I've been working on a scrap yarn granny square blanket for a while now out of many different 100% animal fiber yarns, yarn that was left over from other projects or which I'd bought 1 skein of on sale or because I couldn't leave it behind. You know the way that works. Anyway, I had occasion / necessity to wash it last night -- actual washing, too, not just a gentle soak in some lukewarm water. (Fracking cats and their fracking urine.) I washed it in the washing machine, with cold water, on the delicate cycle, 2 minutes of agitation, with regular old Tide detergent, and then ran it in the dryer on Fluff Air / No Heat for an hour. The different yarns reacted VERY differently to this treatment; some of them are basically unchanged, some of them felted very aggressively. The blankie itself is crocheted, but I figured there was enough felting interest here that the behavior of the yarns was of general use.
Bear in mind, this was, like, the Anti-Felt Wash. The only thing I could have done differently is put enough vinegar in the wash water to make the whole thing acidic. No heat, no temperature changes, not a lot of agitation.
Anyway, the results.
In the Basically Unchanged Category: Schaefer Helene (50/50 merino/silk), Noro Kureyon, some random single-ply heavy worsted yarn I picked up in a remnant bin. All three of those, you can't even tell they've been through the wash; they look exactly the same. Also some Dale Falk double-stranded with laceweight mohair, but that's superwash so I don't think it counts.
In the Enh, Yeah, That Went Through the Washer All Right category: Cascade 220 (dark eggplant heather), Jaimeson Tweed (sage green), Mountain Colors Weaver's Wool Quarters (all colors). These have fuzzed up, fluffed out, lost some stitch definition, but still maintain the integrity of the yarn.
In the Holy Crap Thank God I Didn't Use Hot Water category: Cascade Pastaza, Mountain Colors Mountain Goat, and (the GRAND PRIZE WINNER OF ALL TIME) Mountain Colors 4/8's wool. In all three of these, the squares compacted, the fibers started to lock together, individual stitch definition is gone, it was a struggle to get the hook through the edges to add the next row. The 4/8's wool in particular is practically just a solid piece of felt, and the only reason it's not half its previous size is because the weight of the other squares in the dryer kept it pulled out. The fabric is unbelievably soft and snuggly, and this was a project to use up leftovers, so I'm not terrifically sad about it, but I honestly would not have thought that 2 minutes of cold agitation and then no-heat tumble drying would have affected the wool so much.
Interestingly, all the Unchanged yarns are single-ply, and the 4/8's wool is 4-ply. Pastaza is single-ply 50%wool, 50% llama (I think) and Mountain Goat is 2-ply 55/45 wool/mohair. If I were designing a project to be felted, I would definitely choose multi-ply wool over singles, after this experiment. Density of the work didn't seem to affect the feltability that much; the Pastaza was the densest square, and the Weaver's Wool Quarters squares were the least dense, but the Random Single Ply yarn that was so unchanged was nearly as heavy as the Pastaza, and the Mountain Goat wasn't that much heavier than the Quarters.
Who knows if anyone will find this information as fascinating as I do? But I figured that since we get a lot of questions along the lines of "what yarns are good for felting," the results of this experiment were probably worth committing to pixels.