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Rayon, also known as viscose, is a reprocessed cellulose fiber, made from wood pulp. It was the first manufactured fiber, and yet, since it is made of cellulose, it dyes and feels like a natural fiber. It absorbs moisture from the skin like cotton.
Most bamboo is actually rayon, so you're on the right page for information about how to dye bamboo.
Rayon can be dyed with the same dyes that work on cotton, but it often produces deeper, more brilliant colors. Rayon dyes most beautifully with cold water fiber reactive dyes. It can also be dyed with direct dyes and all-purpose dyes, though the colors from these less washfast dyes will fade quickly unless a cationic aftertreatment is used to fix them. Vat dyes, including indigo, and naphthol dyes can also be used to dye rayon. If you use natural dyes on rayon, premordant it as you would cotton; it will not bond to most natural dyes as well as wool does.
The best way to dye rayon is to use a fiber reactive dye, such as Procion MX dye. See How to Hand Dye for instructions. To dye a single solid color, use a washing machine or bucket, with a large ratio of water to fabric, and constant stirring.
Rayon has one significant flaw, compared to other cellulose fibers: it has very poor wet strength. When rayon is wet, it can be torn or abraded very easily. This is the main reason why rayon is often labeled "dry clean only" (though in some cases the reason may be a water-soluble dye or other fabric treatment).
If you dye rayon, or machine wash it, you must take care to avoid stressing the fiber. It is safer to hand wash separately than to machine wash. If you do machine wash, use the delicate cycle, and expect an occasional failure. Never wash rayon with heavier garments, such as blue jeans. Sorting by weight before washing is very important.
I always machine-wash my rayon clothing, in cold water, and tumble-dry it (at low temperature) as well, which minimizes wrinkles. This is a little risky, since some rayon is too weak to survive machine washing. I've had only one or two failures, in which the rayon showed damage. Generally this damage appeared after the first washing, indicating that the garments that failed were inherently weaker in some spots than other items. Depending on your provider and on how gentle your washing machine is, you might have more problems with shredded rayon fabric.
One of the best available sources for ready-to-dye white rayon clothing blanks is Dharma Trading Company. Many of their rayon clothing blanks are sewn with cotton thread, which takes the same dyes as the fabric.
If you buy rayon clothing that is not marketed as being ready to dye, be careful to avoid finishes that will resist dye, such as permanent press, stain-resistant, or water resistant finishes. Each of these will repel dye and interfere with dyeing.
The book Reactive Dyes in Biology includes a mention of the pH required for rayon to react with Procion MX type dyes. The optimum pH is between 0.5 and 1.0 pH units higher for rayon than for cotton. In practice, however, there is no need to worry about this; the usual amount of soda ash will work just fine.
Most of the newly popular bamboo fiber clothing, fabric, and yarn that is currently available is actually a form of rayon that is made from bamboo pulp instead of wood pulp. Only a small fraction of commercially available bamboo is made by separating and spinnning the fibers that are grown by the bamboo plant. In either case, dye as for rayon. More rarely, you may find bamboo that has been acetylated to form rayon acetate (see below), which I suspect may be responsible for a very small number of reports of bamboo's dyeing poorly. Usually, bamboo dyes wonderfully with fiber reactive dyes.
Don't confuse rayon, which is also known as viscose, with acetate, also known as rayon acetate. Acetate is made from rayon, but it is anything but a natural fiber. Acetate cannot be dyed with the same dyes that are used for rayon. In order to dye acetate, you need to use disperse dyes, just as you would if you were dyeing polyester. One way to tell whether your fabric is made of acetate is to try dissolving a small clipping from it in acetone, which is in nail polish remover. Rayon will be left undamaged, but acetate will dissolve.
Lyocell is, in practice, extremely similar to rayon. It is supposed to have greater wet strength, but I have not noticed that it wears any better in the wash than rayon. Dye Tencel just like any other reprocessed cellulose.
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