The Dyeing of Woollen Fabrics Part 27

CHAPTER IX. (p. 218)

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 sample 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 acc.u.mulate 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 do 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 (p. 219) 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 aid 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 20 times its volume of water.

3. Concentrated hydrochloric acid as bought.

4. Dilute hydrochloric acid, 1 acid to 20 water.

5. Concentrated nitric acid as bought.

6. Dilute nitric acid, 1 acid to 20 water.

7. Acetic acid.

8. Caustic soda solution, 5 grammes in 100 c.c. water.

9. Ammonia (strong).

10. Dilute ammonia, 1 strong ammonia to 10 water.

11. Carbonate of soda solution, 5 grammes in 100 c.c. water.

12. Bleaching powder solution, 2 Tw.

13. Bisulphite of soda, 72 Tw.

14. Stannous chloride, 10 grammes 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 (p. 220) of the fabric 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 cla.s.s, 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. Pa.s.s 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 (p. 221) 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 pota.s.sium 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 a.n.a.lysis 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 ent.i.tled to be called fast. The degree 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 (p. 222) is no doubt rain, hail and snow have some influence on the fading of the colour. If the piece is exposed under gla.s.s 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, the 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 with 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 grammes 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 tests 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 (p. 223) 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.

THE END.