Thanks for the advice. This is a lot more than a stain. There is material caked on. I may try beating the cloth or even just working the cloth with my hands to break the material up better. I also like the idea of washing in water -- wool safe soap -- and slow drying (not in clothes dryer).
I also love wool for hunting. It is silent. It is warm when wet. Additionally -- and this is something I haven't heard from others, maybe I am deluding myself -- it seems to me it has a broad temperature comfort range. I've worn these M-51 trousers over wicking long underwear in 20 degree weather hunting elk at 11,000' and I've worn the same trousers over long underwear at 70 degrees hunting deer at 500'.