Evaluation of color transfer and of color variation.
There are two kinds of gray scales. The first is used to express an objective valuation with regard to the transfer of color from the test piece to another material. The second scale is used to give a numeric value to the variation of color, i.e. to the degradation and color change of the test piece due to factors such as light, humidity, heat, etc.
SCALE TYPE 1: This gray scale for assessing color transfer is used when measuring the color fastness of a certain material and of its resistance to the transfer onto another material, by comparison with standard parameters valid in every place and under every condition. The scale comprises 10 pairs of plaques: a pair of identical white plaques, and 9 pairs made with a white plaque and a gray one, with the gray changing in intensity, with different gray tones indicating different fastness. The two white plaques have a fastness of 5, so the difference of color between them is zero. Fastness values from 4/5 to 1 correspond to pairs of plaques of which one is white and the other gray. The increasing contrast between the two plaques corresponds to a decreasing value of fastness; therefore, the pair of plaques formed by white and the darker gray represents a fastness of 1. After submitting a sample to test, the gray scale will serve to quantify color transfer. So we compare the felt used for rubbing against a new one, then compare them to the 10 pairs forming the scale, we look for the pair with the contrast closest to the two felts. In this way, we can provide an objective assessment of the quantity of color that was transferred to the felt during rubbing.
SCALE TYPE 2: This color-variation gray scale is used to compare the fastness of the color of a material and its resistance to agents that can change its intensity, such as sunlight. The scale is composed of 5 pairs of gray plaques. In each pair, the difference of intensity and contrast between the plaques is variable, with different contrast corresponding to different values of fastness on a scale that goes from 5 to 1. A fastness value of 5 corresponds to an identical pair of plaques, with a color difference of zero. On the other hand, a fastness value of 1 corresponds to the highest degree of contrast between the two gray plaques. Use of this gray scale is analogous to the preceding one: after testing a piece of leather, this piece is compared with the original material, and the contrast between the two is compared with the contrast within each pair of the scale. In this way, we can provide an objective assessment of the color variation.