Felting yarn refers to any type of yarn that works for the process of felting. Felting is used on finished crochet and knit pieces to help blend the stitches together to form a piece of material with a texture much like felt. For felting to work, the piece must be made from either 100% wool yarn or yarn that contains a high amount of wool. When the yarn is put into a washing machine with hot water and lots of agitation, the fibers appear to melt together and fuse into a solid piece that won't unravel. The final piece also shrinks from its original size, just like wool clothing does if a person forgets and washes it in the washing machine.
Different yarns create different felted textures. A 100% wool yarn, often referred to as a felting yarn, will create a completely felted texture. Depending on how long the creator washes her piece, the stitches can disappear completely and leave a flat, fuzzy surface. Yarns that include part wool and party acrylic or other fiber will felt the wool and create an unraveled affect with the other fibers. Too little wool can simply cause the piece to fall apart or cause no change when felted.
The process of felting can be done with or without a washing machine, though a washing machine is recommended for best results. The creator should first stitch a small square using her chosen felting yarn and felt it to see the results. This lets her know how much the piece will shrink and how many times she must felt it to achieve the desired results. After learning how much the yarn square shrinks, she can adjust her final creation so that after felting it will shrink to the desired size.
When she is ready to felt, she chooses a felting yarn and creates her piece from it. She then places it inside a pillow case or protective cloth bag and throws it into the washing machine with a few pairs of blue jeans. She sets the washing machine on a low water setting and sets the water temperature as hot as her machine will allow. The blue jeans are there to cause agitation which encourages the felting yarn to become fuzzy and fuse together. Finally, she adds a small amount of laundry detergent or soap and lets the machine wash for one cycle.
After the cycle finishes, she stops the wash and removes the felted piece. She should rinse it in cold water once she is satisfied with the felted quality as the cold water will help lock the fibers in place. Then she should pin up the fabric in the desired shape. Once it air dries, it will hold its shape and will not become lopsided or shrink any more.