I have been collecting old pennies for a science experiment. (The composition of the penny changed in 1982, which changed the weight slightly, and I will soon have a student exercise that makes use of that weight difference. Stay tuned…) I wanted to clean the pennies enough that they were recognizable as pennies, and so that you could clearly read the date, and also so that at first glance you didn’t know if they were older pennies or not.
Doing some reading on the Internet I found that you should not clean pennies if they are old and potentially valuable. So the one 1940 “wheatie” that I found in the pile will not be the subject of today’s experiment. The suggestions I found for just getting the oxide layer off were to use a weak acid, like vinegar, and perhaps throw in some salt, which somehow makes the acid work better.
Then I found someone who pointed out that these two ingredients, vinegar and salt, are key components in ketchup, so you should be able to clean pennies with ketchup. Or they had actually done so. I don’t remember which, and I don’t have a link, because it doesn’t matter, because it’s a testable hypothesis. Only I didn’t have any ketchup available in the lab.
But I did come across some taco sauce at dinner, so I decided to put that to the test. I took 16 rather tarnished pennies and put 8 each into two different brands of taco sauce for 5 or 10 minutes. See Figure 1. (I didn’t watch the clock – I had an intervening conversation with a colleague so that’s only an estimate.)
Figure 1: Pennies soaking in taco sauce
During the process it seemed to me that the brand on the right was doing a better job, but in the end I’m not so sure. I rinsed them off with water and dried them and arranged them with the worst side up (if there was a worse side), and from the photo in Figure 2 I can’t really say one did a better job than the other.
Figure 2: Pennies after cleaning.
What is more notable is that there is a wide variation of the results within each treatment group. Maybe some of those pennies needed more time, or need a second round of hot sauce?
I have to admit that I was not careful enough to document the pennies before the treatment. The lines of pennies above each packet were taken from the same source of pennies and show about the same levels of oxidation as the pennies I used, but they are not the same pennies. So what you can clearly see is that the treated pennies are cleaner and shinier, which was the goal, and both brands of taco sauce did about the same job.
Further investigation is clearly warranted, so I may stop by Taco Bell tonight…