A recent addition to the spectro-guide’s list of color indices is the Grey Scale. The Grey Scale is one of the most important tools to evaluate color shading and staining, and is widely used by the automotive industry to measure the fade of the fabric in a vehicle’s interior. It is based upon both ISO and AATCC specifications.
Typically Grey Scale evaluation is performed by comparing dyed or weathered fabric samples with a set of sample tiles with varying shades of gray on them. Grey Scales have grades of 1-5, increasing by half grades (1, 1-2, 2, 2-3, 3, 3-4, 4, 4-5 and 5) with a Grey Scale grade of 5 being the best. Usually a grade of 4 is acceptable, with some customers accepting a grade of 3-4.
Un-weathered fabric is measured as the standard, with Grey Scale turned on in the instrument. The GreyScale is found at the very end of the installed Index list, found under the Option header. The instrument must be set up in the difference mode. When measured as the standard, the Grey Scale on the instrument shows a 0 (zero). If that same piece of fabric is measured again, without weathering, the Grey Scale will appear as a 5. This indicates no fading has occurred. However, fabric is checked after various periods of weathering in a sunny environment (Arizona or Florida), or in a weathering cabinet with xenon lamps, and the Grey Scale is again measured against the stored, un-weathered standard. The fabric fade must show better than a 3-4 or a 4 Gray Scale rating in order to pass. These standards are company specific.
Another test frequently run using the Grey Scale index is a staining test. Wet and dry pieces of dry cotton fabric are rubbed over a piece of dyed fabric, 10 times with a force of 1 KG. The Grey Scale difference between the original unstained white cloth and the dry rubbed cloth should be no worse than a 4. The Grey Scale of the wet cloth (dried again before measuring it) should be no greater than a 3-4 when measured against the original white cotton fabric.
Using the spectro-guide takes a lot of the guesswork out of determining the Grey Scale difference between two samples. Most fabric in automobiles is not a shade of gray, and evaluating the color change of a blue or beige, using grey standards, requires a skilled inspector. The instrumental calculation of the Grey Scale takes subjectivity and experience out of the equation. An untrained operator and a trained inspector should get the same results.
Jim Roberts is a Technical Support Manager at BYK-Gardner USA, Columbia, MD.
Color, Gloss and Weathering Seminar
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