Natural fibers can vary greatly in fineness. To grade a set of fleeces/fibers, it is helpful to have the results of a histogram test, and specifically the Average Fiber Diameter (AFD). The AFD is the measurement commonly known as the micron count (because the equipment used measures each fiber in microns) and represents the average diameter of the fiber sample measured in microns. It is also possible to sort and grade fibers without this test. The professionals who do this are able to sort raw fiber into ranges of only 3 microns at the rate of 150-200 pounds a day! That takes a lot of practice! So, either with the help of the AFD from the histogram test or the help of a professional sorter/grader, natural fibers are categorized by their micron. There are six categories, or “grades,” used in this process. 

Grade

Name

Microns

Use

Grade 1 Ultra Fine (Royal baby) < 20 microns Next to skin
Grade 2 Superfine (Baby) 20-22.9 microns Next to skin
Grade 3 Fine 23-25.9 microns Most versatile grade Yarn and textiles
Grade 4 Medium 26-28.9 microns Socks and blankets
Grade 5 Coarse 29-32 microns Mattress pads, quilt batting, rugs, seat cushions,
Grade 6 Robust 32.1-35 microns stuffing for pet bedding, insulation

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