Grading systems have been developed to explain the diameter difference in wool fibers from different breeds of sheep, and even the variations within the same fleece. The commercial wool industry depends on grading for sales and manufacturing. Hand spinners can look to grading systems for selecting the right fiber for different projects. They are also a helpful guide for flock improvement.
The Blood System was an early method of grading fleeces in Colonial times, and was based on the amount of Merino breeding or blood in the sheep. Merinos were highly prized then, as now, for thier fine diameter fiber. Wool from a sheep containing 1/2 Merino blood and 1/2 common blood was called "1/2 Blood", etc. The Blood grades eventually designated fiber diameter (fineness), regardless of the breeding. The limitations of this system were exceeded by the development of new sheep breeds.
The Counts System (also called Bradley System) indicates the average diameter of a fleece's fibers. They range 80's to 38's and are equal to the number of hanks of yarn (560 yards) that can be spun from one pound of clean wool. For example, Grade 44's yield 44 hanks per pound of clean wool, or about 14 miles of wool!
The Micron Count System is now used extensively and is very precise and accurate. Sixteen grades in total are used and are based on the average fiber diameter measured by a lab micrometer. Limits are set for how much variation is allowed within each grade. A micron is 1/25,400 of an inch, so an 80's by the Count System averages approximately a8 microns, while a 36 averages about 39 microns in fiber diameter. Wool that varies too much is placed down one grade.
None of the grading systems measure characteristics such as staple length, uniformity, yield, crimp, color, stregth, purity or character. These eight factors, along with the grade, determine the market value of the wool.