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 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 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 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 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.