In an effort to shrink my wool slippers to make them fit better and with much help from others on this site, I took the plunge and decided to throw caution to the wind and tossed the slippers in a hot wash/warm rinse and let the washing machine run its course. When they came out they looked smaller and a bit more densely packed, which is a good thing in slippers. Since they are knit with two strands of yarn, I didn't want them to stay damp too long, so I put them in a low dryer for about 15 minutes. They shrunk a bit more and fit even better now. Since they are 90% wool and 10% other fibers, I imagine they didn't shrink as much as 100% wool might.
I am now making a pair for my son and am making the smaller size, so perhaps they won't need shrinking to fit him. I have also decided to use 100% pure wool in a nice bright blue...his favorite color. I have also changed the yarn to one strand instead of two and chose the #5 bulky (100g/3.5oz, 109m/120yds) instead of the #4. It's a lot nicer to work with and looks great. I just finished the sole this evening. The wool yarn is by Patons and is called Classic Wool - Roving, which is smoother than the Tweeds version from the same company. I'll post a picture once they are finished.
In the interest of increasing my knowledge of the science of yarn, I did a bit of research and present three pictures for all who may not know what wool does when it is 1) tumbled about in a washer and dryer, and 2) washed in hot water and rinsed in warm. I also offer two articles I stumbled upon that explain why wool shrinks as it does, as well as an article from 1938 about the "newly discovered" chemical treatments that prevent wool from shrinking.
Below are pictures of the yarn and the slippers. The last picture is a side-by-side comparison of a slipper before and after shrinking - top= preshrunk, bottom=shrunk (felted).
Thanks to the guys who gave me some good information before I risked the shrinking process.
Okay, class dismissed.