Dear Dr. Vinny,

I bought a very dark red wine. A few drops remained in the bottom of the glass. I added water to rinse, and the water turned a definite blue, like a diluted indigo. I later spilled—oy!—about 5 ounces on the counter, and on the countertop (white melamine) and sink (porcelain) the residue was blue; the stain on the cotton towel was purple-red, like the wine. The wine in the glass was the darkest I’ve ever seen. Can you explain?

—M.B., Roanoke, Va.

Dear M.B.,

Sure, it’s chemistry!

Let’s start by noting that the color in wine comes from pigments called anthocyanins, which come mainly from the skins of the grapes. As you might imagine, some grapes—and the wines made from them—have more anthocyanins than others, and certain winemaking practices can coax more pigment out. Even getting the grapes extra-ripe can do this—the cell walls start to become fragile and release more color. (Similar anthocyanins are also found in hand-staining foods like berries, plums and cherries.)

Anthocyanins also work as acid-base indicators, which is the same chemistry behind litmus tests. The color of the anthocyanins changes depending 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, it will start to turn blue.

I’m guessing your tap water is alkaline, as are the cleaning products you use on your countertop and sink—if you had softer water, you’d be less likely to experience this phenomenon. I like to use baking soda (which is alkaline) on stubborn wine stains, so I often come across red wine turning blue.

—Dr. Vinny