I wrote to Greg Driscoll at Still River Mill/GreenerShades, and he gave me more information about their approach to working with dyes and fibers. As owners of Still River Mill, they process all sorts of fibers. Here is some of what he had to say:
"In keeping with our environmentally friendly philosophy, we really try to do things as nontoxicly as possible here in the mill, hence the GreenerShades dyes in the first place. The dyes have been tested on a lot of different fibers, including nylon swimsuits, and they outperform other dyes in most cases. "
(Sheila's note: I wrote to Greg and mentioned that in my experiment I had used some superwash/nylon yarn, and his answer gives us more insight into the reactions of fiber to process. Read on ...)
"Thank you very much for the writeup on your blog and the link on your site. The superwash process is quite toxic, so we don't purchase superwash fibers. The process uses several chemicals to clean the wool and then applies a synthetic resin to essentially glue down the scales of the wool. (This lessens the felting that happens when you wash and dry wool.) This process adds many more dyepoints to the fiber, allowing the fiber to absorb a lot more dye. Hence your deeper shade on the superwash.
The light spots on your yarns are usually caused by the skeins not being clean enough before you dyed them. There is something on the wool and usually it is the oils applied in the spinning process, natural lanolin, or scouring agents that prohibit absorption of the dye."