I’ve been dyeing wool fleeces and handspun yarns with natural sources for nearly 20 years. One thing that perplexed me was the way the yarn would often appear somewhat variegated, with lighter and darker segments. A few years ago, it finally dawned on me what the cause was. It’s urine staining. Urine was often used as a mordant, so it stands to reason that the parts of the sheep’s fleece that got soaked with his own urine (the yellowy parts) would take dye differently from the fibers higher up on his back. This can be avoided by making certain not to card urine soaked portions with the cleaner parts of the fleece. Unless, of course, you’re like me and actually prefer the slight variations, often a hallmark of handmade items.