When creating pictures or three dimensional forms, the traditional thing to needle felt with is wool fiber and that's because the results are consistent and beautiful. Accents can be added with synthetic yarns, etc. while the main body of the work is wool. Needle felted items are very tactile, good to touch or hold in the hand. Wool is available in a wide variety of colors and types, so how to choose the perfect kind?
You can needle felt anything, right? Well, you can certainly try. Those magic needles will penetrate many things, including your fingers! Some things will melt to each other easily, some not so easily. Industrial needle felting is used to create things like carpet underlay and boot liners and quilt batting, to name but a few. These, as you may know, are not often wool.
Crafters and artists choose the wool which will give the best results for the project they are doing, so here's a guide to wools and what I have found works best for pictures and 3-D forms. Each breed of sheep has a different kind of wool. It depends where the sheep lives, what it eats, etc. Wool is measured in microns. The finest wool would have the lowest number, 10 microns being very fine merino. This fine fiber is fabulous for spinning and wet felting, but hard work to needle felt. The longer, finer fibers take a long time to melt together with the needle and the work retains a fluffy look.
My preferred wools are Corriedale, mixed bred, Romney and coarse merino. Corriedale wool is soft, yet needle felts well. This fiber will work up fast and produce a smooth finish to the piece. Mixed bred is just as it sounds, coming from a mixed breed of sheep. The wool is perfect for needle felting, being fairly coarse. Finn wool (from Finland) and Gotland behaved similarly. I mentioned merino as being too fine, but merino comes in many microns. The heavier merino, which has been carded and made into batts, has a rougher appearance and needle felts very well. This is what I use for details like facial features or hat bands on Santa figures. Leicester and alpaca (which is not a sheep, being related to llamas) are also good for needle felting. Alpaca tend to be expensive, but if someone offers you some, give it a try!
Wool referred to as core fiber is generally un-dyed wool in a natural color like white or gray. It can be coarser and often has bits of grass in it. Use the cheaper kinds of wool as core fiber. Coarser wool will work up more quickly into a shape, which can then be covered with your chosen color of fiber.
When buying fiber, it is available as batts or sliver (or roping / roving). If you can find batts, they are easier to work with. Sliver and batt will both needle felt, it's just that sliver has been combed so all the fibers lay in one direction, ready for spinning or wet felting and batts have a tangled, loftier layout of fibers. I prefer batt, because you can peel it apart in thin layers and build your shapes up gradually. With sliver, one must be careful to keep the layer even and not lumpy.
All the fibers I have discussed will also spin and wet felt beautifully, so if one does not work for your needle felting, try wet felting it! It's an interesting exercise to try out the different methods of working with different fibers.
I hope this article has given you a guide to use when selecting fiber for needle felting. It's by no means a comprehensive list of fibers, just those I prefer to work with. Have fun and experiment with this very satisfying craft.