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If one color change is taking a while to happen on one bobbin, edit it out -- break off that single and move up to where you want the color to be. I like a little unevenness for heathering/marling between the color changes, but more than a few yards and I'm breaking my single, editing out a bit, and spit-joining the two ends of that single (or actually really joining them, if you can manage it) back together again.
In the picture, the right glove (with the heathered color changes) was spun this way -- with editing! -- and the other was Navajo plied to keep colors separate.
Fractal plying! This technique is described by Janel Laidman in the Summer 2007 Spin-Off; I've done it twice now, and love it. Split your roving in half lengthwise, spin one half onto one bobbin as-is, drafting-drafting, drafting. Take the other half and split it more -- I try to split it into at least 4 pieces lengthwise, then spin those lengths onto a second bobbin. When you ply the two bobbins, you get pleasing color changes where one ply changes more slowly than the other, leading to dominance of the color in the "slower" single -- it looks really cool, like subtle background striping, knit up.
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The picture shows handspun singles.
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Well, I bet there are more ideas than this even. A Navajo 3-ply is the best way to keep the color changes "clean", or to at least choose how clean you want them to be
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See earlier posts on this topic and material mentioned here:
How can I preserve color in a 2-ply?
How can I preserve the color in my space-dyed roving when I spin it?
How do you organize your spinning?
Do you want to play with thread?
posts on spinning singles
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Do you have suggestions on color handling in space-dyed roving, links to interesting samples of this, or questions on the topic? Please post a comment here or contact me. Thanks!