This post is the first in a series of three I plan to write targeting the properties of wool and how they affect a finished yarn. Most knowledge is derived from my own experience, and these posts are not meant to be scientific in nature. They are meant to be loosely informative and useful to the home handspinner.
They're not midichlorians (watch: Star Wars), but they're still pretty magical. Or mystical, whatever. Microns are a measure of fiber diameter, and a typical indicator of the fineness and softness of a fiber. The smaller the micron count a particular wool has, the softer it is to the touch. The larger the micron count, the more hairy or wiry the wool feels.
Micron counts typically range from around 18 to 35, but can be as low as 10 (the record for an entire fleece), and higher than 40. Micron counts are usually an average across a particular fleece or batch of fleeces, and can have a large range within that sample. The quality of a fleece, no matter the micron count, is high if that fleece has a consistent micron count (the range of fiber diameter in the fleece is not large). Table 1 shows the differences between breeds of sheep and their average fiber diameter, and where they fall in the spectrum from fine to coarse.
The reason these numbers are important is because wool eventually becomes yarn that is made into fabric (knitted, crocheted, or woven). The softness of the wool, as well as staple length and yarn construction, determines what the finished yarn should become. For instance, a super fine Merino wool with a micron count of 18 would not make a good sock yarn (for a variety of reasons, and despite those reasons, it is the most common breed used for socks). Similarly, a Lincoln fleece made into yarn would likely not make a good baby sweater due to the micron count being 35-40. The highest of micron count fleeces are used to make rugs, blankets, and for insulation.
The contruction of your yarn, while it will not change the micron count, will change the appropriateness of a fiber to be used in certain projects. It may make an Icelandic wool feel softer for a hat, or a Merino/Rambouillet wool feel more durable for use in socks. I implore you, as a handspinner or knitter, to work with all kinds of fiber, and not just the soft and common Merino that is widely available. It is fun to work with both a Merino and a Lincoln, then realize that those two sheep were bred to form another breed, Corriedale (one of my favorites to spin), and then spin that fiber too, to see which characteristics it has in common with its parent fibers.
It is important to add that the micron system is not the only system in the United States that is used to measure fiber diameter. The American Blood Grade System and the English Spinning Count system are also used, which is sometimes you'll see things like "80s Australian Merino" in item descriptions. The micron system is the most widely used, and also the one I'm the most familiar with and comfortable with using.
Microns, for the hand spinner, are only a piece of the knowledge necessary to competently design yarn. They determine the fiber's limitations (or lack thereof - I'm looking at you, BFL) in terms of softness and comfort. For the handspinner, spinning a yarn that is the appropriate softness for the project at hand starts with micron count. The next property of wool we will talk about is staple length, and after that, crimp. These properties are important to understanding your finished yarn, and to design yarn that you will use with intent.
Is the yarn you like to use different in micron count from the fiber you like to spin? What is your favorite micron range to knit and spin with? Let me know in the comments.
Wool Grades: http://aces.nmsu.edu/pubs/_b/B409/welcome.html