What is the science behind the effect of salt when rinsing garments with loose dye?

Name: Mike

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Unlike salt, iDye Fixative and Retayne do stop dye bleeding in purchased clothing.

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Message: Hi, I have noticed when rinsing garments that have loose dye in a saline solution that the dye stops bleeding dramatically. I was just wondering what the science is behind this. Is the salt forcing the dye back into the garment? If the garment was then dried, would the dye be less likely to rub off temporarily? Of course next time the garment is washed with detergent the dye starts to flow again. I realise that salt doesn't set dye but was wondering also if this inhibiting effect is the reason behind the old wives tale. Thanks.
P.S. & thankyou for your wonderfully informative site.

I am sure that you are correct about this being the source of the claims that salt will set dye. It's unfortunate that the effect is so temporary.

Salt reduces the solubility of dyes in water, so adding salt turns water into a less effective cleaner. When the same item is again placed in clean water, the unattached dyes can once more bleed easily into the water.

(Salt is also useful for reducing the amount of dye needed to be used in high-water-ratio immersion dyeing, but in that case it's not doing anything to set the dye, it is just making it easier for the negatively-charged dye molecules to find the negatively-charged fiber molecules.)

Rinsing in salt water does not reduce the incidence of crocking (which is what it's called when dry particles of unattached dye rub off onto other items, often ruining them). The only way to reduce crocking is to remove the excess unattached dye--or, depending on how bad the situation is, to start over with a new piece and apply the dye more correctly.

Two other factors that temporarily affect how well unattached dye washes out are the temperature of the washing water, and how hard the water is.

Hotter water is far more efficient than cooler water at washing out unattached dye; this is why we wash out the excess from good permanent reactive dyes with the hottest water available, as it saves water overall and works better, but we wash woolens that have been dyed with more weakly-attached dyes only in cool water, to make the color last as long as possible.

Hard water is water that contains mineral ions, such as calcium or magnesium. Garments that have been dyed and then washed in hard water can appear to have had all of their excess dye removed, and yet then bleed dye once more when washed in softer water. It is best to wash out excess dye with water that is not hard, or water that has been softened by replacing the calcium ions with a little sodium, or water to which a water softener such as sodium hexametaphosphate has been added. Many dyers always add a small amount of the water softener sodium hexametaphosphate to their dyebaths and to their washing-out water.

One of the ways to exploit the reduced solubility of dyes in salted water is an answer to the problem of after-setting Procion dyes. It is possible to use soda ash to set the dye by adding it to the fabric in one of three ways: before the dye, with the dye, or after the dye. For tie-dyeing or dye painting, it is best to either presoak the fabric in soda ash, or to mix the soda ash directly with the dye mixture and use it quickly. The third method, after-fixing by first painting the dye onto the fabric, and then soaking it in dissolved soda ash, will tend to destroy any detailed design, as the dye on the fabric will dissolve in the soda ash mixture and transfer to parts of the fabric where it is not wanted. However, sometimes people forget to use the soda ash until after they have already painted on their dye. To salvage the unfixed design, you can fix it by painting on a water glass solution (this is sodium silicate dissolved in water), or you can add as much salt as can possibly be dissolved to the soda ash, so that most of the dye will stay in place. It's not perfect, but it is much better for preserving the design than after-soaking in just plain soda ash without the salt. (The recipe is 359 or more grams of salt [which is 166 ml, or about 2/3 of a US cup]—use a little too much salt, to be sure that you have as much as can be dissolved—plus a quarter cup of soda ash (60 ml), per liter of water.)

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Posted: Thursday - February 13, 2014 at 03:52 PM

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What is the science behind the effect of salt when rinsing garments with loose dye?

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