The property of a dye to retain its color when the dyed (or printed) textile material is exposed to conditions or agents such as light, perspiration, atmospheric gases, or washing that can remove or destroy the color. A dye may be reasonably fast to one agent and only moderately fast to another. Degree of fastness of color is tested by standard procedures. Textile materials often must meet certain fastness specifications for a particular use.
It is always useful and interesting to test the dye which is to be used on a sample of the yarn or fabric to be dyed. The outcome will depend on the fabric, the mordant that has been used and dye that has have been chosen. Testing is best carried out on a series of Groundnuts marked (for identification) samples, which have been mordanted with a number of different mordants. Tests can be carried out for light, water and washing fastness using simple standard test methods.
To Test for Washing Fastness Follow these Steps :
- Take two pieces of fabric about 5cm by 5cm, one of which is undyed cotton and the other undyed wool. Stitch them together along one side.
- Take some sample strips of the dyed yarn and spread them evenly between the two pieces of cloth so that they overlap both sides. If dyed fibre is being tested a combed sample can be used in place of the yarn.
- Sew around all four sides of the cloth so that the yarn is held in place.
- Prepare a similar specimen with dyed materials that has satisfactory properties and place them in two jars with screw lids containing a solution of 5gm per litre soap or detergent solution at 30oC.
- Agitate the two jars gently for 30mins, then remove the fabrics and wash them gently in clean water for 5mins. Open the stitching and separate the pieces to dry in air. Examination:
- Place the dyed yarn next to a sample of the same material which has not been tested, and compare the change which has taken place. Compare also with the control sample with satisfactory properties. If the dyeing being tested shows equal or less change than the satisfactory sample, then it is as good as the satisfactory sample.
- Place the wool and cotton cloths next to samples of the same material which have not been tested and compare them with the cloths that have been tested with a satisfactory dyeing. Equal or less staining shows equal or better fastness.