0·5 gramme of each of the samples of dyes should be weighed out and dissolved in 100 c.c. of water, care being taken that every portion of the dye is dissolved before any of the solution is used in making up the dye-vats. Care should be taken that the skeins of yarn or swatches of cloth are exactly equal in weight, that the same volume of water is placed in each of the dye-pots, that the same amounts of sulphate of soda or other dye assistants are added, that the quantities of dye-stuffs and solutions used are equal, in fact that in all respects the conditions of dyeing are exactly the same, such in fact being the vital conditions in making comparative dye-tests of the actual dyeing strength of several samples of dyes.
After the swatches have been dyed they are rinsed and then dried, when the depths of shade dyed on them may be compared one with another. To prevent any mistakes it is well to mark the swatches with one, two, three or more cuts as may be required.
It is easier to ascertain if two dyes are different in strength of colour than to ascertain the relative difference between them. There are two plans available for this purpose; one is a dyeing test, the other is a colorimetric test made with the solutions of the dyes.
Dyeing Test.--This method of ascertaining the relative value of two dyes as regards strength of colour is carried out as follows. A preliminary test will show which sample is stronger than the other; then there is prepared a series of dye-vats, one contains a swatch with the deepest of the two dyes, which is taken as the standard, the others with the other dye but containing 2, 5 and 10 per cent. more dye-stuff, and all these are dyed together, and after drying a comparison can be made between these and the standard swatch, and a judgment formed as to the relative strength of the two dyes; a little experience will soon enable the dyer to form a correct judgment of the difference in strength between two samples of dye-stuff.
The colorimetric test is based on the principle that the colour of a solution of dye-stuff is proportionate to its strength. Two white glass tubes, equal in diameter, are taken; solutions of the dye-stuffs, 0·5 gramme in 100 c.c. of water, are prepared, care being taken that the solution is complete. 5 c.c. of one of these solutions is taken and placed in one of the glass tubes, and 5 c.c. of the other solution is placed in the other glass tube, 25 c.c. of water is now added to each tube and then the colour of the diluted liquids is compared by looking through in a good light. That sample which gives the deepest solution is the strongest in colouring power. By diluting the strongest solution with water until it is of the same depth of colour as the weakest, it may be assumed that the length of the columns of liquid in the two tubes is in proportion to the relative strength of the two samples. Thus if in one tube there are 30 centimetres of liquid and in the other 25 centimetres, then the relative strength is as 30 to 25, and if the first is taken as the standard at 100 a proportion sum may be worked out as follows:--
30: 25 :: 100 : 83·3;
that is, the weakest sample has only 83·3 per cent. of the strength of the strongest sample.
CHAPTER IX.
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.