community of dyers

Viscose rayon, which is what we usually see labeled as rayon, takes up fiber reactive dyes extremely well, better than most cotton, at least if you follow the recipe. (See How to Dye Rayon.) I suspect that your rayon dresses might not be 100% viscose rayon. Maybe they are part rayon acetate, which can't be dyed well except with disperse dye, the same type you use for polyester. Disperse dyes were originally developed for acetate. Acetate has been chemically altered to the point where it is no longer a cellulose fiber, unlike viscose rayon.

Another possibility is that your rayon dresses may have a surface finish that is blocking the dye. This finish could be there for the purpose of stain resistance, or to make the fabric permanent press. Sometimes you can remove these finishes by scouring in very very hot water with detergent plus extra soda ash, but a lot of these finishes are essentially permanent, impossible for you to remove.

In either case, if your piece of cotton dyed well, then you're probably doing everything right. Did you closely follow ProChem's recipe for immersion dyeing, using the right amounts of soda ash and salt? (Here's a link to their page of product instructions.) Was your dyeing temperature as warm as their recipe requires? If so, then the dresses are simply not dyeable. I'm sorry that you invested in them, because there's not much you can do about the problem now. Don't buy dresses from that source again. Look for PFD dresses in the future (PFD stands for "Prepared For Dyeing").

The only hope is if you realize in retrospect that you omitted the soda ash or some other requirement of the recipe you used. (Were you using ProChem's immersion dyeing recipe?) If you left out the soda ash, say, or the salt (which is required for high water ratio immersion dyeing, but not for low water immersion dyeing or tie-dyeing), then the solution to the problem will be simple.

When you say that you use ProChem's Deep Black, do you mean that you used their Procion MX dyes? They call them their "PRO MX" dyes. This is the best sort of dye to use for viscose rayon. ProChem sells a lot of different dyes with similar color names, so it's not completely clear, but you said that it worked well on cotton, so that means it probably was their Procion MX type dye.

You absolutely should not try ProChem's WashFast Acid dyes on rayon. Since rayon is made of cellulose, it will not dye well with acid dyes, which are intended only for polyamide fibers. Silk and wool are polyamides, and so, surprisingly, is nylon, but no cellulose-based fiber can be dyed with acid dyes. Acid dyes might stain rayon a little, depending on the dye, just as they might stain cotton, but they will not bond to it properly. Boiling water won't help, since WashFast Acid Dyes are unsuitable for cotton and rayon at any temperature. If the dresses contain any acetate, or any polyester, then that portion of the fiber can be dyed only with disperse dye, which would in my opinion be far more trouble than it's worth.

Since the dresses did take some of the dye, maybe you should try again with purple, to counteract the yellowish hue. Even if you can't obtain a true black, you might be able to get a much nicer gray.

I'm sorry I don't have any better suggestions.

-Paula