Why did Rit all purpose dye come out so differently on swatches of cotton, silk, linen and wool?

Name: Aaren

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Country or region: USA

Message: Hello! My daughter did an experiment for school and she still is not convinced she understands why her all purpose Rit dye came out so differently on her swatches of cotton, silk, linen and wool. The basic purple liquid dye came out navy blue on wool, magenta on linen and cotton and a true purple (like the bottle) on the silk. Should she detail the chemistry of the fabric to explain it or is there something a little simpler she can tell her class that explains the varying results? She followed the directions exactly as it said to do for the different fabrics according to the bottle's instructions (water temperature, stirring time, etc.)
Thank you! Any help would be greatly appreciated!

The dye on your daughter's wool is a completely different dye than that which is on her cotton and linen! The dye on the silk is a mixture of the two.

Rit all-purpose dye is a mixture of two unrelated dyes. Neither is a long-lasting dye, as both tend to bleed in the laundry, but they are inexpensive for the manufacturer to use.

One of the two types of dye in Rit all-purpose dye is a type of dye called acid dye, which sticks to protein fibers but not to cellulose fibers. That means it sticks to wool and silk, which are both animal fibers, made from proteins, but not to cotton or linen, which are plant fibers, composed of a carbohydrate called cellulose. The acid dye in the Rit dye mixture washed out of the cotton and linen but attached to the wool and to the silk.

The other dye in Rit is a type of dye called direct dye, which (rather loosely) sticks to cellulose fibers and to some proteins. As you can see, the direct dye in this particular color happens to stick to silk pretty well, but not much to wool. Direct dyes often attach well to protein fibers, but generally better to silk than to wool. The direct dye in the Rit dye mixture washed out of the wool but not out of the silk or the linen.

From your description, it seems as though the two dyes in this particular Rit dye mixture are not very well matched for color. It seems that the acid dye is a navy blue, while the direct dye is a magenta color. Of course, mixing blue with magenta results in purple. A particular dye may produce a slightly different color on one fiber that it does on another, if it is able to bond to both fibers, but in this case the difference in coloration on the different fibers is great enough that it may be that the different dyes in the mixture are not quite the same color as one another.

For more information, see the following pages on my website:

"What kinds of chemical bonds attach dyes to fibers?"

"All Purpose Dyes"

"Acid Dyes"

"About Direct Dyes"

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Posted: Tuesday - January 21, 2014 at 01:19 PM

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Why did Rit all purpose dye come out so differently on swatches of cotton, silk, linen and wool?

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