Cover Image
close this bookDyeing of Sisal and other Plant Fibres: A Handbook for Craft Instructors (NRI)
close this folderPart 1: Basic information and essential requirements
View the documentMaterials and equipment
View the documentTechnique
View the documentFastness testing

Fastness testing

Introduction

Many chemicals and environmental factors can cause colours to change their depth of shade and also, occasionally, their hue. Some changes are caused by the physical removal of dye and, whilst the visual colour may not be affected, this removed dye may stain adjacent materials.

Colour fastness is not solely a property of the dye. Fastness properties are also affected by the material on which the dyestuff is used. Sisal, for example, unlike cotton, contains substances which contribute to the action of light in fading the dyes and most dyes are, therefore, less light fast on sisal than they are on cotton. The substances in sisal usually turn brown, causing the colour of the dyed material to change. Colour fastness of a dye is also dependent on depth of shade. Deep shades resist light better than pale shades of the same dye, but dye is more readily lost by physical means from deep shades, leading to reduced water fastness and greater risk of staining onto other materials.

When evaluating dyes it is important that the depth of colour can be described accurately to allow fair comparisons between dyes. This is usually done with the aid of pattern cards which illustrate and describe hues in terms of internationally agreed standard depths of colour. Such pattern cards conform to ISO Recommendations R105 and include British Standard BS1006: Section A01: 1978, Standard depth: matt. Only colours of equal visual depth can be usefully compared.

When choosing dyes, the dyer must consider colour fastness from two aspects: firstly, the changes of colour that could occur during subsequent processing of the dyed material; and secondly, changes that could occur during use of the finished article.

After-treatment of some dyes (e.g. directs) to improve their water fastness, or subsequent application of lacquers to the finished goods, may cause change of colour. The change, of course, is important if the dyer is aiming to produce a specific colour on the finished goods and the dyer must then know what changes will take place so that compensation can be made for them. For craft workers who make hats, fastness to dry heat is of importance: hats are often formed on hot moulds, or by ironing, and these processes could cause loss of dye.

Colour fastness in use is far more important. Many dyed goods are necessarily exposed to adverse treatment during use, for example, by cleaning. With hats and bags, a shower of rain could cause colours to run and stain the user's clothing. Table mats could become wet from spillage and stain the table cloth. Therefore, it is the dyer's responsibility to ensure that the fastness properties of the colours produced are suited to the goods for which the material is to be used.

All colours fade in use, but provided that this is consistent with age it does not worry the user seriously. However, if each colour changes to a different extent, or changes in the hue occur, the art design may be spoilt and this the user will find unacceptable. Therefore, colours of similar fastness must be used on individual articles and blends must be prepared from dyes with similar fastness properties.

Fastness requirements of sisal goods can be expected to be less stringent than those for cotton and woollen goods. It is very unlikely, for example, that sisal will be washed with hot water and detergents, except when it is used in matting, or that it will be dry-cleaned or bleached, so fastness to these treatments need not be considered. However, fastness to light and cold water (e.g. rain) is of great importance - especially for more expensive durable goods.

Since fastness properties are important, craft workers should test the quality of new dyes before they are used for durable goods. Test methods are described in this section. However, provided that dyeing conditions are not altered it is unnecessary to test subsequent dyeings with the same dye.

There are a number of recognised tests for colour fastness, the results of which can be expressed in numerical terms. For workers who are interested in this subject, a complete set of colour fastness tests (BS 1006: 1978 Methods of test for colour fastness of textiles and leather), based on the recommendations made by the International Organization for Standardization, is available. As it is unlikely that craft workers will have the equipment or expertise to follow these special tests, some alternatives are given here. The basis of these alternative tests is the comparison of performance given by the new dye with that given by one that has proved satisfactory in service.

Water fastness

Tests for water fastness are designed to determine whether water will cause fading, running of colour with spoilage of design, and staining of other goods. To obtain this information the craft worker must carry out two separate tests, each one run concurrently with a similar test conducted on dyed sisal (or other material) that has satisfactory water fastness.

Loss of colour and staining onto undyed sisal (see Figure 1)

The test is done as follows:

1. Take a small portion of sisal (about 10 cm long) dyed with the new dye, and plait it with an equal weight of undyed sisal. Secure the plait at each end, for example, with rubber bands.

2. Immerse the plait in thirty times its own weight of cold, preferably distilled, water for 4 hours (1g water = 1 ml).

3. Remove the plait from the water, separate the dyed and undyed fibre and leave to dry

4. Compare the tested dyed sisal with the untested dyed sisal to assess the degree of change in or loss of shade (if any).

