I’ve used both the disposable and the reusable cloth ones. It seems that they do what they advertise, but is it just placebo effect? And if they do work, how? The cloths are just cotton I think, so how can they actually absorb dye? The link above explains that they act like a “magnet”, but that can’t be right.
I’d be tempted to run a washing machine with nothing in it but one of these “dye grabbers” just to see whether it changes color.
Dyes generally have a fairly low binding rate to materials - if you have ever dyed something, you can see how much dye remains in the liquid after dyeing. Fixatives are often used to ensure that dyes stay in the cloth fibers, otherwise the dye can wash out. A certain proportion of the dye in (particularly newly purchased) clothes has not actually fixed to the fibers and can be washed out.
In a washing machine, some of the loosely bound dye washes out into the wash water. This may lodge in undyed clothing, colouring it. Dye catchers have a much higher affinity for absorbing dye, and act to reduce the amount of dye in the wash water, to prevent (well, reduce) staining. As the dye in the stain is in an equilibrium with the dye in the wash water, removing dye from the wash water will reduce the amount of dye in the stained cloth. Additional factors such as ionic strength (managed by the detergent additives) also influence dye solubility and absorption. Bleaches break down dyes.
Si
Actually, I do use these, and they clearly change colour. If I put one in with (mostly) whites, there may be no discernible difference, but if one goes in with a red or black load, it will look mottled pinkish, or dark gray. So clearly something is happening. In fact, even though they say one use, if it still looks fairly light I just use it again, until it gets very dark.
Thanks,** si_blakely**. What you’re saying does sound kind of similar to their ‘magnet’ explanation. But I still don’t get how the excess dye in the wash water knows to attach itself to that piece of white cotton, rather than the white cotton washcloths.
Dye is still settling in other garments, but hopefully to the point that you can’t notice, since a huge proportion of the freed-up dye has found its way onto the sacrificial cloth that has an affinity for grabbing the dye.
Now that the original question has been answered (and nice job, si_blakely), are such things still needed now that most clothing isn’t dyed using vegetable dyes but synthetic ones instead? In other words, do my dark clothes from The Gap still bleed dye?
The dye in the wash water just randomly attaches itself to the dye catcher. But once it does, it stays stuck to it.
Example: In your wash load, 1000 dye molecules fall out of your new red socks, and mix with 50 litres of water and 5 white tee-shirts. Assume that each tee is the equivalent of 10 litres of water, and that each tee has a dye affinity about the same as water.
Thus, at the end of the wash, 500 dye molecules get washed away, and each tee has 100 dye molecules, and are thus now pink.
Now, suppose we do the same thing with our dye-catcher. If the dye-catcher grabs and holds 500 dye molecules, there are now 500 dye molecules in the wash. They get evenly distributed through the water and the shirts, so the water now has 250 dye molecules, and each shirt has 50 - a mild puce. If you go through another cycle, another 250 dye molecules get held by the dye-catcher, 125 end up in the water, and 25 in each tee - almost white. This keeps repeating, though as the dye percentage goes down, there needs to be more time for the dye-catcher to grab all the dye in the water.
It is this equilibrium exchange with removal that reduces the staining when a dye-catcher is present.
Si
thanks
It depends on the dye and the substrate. Dyes usually involve a chromophore (aromatic rings or conjugated double bonds) which is pretty unreactive at substrate-stable conditions, and something to bond to the substrate with. A synthetic dye can have it’s chemistry tinkered with to modify it’s affinity to a substrate, but for many natural fibres, you are relying on insoluble dye particles being trapped in the fibres. Synthetic materials can be worse, because they are smoother and possibly less reactive (long chain polymers), but you may be able to add the dye early in the process (at the feedstock) if the dye is stable at the synthetic forming temp and does not affect the material properties adversely.
So…
YMMV 
Si
This is the gist of what I’m trying to figure out. Why does this plain white cotton cloth have more of an affinity for grabbing dye than my plain white washcloths? Again, si_blakely, thanks for the explanation. I do understand what you’re saying. But what elusive property does the cloth have that makes it attract the dye more than the white t-shirts? Is it treated with some chemical? Or are the fibres rougher? Or something else maybe?
What fraction of dye molecules are removed from the water by the cloth? If it’s only ten percent, for example, the dye grabber will still turn dark, appearing to work, but it won’t do much to actually save your clothing.
I use them when I first wash brand new clothes, especially black jeans.
Anecdotal only, but mom uses one with her newly-made quilts to prevent the deep colors from dying the light ones and it does the trick.
A combination of things. The dye-catchers we use are disposable - longer, coarser cellulose fibres to make them hold together, unpolished, and probably some chemical treatment as well to make the fibres fluffier. You can almost certainly tune this. Remember, that at this stage the dyes are mostly insoluble and unreactive, so physical capture is paramount. Your cotton ones will be similar. Basically, similar to your white washclothes, but just a bit better at holding on to the dye.
Si