Gray Scale for Color Change and Color Staining

What is Colorfastness?
Colorfastness is the resistance of a textile material to specific chemical agencies. Poor colorfastness in textile products is a major source of customer complaint. The fastness of a color can vary with the type of dye, the particular shade used, the depth of shade and how well the dyeing process has been carried out. Dyes can also behave differently when in contact with different agents, for instance dyes which may be fast to dry-cleaning may not be fast to washing in water. It is therefore important to test any dyed or printed product for the fastness of the colors that have been used in its decoration.

There are a number of agencies that the colored item may encounter during its lifetime which can cause the color either to fade or to bleed onto an adjacent uncolored or light colored item. These factors vary with the end use for which the product is intended. For instance carpets and upholstery are cleaned in a different way from bed linen and clothing and therefore come into contact with different materials. The agencies that affect colored materials include light, washing, dry cleaning, water, perspiration and ironing. There are a large number of color fastness tests in existence which deal with these agencies and a full list will be found in the British Standard. A further group of tests is connected to processes in manufacturing that the colored material may undergo after dyeing but before completion of the fabric, processes such as decatising or milling. Despite the fact that the list of color fastness tests is very long, most of them are conducted along similar lines so that the main differences among the tests are in the agents to which the material is exposed.

Colorfastness is usually assessed separately with respect to:

Changes in the color of the specimen being tested, that is color changing / fading;
Staining of undyed material which is in contact with the specimen during the test, that is bleeding of color.

In order to give a more objective result a numerical assessment of each of these effects is made by comparing the changes with two sets of standard gray scale, one for color change and the other for staining.

Gray Scale:
A “gray scale” version of an image uses a mix of black and white to represent the “value” of the colors. Gray scale is used for visually evaluating changes in color of textiles resulting from colorfastness tests. There are two types of gray scale – one for assessing color change and another for staining. The color change gray scale consists of nine pairs of grey- colored chips, in grades ranging from 1 to 5. The staining scale consists of nine pairs of grey and white-colored chips, in grades ranging from 1 to 5. They are manufactured strictly in accordance with ISO 105- A02 and AO3.

The fastness rating goes step-wise from:

Note 5 = no visual change (best rating) to Note 1 = a large visual change (worst rating).

The gray scale has nine possible values: 5, 4-5, 4, 3-4, 3, 2-3, 2, 1-2, 1.

These two gray scales are described below.

Gray Scale for Color Change:
In colorfastness tests, it is necessary to determine the magnitude of the change in color between the original sample and the sample after testing. Slight changes may be acceptable to the consumer, whereas large changes would certainly be unacceptable. The correct use of the gray scale is important, and the angle of viewing, and the quality and intensity of radiation used to view the samples and the gray scales are specified in the standards, in order to minimize operator bias.

These scales consist of five pairs of grey colored material numbered from 1 to 5. Number 5 has two identical greys, number 1 gray scale shows the greatest contrast, and numbers 2, 3 and 4 have intermediate contrasts. After appropriate treatment the specimen is compared with the original untreated material and any loss in color is graded with reference to the gray scale. When there is no change in the color of a test specimen it would be classified as ‘5’; if there is a change it is then classified with the number of the scale that shows the same contrast as that between the treated and untreated specimens.

Gray Scale for Color Staining:
There is also a second set of gray scales, known as the gray scale for staining. The result of staining from a colorfastness test is rated by visually comparing the difference in color or the contrast between the stained and unstained specimens with the differences represented by the Scale. Here, the first pair rated 5 are white, whereas the contrast again increases with the second shade becoming grayer as the rating decreases. This scale is used to assess the wet and dry rubbing fastness test as well as several others that measure the transfer of color to other substrates, including washing fastness. A chromatic transference scale for assessment of rubfastness tests, with a range of different shades, can also be used.

A different set of gray scales is used for measuring staining. Fastness rating 5 is shown by two identical white samples (that is no staining) and rating 1 shows a white and a grey sample. The other numbers show geometrical steps of contrast between white and a series of greys. A piece of untreated, unstained, undyed cloth is compared with the treated sample that has been in contact with the test specimen during the staining test and a numerical assessment of staining is given. A rating of 5 means that there is no difference between the treated and untreated material. If the result is in between any two of the contrasts on the scale, a rating of, for example, 3-4 is given. Gray scale for color staining which are shown in Figure 2.

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Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.