As all of you know, turning a wet mound of fiber into seamless fabric is a pretty darn amazing process. How does it happen, you ask? It turns out that the unique characteristics of some fibers allow them to felt. Here’s some info about feltable fiber, taken from page 93 in Uniquely Felt.
Felters aim to create a stable, immobile felt with fully entangled fibers, and so they usually lean toward fibers without high elasticity because elastic fibers spring apart, which counteracts the felting process, and can create a hairy felt. (FYI, spinners DO search for elastic wool that will create resilient yarn when spun. They also prefer long-stapled fiber, i.e. longer fiber, which is easier to spin but creates a hairy felt).
Crimp is different from elasticity; crimp refers to how many kinks there are in a fiber, and elasticity means how well it expands and contracts. Sheep fleece often contain three distinct kinds of fiber: the undercoat, the outer coat, and kemp. The outer coat repels water and keeps the undercoat dry. Undercoat fibers are next to the skin, trap the sheep’s body heat, and help keep them warm. Kemp fibers act as spacers and prevent matting and skin disease. In some sheep, undercoat and outer coat fibers are very similar, and the fleece is spongey and elastic. In many other breeds, the undercoat is finer and the outer coat is straighter, coarser, and hollower inside. Kemp fibers (which not all sheep have) are hollow and often even straighter than outer coat fibers. They are very brittle and will often break.
As should be expected, outer coat and undercoat fibers felt differently and at different rates. The finer undercoat fibers felt faster and tighter than coarser fleece. They are often dull in appearance and are soft and nice to wear near the skin. Large-diameter outer coat fibers often have a sheen and create more durable felts that can stand abrasion.
For more information I suggest that you refer to page 95 of Uniquely Felt. You will find information about specific breeds, their micron count, and how they felt. Unsurprisingly, Christine recommends experimenting and blending different kinds of wool for different effects. She also recommends making felting samples for your reference. We’d love to hear about some of your favorite breed combinations.