Region: California, USA

Message: I've been trying to dye a dress that is 65% polyester and 35% rayon:

1) I tried dying it with "iDye by Jacquard Products" black for natural fabrics (silk, cotton, linen,wool and rayon); this left my light gray dress looking somehow darker, and reddish.

2) The second time, I used the same brand but the iDye Poly for 100% polyester and nylon; this left a better and more solid dark gray with reddish tones. 

I'd like to dye this dress jet black. How and what dye to I need to use to acomplish this?
Thanks.

Your choice of dyes so far has been good. iDye is a direct dye that should work to color the rayon portion of the fiber blend, while iDye Poly is a disperse dye that should work to color the polyester portion of the fiber blend. You have to dye the two types of fiber with completely different dyes, because there is no dye that will work on cellulose fibers such as rayon or cotton, and also on totally synthetic fibers such as polyester.

There are two likely reasons why your color is not as dark now as you want it to be:

1. First, are you using the stovetop method? Both iDye and iDye Poly are hot water dyes and do not work nearly as well in hot tap water; iDye Poly in particular requires extended boiling, not just hot water, for best results. To color the polyester threads in your dress, cook your dress in iDye Poly at a boil for at least half an hour, preferably an hour, using any additions recommended in the recipe that came with the dye. To color the rayon fibers, either switch to a cool water fiber reactive dye, such as Procion MX dye, or cook your dress in iDye for at least half an hour at a simmer (187°F) or above. The manufacturers of the dye say that you can combine both dyes in one pot and apply them at the same time.

2. Second, getting a true black generally requires a lot more dye than other colors do, since it is so much darker than any other color. I recommend using two to four times as much dye for black. You should use more than one packet of dye at a time to get a truly dark black.

I personally don't like direct dyes, such as iDye, as much as I do fiber reactive dyes, such as Procion MX dyes, for dyeing rayon, cotton, and other plant-based fibers. Direct dye is not as washfast as fiber reactive dye; that is, it tends to fade more quickly. However, I think that, if you use a dyeing temperature of 187°F and plenty of dye powder, iDye should work well enough, on the 35% of your dress that is made of rayon. You can significantly improve the washfastness  of iDye by treating it after you are finished with dyeing with a cationic dye fixative; two brands of this product are Jacquard iDye Fixative, and Retayne. Don't use this dye fixative until after you have obtained the rich dark black color you want.

The key is to use plenty of dye and hotter temperatures. Will this be different from the way you did it in your first attempt? You need to use a cooking pot that is large enough for the dress to move in freely, in order to get solid-color results instead of a sort of tie-dye effect. Note that you should not plan to reuse a cooking pot for food preparation, though I'll admit that you are unlikely to come to any harm if you wash the pot extremely well after dyeing in it.

One caveat is necessary. It's possible that you did everything right, but that your dress is undyeable. If there is a permanent-press, anti-pill, or stain-resistant finish on the dress, it never will fully take the dye to the deep color that you want. Dresses that are sold as being PFD (Prepared For Dyeing), such as the dress blanks sold by Dharma Trading Company, are more-or-less guaranteed to take dye well, but dresses sold by clothing stores are not. Sometimes a commercial garment fails to dye well. However, I think there's a very good chance that increasing your dyeing temperature and amount of dye will work, in this case, though there can be no guarantee.

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