So what is all this Micron stuff about anyway? In the United States wool is graded using one of three methods:
The "American" or "Blood" System: This is an older system where wool is graded depending upon the amount or percent of "fine wool" sheep breeding (Merino/Rambouillet blood) there is in the animal being shorn. For example, if a "grade" ewe is bred by a purebred Merino, the lamb from that mating will produce a 1/2 blood fleece or wool. Currently, the names refer to the diameter of the fiber. With this standard, "fine" refers to the diameter of the fiber, not the total quality of the wool.
The "British," "Bradford," or "Spinning Count" System: This is based on the amount of "Hanks" of yarn that can be spun from 1 pound of scoured (washed) wool. One Hank = 560 yards of yarn. If a Rambouillet grades 64s (fine grade), then 1 pound of clean wool from that sheep will yield over 20 miles of yarn (64 X 560 yards of yarn). This system’s grade numbers are always even numbers, with the lowest being 36s and the highest 120s. With this system the larger number will be a finer wool. The problem with this system is that there are too many variables that influence the results.
The "Micron Count" System: This system was developed in the United States around 1966-67. It is based on actual micron measurements taken with a microprojection unit (magnification 500 X). (One micron = 1/25400th of an inch). Generally, most major wool producing regions of the world utilize this system. It is considered an objective system because it uses scientific measurements to help with the grading.