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I have seen recipes for easter egg dyes that call for 1/4 cup vinegar per cup of water and others that call for 2 teaspoons per cup of water. That's a pretty wide range--what practical effect does the amount of vinegar have?

More generally, what is the role of vinegar in dye recipes? What would happen differently if it were left out?

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By the way, if it has something to do with acidity, could the vinegar be replaced with some other acid (such as ascorbic acid)? –  amcnabb Mar 31 '13 at 16:00
    
Vinegar is a cheap acid, and most people have some in the house. That's it main advantage. Ascorbic is relativly expensive –  TFD Mar 31 '13 at 21:38
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up vote 10 down vote accepted

Long story short: It's the acid, and any acid will do.

Food colourings are a type of dye known as an acid dye. Their effectiveness relies on hydrogen bonding which only works in an acidic environment.

It's not that the vinegar does anything special - not exactly. Rather, adding a few drops of food colouring to a large amount of water would give you something that's not acidic at all, which will diminish or totally eliminate its effectiveness as a dye. Including vinegar in the mix simply allows you to stretch your rations, so to speak - to dilute the colouring in a whole lot of liquid without making it useless as a dye in the process.

Basically, you're creating a dyebath, which is a well-known term in the textile world, as it's used to dye wool and other fabrics. Warm temperatures (140-180° F) also aid in dye absorption, and apparently - although I'm no expert in textiles - a small amount of Urea also helps.

You could definitely use cream of tartar, citric acid or any other acid, but the recipes using vinegar are generally aiming for a specific pH, so you'll want to adjust quantities of any other acidifier to match. Presumably, vinegar is just the cheapest and most widely available acidifier.

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I have some egg dyes which come with three recipes: in one, you dissolve them with just water to get pastel shades, in another you dissolve them with lemon juice (and water) to get slighly darker shades, and in the third you dissolve them with vinegar (and water) to get the darkest shades (including red, yay! - it used to be impossible to find red egg dye in the US). So it seems that the type of acid does make a difference, but it may just be an issue of strength. –  Marti Mar 31 '13 at 19:21
    
@Marti: That does fit the explanation here, since a higher pH would mean lower absorption and hence lighter colours. I'm a little surprised about the lemon juice, though, since it's generally a slightly lower pH than white vinegar and I would have therefore expected it to come out slightly (but not much) darker than vinegar. Not sure how to explain that, maybe the particular dye you have somehow reacts with lemon juice... –  Aaronut Apr 1 '13 at 2:04
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@Aaronut, its been a while since basic chemistry for me, but wouldn't the relative ratio of Lemon Juice-to-Water vs Vineger-to-Water effectively alter the pH of the dye bath? Allowing the lemon juice bath significantly less acidic than the vinegar bath. –  Cos Callis Apr 1 '13 at 2:19
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