Diane Bunce, professor of chemistry at The Catholic University of America in Washington, said eggshells consist of calcium carbonate, the same chemical that makes up marble chips. However, eggshells differ from marble chips in that they are covered with a "protein cuticle."
The protein reacts with vinegar-based dyes in a way that allows dye to bond to the exterior of the egg.
Bunce, of the American Chemical Society, said most tap water is a bit acidic, so eggs dyed without vinegar will still pick up some color -- but it will have more of a pastel appearance.
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