FAQ: Is there any way to "set" dye in purchased clothing or fabric?
How can I set the dye in the clothes I just bought?
The problem with this question is that you don't know what dye was
used, when you buy clothing. A treatment that will help set acid dyes
will tend to strip off fiber reactive dyes, while the carbonate that will set
fiber reactive dyes won't do any good for union dyes. You must match such chemical treatments to the exact dye type that was used, for acceptable results. Furthermore,
such treatments are best used at the time of dyeing, rather than much later.
Vinegar is not the answer!
Many people who know nothing about this subject recommend 'setting' dye in cotton clothing with vinegar. In fact, vinegar can do nothing useful for cotton dyes. Vinegar will help set some acid dyes, but only if applied while it is gradually heated to a simmer (generally in the presence of salt), solely in cases in which this necessary part of acid dyeing was omitted; acid dyes are used on silk, wool, or nylon, but never cotton.
Salt won't do it, either
Salt can be useful in dyeing, by encouraging the fiber not to repel the dye, or by making the dye less soluble, but it will not itself fix the dye to the fiber. Washing your garment with salt won't hurt it, but it also will not help. The extra washing will help to remove the dye only because it's another run through the washer, which can help to remove excess dye.
Washing Out Excess Dye
In some cases, the clothing may be dyed correctly, and also have excess dye in it. This is true of fabrics dyed with fiber reactive dye that have not been washed adequately. In such cases, the simplest solution is to simply go ahead and wash - repeatedly. The test for whether all excess dye has been removed is simple: dampen the dyed item and iron it dry, while pressing it against a bit of white fabric. If the undyed fabric stays white, the dyed fabric (or yarn) can be trusted. Normally, the most popular fiber reactive dye, Procion MX type dye, requires at least one washing in cold water (without detergent, unless it's Synthrapol), followed by at least two washings in hot water, WITH detergent, to remove the last bits of unattached dye. Hot water is much more efficient than warm water at removing the last bits of unattached dye.
The dyer's detergent Synthrapol SP is often recommended, as it was formulated specifically for the purpose of washing out dye, and moreover lacks the high pH that can damage silk and wool, but there is some disagreement as to how strictly necessary it is. In any case, Synthrapol, like any detergent, cannot fix any dye into fabric; what it does is wash out excess unattached dye.
Many quilters swear by the use of pure sodium dodecylsulfate (SDS, also known as sodium lauryl sulfate, a popular shampoo ingredient), which is sold by feedstores under the name of Orvus, for the purpose of washing horses. Others feel that ordinary laundry detergent is adequate. Dye is always most easily washed out with hot water, though the first washing after dyeing should always be done in cool water, and loosely attached dyes such as direct dye (found, for example, in Rit brand dye and other all-purpose dyes) must never be washed in hot water.
True Dye Fixatives, an all-purpose solution
There is only one type of product that you can buy that will actually set dye regardless of its type. A product called Retayne, sold by local quilter's supply shops as well as by most mail-order dye supply houses (see Sources for Dyeing Supplies),
is recommended for fixing dyes in commercially purchased cotton
fabrics or clothing to prevent color bleeding during washing.
This product is a cationic bulking agent, which acts to seal in the dye
by physical means, rather than the chemical bonds which are so dependant on the type of dye. It seems that the particles of Retayne adhere to the dye molecules, effectively making them larger, so they do not come out of the fabric as easily. Note that Retayne is washed in as a laundry additive, and thus can be used only on things that can be immersed at least once without the dye immediately floating off and ruining other parts of the same item. Retayne may be removed by washing with overly hot water, and thus treated items must be washed in cool water. (Unfortunately, the one situation Retayne cannot help with is the dry crocking of indigo, in which improperly applied dye rubs off of the fabric even when dry.)
Instead of Retayne, G&S Dye sells a similar product called Raycafix, which they say is stronger; another advantage of Raycafix is that it can be laundered in warm water without losing strength.
Dharma Trading Company sells Retayne and also a product called "Dharma Dye Fixative",
which they claim increases the washfastness, wetfastness, resistance
to perspiration staining, and resistance to seawater fading of several
different kinds of dye. I don't know how it compares to Retayne; it may be a generic version of the same product, or a related product, instead.
Aljo Dyes strongly suggests the use of their equivalent product, Aljo
Pro-fix PCD after-treatment, after using
direct dyes, which would otherwise run too much to be at all
desirable. (See Sources for supplies for contact information for all companies mentioned.)
Note that the use of any such aftertreatment, while improving washfastness, may adversely affect lightfastness; dry garments indoors, and
store them in darkness when they are not being worn or displayed.
Some grocery stores carry products called "dye magnets", which are
claimed to soak up any loose dye and thus protect other garments washed
with a garment that runs. The label of the first such product claimed that its only ingredient was natural cotton,
according to the label; I don't know whether it's any more effective
than just throwing a couple of extra white cotton washcloths into a
load of laundry. Newer products apparently attract loose dye via a cationic charge, and may help somewhat, in preventing disasters while washing improperly dyed clothing together with other clothing. One brand of dye magnet is the Shout Color Catcher. I have not tested its effectiveness.
Return garments that bleed unacceptably!
It is the manufacturer's responsibility to ensure that their garments are made with fabrics that have been dyed properly, so that washings done according to the sewn-in care label do not cause undue fading. If dye bleeding occurs in spite of your following the instructions on the care label, you can and should return it, to the manufacturer or retailer, and request a replacement or refund. Quality garments are made with dye that does not need to be set after it is purchased.
Advice for manufacturers: Use washfast dye
If you dye clothing, fabric, or yarn yourself, be sure to select quality dyes that will not wash out or bleed onto other garments. On cotton and other cellulose fabrics, this is best accomplished by using one of the many kinds of fiber reactive dyes.
Wash out excess dye adequately before sale. If you use direct dye, or all-purpose dye such as Rit brand all purpose dye, be sure to use an after-treatment to reduce dye bleeding, and include care labels with anything you sell, indicating that after-treated items should be washed in cool water only.
Washfastness data are presented for many reactive, acid, and direct dyes on the charts on the lightfastness page.