10. Dilute ammonia, 1 strong ammonia to 10 water.
11. Carbonate of soda solution, 6 grams in 100 c.c. water.
12. Bleaching powder solution, 2° Tw.
13. Bisulphite of soda, 72° Tw.
14. Stannous chloride, 10 grams crystals in 100 c.c. water, with a
little hydrochloric acid.
15. Methylated spirit.
Small swatches of the dyed goods are put in clean porcelain basins, and
some of these solutions poured over them. Any change of colour of the
cloth is noted, as well as whether any colour is imparted to the
solutions. After making observations of the effects in the cold the
liquids may be warmed and the results again noted. After being treated
with the acids the swatches should be well washed with water, when the
original colour may be wholly or partially restored.
To give tables showing the effects of these reagents on the numerous
dyes now known would take up too much room and not serve a very useful
purpose, as such tables, if too much relied on, leave the operator
somewhat uncertain as to what he has before him. The reader will find in
Hurst's _Dictionary of Coal-tar Colours_ some useful notes as to the
action of acids and alkalies on the various colours that may be of
service to him.
Alizarine and the series of dye-stuffs to which it has given its name,
fustic, cochineal, logwood and other dyes of a similar class require the
fabric to be mordanted, and the presence of such mordant is occasionally
an indirect proof of the presence of these dyes.
To detect these mordants, a piece of the swatch should be burnt in a
porcelain or platinum crucible over a Bunsen burner, care being taken
that all carbonaceous matter be burnt off. A white ash will indicate the
presence of alumina mordants, red ash that of iron mordants, and a
greenish ash chrome mordants.
To confirm these the following chemical tests may be applied: Boil the
ash left in the crucible with a little strong hydrochloric acid and
dilute with water. Pass a current of sulphuretted hydrogen gas through
the solution; if there be any tin present a brown precipitate of tin
sulphide will be obtained. This can be filtered off. The filtrate is
boiled for a short time with nitric acid, and ammonia is added to the
solution when alumina is thrown down as a white gelatinous precipitate;
iron is thrown down as a brown red bulky precipitate; while chrome is
thrown down as a greyish-looking gelatinous precipitate. The precipitate
obtained with the ammonia is filtered off, and a drop of ammonium
sulphide added, when any zinc present will be thrown down as white
precipitate of zinc sulphide: to the filtrate from this ammonium oxalate
may be added, when if lime is present a white precipitate of calcium
oxalate is obtained.
A test for iron is to dissolve some of the ash in a little hydrochloric
acid, and add a few drops of potassium ferrocyanide solution, when if
any iron be present a blue precipitate will be obtained.
To make more certain of the presence of chrome, heat a little of the ash
of the cloth with caustic soda and chlorate of soda in a porcelain
crucible until well fused, then dissolve in water, acidify with acetic
acid and add lead acetate; a yellow precipitate indicates the presence
of chrome.
A book on qualitative chemical analysis should be referred to for
further details and tests for metallic mordants.
The fastness of colours to light, air, rubbing, washing, soaping, acids
and alkalies is a feature of some considerable importance. There are
indeed few colours that will resist all these influences, and such are
fully entitled to be called fast. The decree of fastness varies very
considerably. Some colours will resist acids and alkalies well, but are
not fast to light and air; some will resist washing and soaping, but are
not fast to acids; Some may be fast to light, but are not so to washing.
The following notes will show how to test these features:--
=Fastness to Light and Air.=--This is simply tested by hanging a piece
of the dyed cloth in the air, keeping a piece in a drawer to refer to,
so that the influence on the original colour can be noted from time to
time. If the piece is left out in the open one gets not only the effect
of light but also that of climate on the colour, and there is no doubt
wind, rain, hail and snow have some influence on the fading of the
colour.
If the piece is exposed under glass, the climatic influences do not come
into play, and one gets the effect of light alone.
In making tests of fastness, the dyer will and does pay due regard to
the character of the influences that the material will be subjected to
in actual use, and these vary very considerably; thus the colour of
underclothing need not be fast to light, for it is rarely subjected to
that agent of destruction. On the other hand it must be fast to washing,
for that is an operation to which underclothing is subjected week by
week.
Window curtains are much exposed to light and air, and, therefore,
colours in which they are dyed should be fast to light and air. On the
other hand these curtains are rarely washed, and so the colour need not
be quite fast to washing. And so with other kinds of fabrics, there are
scarcely two kinds which are subjected to the same influences, and
require the colours to have the same degree of fastness.
The fastness to rubbing is generally tested by rubbing the dyed cloth on
a piece of white paper.
=Fastness to Washing.=--This is generally tested by boiling a swatch of
the cloth in a solution of soap containing 4 grams of a good neutral
curd soap per litre for ten minutes and noting the effect--whether the
soap solution becomes coloured and to what degree, or whether it remains
colourless, and also whether the colour of the swatch has changed at
all.
One very important point in connection with the soaping test is whether
a colour will run into a white fabric that may be soaped along with it.
This is tested by twisting strands of the dyed yarn or cloth with white
yarn or cloth and boiling them in the soap liquor for ten minutes and
then noting the effect, particularly observing whether the white pieces
have taken up any colour.
Fastness to acids and fastness to alkalies is observed while carrying
out the various acid and alkali tests given above.