Anyone who's seen an old penny or the Statue of Liberty is familiar with what happens to copper as it ages. It changes colors. This is the result of copper reacting with stuff in the air to form what's called "verdigris", one of a handful of beautiful bluish-green copper compounds.

In ancient times, verdigris was very useful as a green pigment, and so people had an interest in accelerating that aging process. One neat trick is to use acetic acid (a natural product of fermentation), which reacts with copper and air to form cupric acetate. Greek artists who wanted to paint something green would hang copper plates over hot vinegar in sealed pots (sort of a vinegar steam bath) and then scrape off the resulting green crust.

So I experimented with this by boiling some cider vinegar in a jar and then leaving a shiny new copper fitting sealed in the jar overnight. Here's the result, with less than 10 hours elapsed between photos:



The "clean" spots are where it was actually under the surface of the vinegar (needs exposure to air) and where I smudged it with my finger when I was picking it up (needs time to dry before it forms a more permanent crust).

Science!