I tried to clean a red wine stain and it turned blue. What gives?

Dear Dr. Vinny,

I spilled red wine on my white carpet. I used a mixture of water and vinegar and a bit of dish soap to blot, after I frantically blotted with a paper towel first to soak up as much wine as I could. I used a dry towel to blot a bit more, then I sprinkled a lot of baking soda with a bit of water on the entire area and left it to dry. I checked, and it’s a blue-gray color now. Is this normal? Have I wrecked my carpet?

—Cassie, Cold Lake, Alberta, Canada

Dear Cassie,

I don’t know how your carpet will turn out, but you did perform a pretty impressive science trick!

Red wine gets its color from grape-skin pigments called anthocyanins; it's the same stuff that stains your hands from berries, plums and cherries. Anthocyanins work as acid-base indicators, which is the same chemistry behind litmus tests. The color of the anthocyanins depends on the pH of what they come into contact with. Acidity turns anthocyanins red, while alkalinity shades them toward blue. Because wine already has acid in it, its anthocyanins are red. But as soon as you expose those anthocyanins to more alkaline factors, they will start to turn blue.

Baking soda is crazy alkaline (and your water could also be mildly alkaline), so you basically used your carpet as litmus paper.

Here’s what I’d do next. Start by vacuuming up what’s left of your wine/baking soda concoction. Then I’d go back to the original plan of diluting the stain with water and using detergent, vinegar (or some combination thereof) to continue to lift the stain out of the fibers. You can also try salt, club soda or, if you’re desperate (and can do a test in an inconspicuous area), you might also want to try out dish soap and hydrogen peroxide.

Otherwise, let me know when you’d like me to come over and help rearrange the furniture.

—Dr. Vinny