Posselt's Textile Journal 7, 1908
One of the most important properties of a dyestuff is its fastness to light, sun proof as it is sometimes called, and which property is more important than those fast to washing or to ironing. The test of a dyestuff with reference to in fastness to light refers naturally to an exposure of the dyed fabric to the direct rays of the sun, but it is very difficult to get comparative results, on account of the long time required for the test, and during which the intensity of the light constantly changes. To ascertain the fastness to light of a dyestuff, it is necessary to expose the fabric dyed with the dyestuff in question, to the rays of the sun, side by side with the same make of fabric but dyed with a dye whose behavior to light is well known. Such a dyestuff as indigo carmine, and where repeated experiments have shown that this dye changes on exposure to varying daylight with more uniformity than any other known dye. If, then, the fabric to be tested is exposed to the direct rays of the sun, together with another piece of the same fabric, but dyed with indigo carmine, the fastness of the dyestuff to be tested can be judged according as the color alters more or less quickly than that of the indigo carmine. If the behavior of the two samples is the same, the dyestuff to be tested can be considered as satisfactorily fast, whereas if the indigo carmine sample changes first, the dyestuff to be tested can be considered as extremely fast, while, if the reverse is the case, the dyestuff may be fast enough for some purposes, but not for others, and when further experiments may be necessary to be made.
- (Dr. Jacobi in Chemiker Zeitung.)
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