5. Compare the undyed sisal used in the test with some undyed sisal that was not tested, to assess the degree of staining.

Repeat steps 1 - 5 with dyed sisal {or any other material) that is known to have satisfactory water fastness. If the degree of change in shade and staining for the sisal dyed with the new dye is the same or even less than that of the satisfactory dyed sisal or other material then it is safe to use fibre dyed with the new dye.


Figure 1: Water fastness testing: loss of colour and staining onto undyed sisal

Loss of colour and staining onto wool and cotton (see Figure 2)

The test is done as follows:

1. Take two pieces of cloth of approximately equal size (a 5 cm or 2 in. square is suggested), one of which is undyed wool, and the other is undyed cotton, and stitch the cloths together along one edge.

2. Take a portion of sisal dyed with the new dye weighing about half of the combined weight of the cloths and spread this evenly between the two cloths.


Figure 2: Water fastness testing: loss of colour and staining onto wool and cotton

3. Wet the test specimen with cold, preferably distilled, water.

4. Place the wet test specimens between two glass plates weighing approximately 50 9 each and of a size similar to that of the cloths (a 58 mm square of 6 mm thick glass plate weights approximately 50 9).

5. Place the 'sandwich' in a dish and cover with cold, preferably distilled, water.

6. Press the top plate evenly and lightly to remove bubbles of air from between the plates, and then allow to stand for 15 minutes.

7. Without disturbing the plates, pour off the water from the dish (a film of water should remain within the plates).

8. Leave for a further 4 hours, then separate the sisal from the cloths and allow to dry.

Repeat steps 1 - 8 with dyed sisal (or any other material) that is known to have satisfactory water fastness.

9. Place the dyed sisal from the test alongside a portion of the dyed sisal of similar size that has not been tested, and compare the contrast (see Glossary) with that shown by similar portions from the test on the satisfactory dyed sisal. If the colour under test shows equal or less contrast than the proven colour, the fastness with respect to loss of colour is equal to or better than the proven colour.

10. Similarly, place the cloths from the test alongside similar-sized portions of the same materials that have not been used in the tests and compare the contrasts shown with those shown between similarly arranged cloths from a test on the proven colour. Equal or less contrast demonstrates equal or better fastness with respect to staining.

Wool and cotton have been chosen because these two materials are commonly used in clothing. However, if it is known that the fibre will be in contact with other materials, such as nylon or polyester, then a similar test using cloths made of these materials should be carried out.
If in either of the water fastness tests the fibre dyed with the new dye produces a worse staining or has a greater loss of or change in shade than the satisfactory dyed sisal (or other material), then an alternative dye should be tested.

Light fastness

It is important that, having spent much time and trouble on an article, the dyes do not fade quickly.

There are a number of standard tests for light fastness which are used by manufacturers and test houses. Basically these involve comparing the fastness of the test material with that of materials dyed in order of increasing fastness. Materials which are to be used for window curtains have to be of the highest light fastness (8 on the International Organization for Standardization (ISO) scale) whilst for cheap 'throw away' goods such as party hats, light fastness is unimportant. These international standards require special equipment which the craft worker may find difficult to obtain. Fortunately adequate results can be obtained with quite simple equipment (see Figure 3).

A satisfactory test may be carried out by taking a bundle of dyed fibre (about 4 cm long) and mounting it on a card next to a similar-sized bundle of coloured fibres which are known to have satisfactory light fastness. About a quarter of each fibre is covered with an opaque card or aluminium foil and the whole assembly placed under glass in a position facing the mid-day sun. The glass should be at least 5 cm away from the fibre and should allow free circulation of air over the fibres. The specimens are examined daily for fading (shown by a sharp contrast (see Glossary) between the covered and exposed portions) and the performance of the new colour is compared with that of the proven colour. During the test a further cover may be placed over some of the exposed portion, so that the performance over the short term can be compared with that over the long term.


Figure 3: Light fastness testing

The colour used for comparison purposes can be on any fibre, it need not be on sisal. As with the water fastness tests, the dyer can judge from the results whether the new dye is likely to have an acceptable light fastness on sisal.

Light fastness varies with the quality of light. Therefore, in different locations or with different light sources, different light fastness ratings could arise.

Fastness to processing

As mentioned previously, lacquering, hot pressing, and after-treatment of dyes could cause colour changes. If it is important for craft workers to know of these changes it is suggested that a small portion of the dyed fibre be subjected to the processing involved, and compared with the original dyed fibre. Any serious change of colour will then be apparent.