I’ve got a bit of a conundrum. When creating a Penny Portrait, it’s fairly easy to make pennies shinier (any acid will do the trick) but I’ve yet to find a way to artificially speed up the hands of time and make pennies darker…

Ideally, for a Penny Portrait you need a lot of chocolaty dark colored pennies but they can often be tricky to find.  It would be fantastic if there were a way to speed up the chemical process that causes pennies to darken naturally.

So if there any students/teachers/hobbyists out there who want to take on the challenge, I’ve got a $50 prize and a free Penny Portrait Kit for you. I can imagine this would be the ideal project for a student who is stuck trying to figure out a unique and clever science fair project.

Here are the rules:

1. Only safe, household chemicals/materials may be used.

2. The process should take no longer than 2 weeks to transform a shiny penny into a darker colored penny. (Obviously, the less time it takes the better.)

3. I’m looking for the end result to be a dark brown penny. (Not black) See the image above as an example of what I’m looking for.

4. You need to be able to darken a bunch of pennies. (The poster needs about 300.)

That’s about it.  First person to e-mail me a solution at info@PennyPortrait.com wins the prize!  (I’ll post the winner in the comments below.  If no winner has been posted, the contest is still open.)

One thought I’ve had is that since we know acids like vinegar can reverse the chemical reaction that causes a penny to turn dark, perhaps the opposite of an acid (a base) can be used to speed up the oxidation process?
Additionally, there are some products on the market that are used to darken copper: Brimstone Coin Darkener, Deller’s Darkener, and I know folks in the jewelry trade use something called “Liver of Sulfur”.  One ingredient all these have in common is sulfur, so maybe that can be a starting point.  I know egg whites are a great source of sulfur, so something to think about…

I’m including a link to a great article on why copper coins change color to get you started on your research:

Why do Copper Coins Change Colors?