When I was in high school back in the 80s, I worked at Taco Bell.
Work was not really tough at Taco Bell, so I had lots of time to goof off.
One of the things I discovered was that the hot sauce would clean pennies.
I've showed the trick to people many times over the years, but never
really understood why it works so well.

I looked on the internet and found a wide variety of explanations from the silly ideas,
like the idea the somehow hydrochloric acid is forming to the more reasonable ideas
involving the acid already in the sauce.  None of them seemed to make complete sense to me.

I decided to do a little investigative journalism and find out.

First I gathered up a bunch of fairly tarnished pennies.
The ones on the right are tarnished and the ones on the left are clean and shiny.



Here is a close-up to compare the clean penny with the dirty ones.
I knew that oxygen in the air reacted with the copper on the penny and
formed a copper oxide that appears as the dull finish.

Here's a close up to show how a partially cleaned penny compares to
both clean and dirty pennies.

So here is the good stuff, both hot and mild Taco Bell hot sauce.
Back in the day, the sauce used to come in little tubs you could drop the
coins into.  Today it's all about the mini-packet. 

The sauce tastes the same to me, so I don't think they changed much.

Taco Bell hot sauce is made from the following ingredients:
Water, Tomato Paste, Jalapeno Peppers, Vinegar, Salt, Spices,
Dehydrated Onions, Xanthum Gum, Sodium Benzoate, and Natural Flavor.

Now I think we can rule out water, but somehow, the other ingredients clean pennies.
Looking at the tiny amounts of Xanthum Gum, Sodium Benzoate, and Natural Flavor,
I'm going to rule those out too.

That leaves the following things to create the copper cleaning action:
Tomato Paste, Jalapeno Peppers, Vinegar, Salt, Spices, and Dehydrated Onions

Here is the method I used to test the cleaning power of the various concoctions.

1. Open sauce

2. Apply thin layer of sauce onto half of the penny

3. Let sauce sit on penny for two minutes

4. Wipe sauce off with paper towel

5. Take photo of penny compared to clean and dirty pennies

I followed this method for each of the following sauces.

 
Here we go.  The baseline.

Taco Bell Hot Sauce
Good cleaning power, but more time would have made it much cleaner.

The mild sauce doesn't appear to do much better or worse than the hot stuff.
The amount of chili in the sauce probably doesn't make much of a difference.





This is Jack in the Box taco sauce.  IMHO, their label kinda sucks.
Taco Bell at least said Taco Bell on it and had a witty saying.
This packet looks like generic sauce you get in a cafeteria.

The performance is OK, but not great.
Less cleaning than the Taco Bell sauce.
Whatever the causative agent, Jack in the Box sauce has less of it.

Don Sonora is the taco sauce you get at Burger King.
Friggin' Burger King is so lame they don't even make their own sauce.

The cleaning power is average, similar to Taco Bell sauce.
 



Next I tried McDonald's ketchup.
McDonald's doesn't even have hot sauce, so I used ketchup instead.

The ketchup did a good job.  I think this rules out the impact of chili on cleaning.  It also rules out the possibility of Spices and Dehydrated Onions.

Let's review.  Out of the possible ingredients that make hot sauce a copper cleaner
and that are in each of the effective sauces above, we are left with this list:
Tomato Paste, Vinegar, and Salt.

Kinda narrows it down, eh?

I remember something about tomato sauces being bad for copper cookware
or some other nonsense like that being talked about on FoodTV.
Could tomato sauce be the causative agent?

It's time to try them out individually

This is pure tomato sauce.  No salt, as you can see.

Lookie here.  Absolutely no cleaning effect at all.

I did a second test and let the tomato sauce sit on
the penny for 5 minutes and still no cleaning.

It appears that tomato is not the causative agent.  But let's be sure it's not a catalyst.

Probably the most well known American hot sauce is Tabasco Sauce.
Tabasco is made of water, peppers, salt and vinegar.  No tomatoes.

The Tabasco Sauce did a good job, but again, no better than the other hot sauces.
And it has no tomatoes, ruling out tomatoes as being involved in the cleaning.

That leaves vinegar and salt.

This is a simple household vinegar, similar to what's in hot sauce.
Vinegar is also known as acetic acid, a weak acid used for
 cleaning, cooking, and even food preservation.

Is this acid the agent that cleans the pennies?

No significant cleaning.

Vinegar itself doesn't clean pennies.

And rubbing salt on pennies doesn't do it either, I tried that...

So it must be a combination of the two that does the cleaning.

I made a mixture of vinegar and salt as a cleaning agent.



 
 Bingo!  Vinegar and salt are the ingredients that in combination can clean pennies.

A quick Google search reveals that I'm not alone in determining this.

But no one can explain why the vinegar and salt work together in this.
Lots of suggestions, but no one that is an authority on the subject.
We need to know exactly why vinegar and salt do this.

To do this, I did what any person would do, I googled.
Since I am a web search ninja, I found this article by Professor Laurence D. Rosenhein
at Indian State University about The Household Chemistry of Cleaning Pennies.
  Since I could only read the abstract, I sent an email to Professor Rosenhein.

I wrote:

A lengthy internet search led me to you. You wrote a paper "The Household Chemistry of Cleaning Pennies" but I am unable to read it since I am not a member of the JCE.

As far as I can determine, the acids (usually vinegar - acetic acid) break the copper oxide free from the penny. As I have seen experimentally, acid alone does not clean as well as acid and salt. As best I can tell from your abstract and other sources, the salt breaks down somewhat, freeing chlorine ions into the solution.

The chlorine ions bond with the copper in solution forming some sort of copper chloride, allowing the acid to break more of the copper oxide free from the penny.

He replied

Any university library will have the hard copy publication, if you are really interested in reading it. (Or I could send you a reprint.) But it is written for chemists, so you might not get much more out of the longer version than the abstract.

In water, under any conditions, salt breaks down into sodium ions and chloride ions. (Even in the solid, it is in that form, so it would be better to say that the water just allows the ions to separate.)

 It's a little more complicated than that. The copper in the tarnish would be in the 2+ form. Chloride ions do not bond strongly to this form. However, they do bond fairly strongly to the 1+ form. A reaction that would be unfavorable in the absence of chloride ions, Cu + Cu2+ *--> 2Cu1+, becomes favorable when the chloride is present to tie up the Cu1+ form (as [CuCl2]-).

The first process can be viewed as transfer of an electron from Cu (metallic copper) to  Cu2+. This is followed by reaction of the Cu1+ with the chloride. The resulting copper-chloride complex is soluble in water. It's the electron-transfer aspect of the chemistry (the conversion of metallic copper and the Cu2+, to Cu1+) that makes the role of the salt a little surprising.

So there you have it.  It truly is the combination of vinegar and salt that allows
taco sauce to clean pennies so well.  Shown both in my empirical data
from the experiments and in theoretical chemistry terms by the Professor.

I hope this clears up any questions or concerns you may have had
regarding the cleaning power of taco sauce.