Below are the tips/tricks and techniques I use for machine felting upcycled sweaters, blankets, skirts and pants. I have credited the sources where applicable!
1. Begin with animal fibers. Only animal-fiber sweaters will felt, so look for wool, alpaca, cashmere, or a blend of these. Plant-based fibers, like cotton or rayon, will never felt. Nor, sadly, will the ubiquitous acrylic. If you find a great sweater that’s at least 80% animal fiber with a little synthetic thrown in, give it a try – sometimes, they’ll felt just fine. http://www.craftstylish.com/item/2259/how-to-felt-sweaters
2. The best assurance that the wool will felt is if it has a tag that says “Dry Clean Only.” Some wool sweaters are made using wool that has gone through a process that reduces the likelihood that it will felt. You can identify those sweaters because they will usually say that they are Machine Washable or the tag will use the word “Superwash” somewhere in the description of the wool. http://www.tryourdesigns.com/Tips%20and%20Tricks/FeltingSweaters.htm
3. Different sweaters felt differently. If you have a specific project in mind for your felted sweater, then it’s a good idea to think about how your sweater will look after felting. Heavier weight, 100% wool sweaters, such as the traditional fishermen’s sweater, will felt into something very thick and stiff – great for a laptop case, but not for a stuffed toy. A lightweight cashmere sweater will be very soft and drapey after felting – which is lovely for a scarf, but won’t have enough body for a tote bag. http://www.craftstylish.com/item/2259/how-to-felt-sweaters
4. How do I know it’s felted? When you pull your sweater out of the dryer, look for two things:
– Did the sweater shrink significantly?
– Can I still see the texture of the knitted stitches?
If the answer is “No” to the first question and “Yes” to the second, try running the sweater through the washer and dryer again. And maybe even a third time. Some sweaters just need a little extra encouragement. http://www.craftstylish.com/item/2259/how-to-felt-sweaters
5. It’s not an exact science. The thing about thrifted sweaters is, you may not know their whole history. Some 100% wool sweaters have been treated to be washable, which means they won’t felt. But the tag that would tell you this has been cut off the sweater long ago. So, just be prepared: some sweaters will resist your felting efforts. And if they do, you can still cut them up and make things from them. You’ll just have to finish the edges. http://www.craftstylish.com/item/2259/how-to-felt-sweaters
6. For your wool sweater to felt evenly, you first must cut it apart at the seams (completely cutting away the seam–sometimes they can be thick) and cut off any ribbing or reinforced seams. You can still felt and use the ribbing, but it will felt differently from the rest of the sweater. http://craftingagreenworld.com/2009/02/17/felt-your-sweater-a-felted-wool-tutorial/
7. Set aside any parts of your sweater that you might want to incorporate into a project without felting–pockets, for instance, or buttoned cuffs. Also cut off any snaps and buttons to reuse in another project. http://craftingagreenworld.com/2009/02/17/felt-your-sweater-a-felted-wool-tutorial/
8. Put them in a fine mesh laundry bag or into a tied off pillow case. When felting/fulling the sweaters in your washing machine, put the sweater into an old pillow case and tie the top closed. You can wash a number of sweaters at the same time this way and not worry about excessive fiber migration. Just make sure that the colors of the sweaters are close. I basically separate my loads into darks and lights. (Don’t wash one white sweater with a load of black sweaters, but you can add a navy sweater to a batch of black sweaters with good results.) The old pillow case will catch most of the fibers that come out of the sweater during the felting/fulling process so that they do not clog up your washing machine drain hose or pipe. http://www.tryourdesigns.com/Tips%20and%20Tricks/FeltingSweaters.htm
9. Machine-wash your pile of wool. The best results come from heat (choose a hot setting), agitation (choose a setting appropriate for heavily soiled clothes), and a little detergent. http://craftingagreenworld.com/2009/02/17/felt-your-sweater-a-felted-wool-tutorial/
10. I use hot water for the wash and warm water for the rinse when I am felting/fulling old sweaters. I use about 1/2 of the laundry detergent that I normally use in a load of wash. Do not use fabric softener as it will interfere with the felting process. Some people like to check their sweaters to see when they reach the exact level of felting/fullling that they are wanting. I usually just run the entire load and see what I get out of it. To me, that is part of the fun. But, if you are knitting something with the intention of felting it, you may want to stop and check the item more often. For this, it is nice to have a zipper closing pillowcase to make it easier to check as your item is felting/fulling. http://www.tryourdesigns.com/Tips%20and%20Tricks/FeltingSweaters.htm
11. I almost always dry the recycled sweaters in the clothes dryer. I do not remove them from the pillow cases, again to try to trap as much stray fiber as possible. I do not let them get completely dry. When they are almost dry, I remove them and I shape them and lay them flat until they are dry. You can also take your sweaters from the washer and lay them out to dry on a flat surface. http://www.tryourdesigns.com/Tips%20and%20Tricks/FeltingSweaters.htm
12. If your wool comes out of the dryer felted to your satisfaction, you’re all set! Otherwise, you can repeat the washing and drying steps to continue felting your wool. http://craftingagreenworld.com/2009/02/17/felt-your-sweater-a-felted-wool-tutorial/