My questions concern finishing and care of pieces, made from yarns that are a blend of fibers with different properties, such as a blend made of cellulose-based and protein-based fibres. I have a cone of rayon and wool blend that appears to be roughly 40% wool and 60% rayon, and I am not sure whether to just treat it as if it were 100% rayon.
Along the same lines: assuming I don’t want a differential shrinking effect, are there some combinations of yarns that I would want to avoid using together in either the warp or weft. I am particularly interested in rayon yarns and what other fibres “play nicely" with rayon.
It’s hard to make sweeping generalizations about finishing methods. Unless I’m weaving a dish towel, which I want to continue to wash by machine in hot water, I usually wash everything by hand in room-temperature water. That way, I can observe changes in the yarns. If I see something happening that I don’t want, I rinse and dry. If I see something that I’d like to happen more, I increase the agitation (for wool) and for cellulose yarns, if I want something to happen “more,” which would usually mean shrinkage, I’d use hot water (this would be an unusual goal).
So, if you use yarns that are blends, even unknowns, you are often safe just washing by hand in cold water with little or no agitation. Wool fulls, thereby shrinking, and rayon shrinks, so I’d treat them carefully but similarly. The wool will do its shrinking and fulling with agitation, so avoid agitation unless you want that effect.
Differential shrinkage is achieved by using yarns that do not shrink with those that do. My experience is that rayon shrinks, but it might not do so enough to contrast much with another non-shrinking yarn (linen or cotton, which shrinks, but not as much).
In general, you are going to have to sample to be sure how your fabrics will react to washing. You can wind a simple sample warp of stripes of yarns (maybe 1/2 to 1 inch wide each), and finish a number of samples in different degrees as an instructive experiment!