|Zinc penny, modern.
This is an amazing object: A modern (1990) penny from which the copper plating has been completely removed. US pennies made after 1982 have a zinc core with a copper plating over it. Removing the zinc from inside, leaving a thin foil of copper, is actually quite easy: Just sand off some of the copper around the edge and then soak the penny in hydrochloric acid (HCl, also called muriatic acid in hardware stores). I have an example of that listed under copper.
But doing the converse is much harder. Any acid that will dissolve copper will dissolve zinc much faster, so it cannot be done using acid. I was at a loss, and the internet did not reveal anyone else who had done it. Fortunately Tryggvi, my chemist friend, took on the project, and found a way using cyanide complexation. Cyanide ions wrap themselves tightly around copper ions in solution, forming what's known as a complex. They do not, on the other hand, care much for zinc at all. In a concentrated solution of potassium cyanide copper is rendered slightly more vulnerable to oxidation than zinc. Under just the right conditions (cool temperatures and gentle agitation) the oxidizer potassium persulfate, combined with concentrated potassium cyanide, will slowly remove copper without touching zinc. It took Tryggvi many tried, and many ruined pennies, to get the procedure just right, but the result was perfection itself.
See the next sample for a clever example of what you can do with disassembled pennies.
Source: Tryggvi Emilsson and Timothy Brumleve
Contributor: Tryggvi Emilsson and Timothy Brumleve
Acquired: 4 May, 2007
Text Updated: 9 May, 2007
Sample Group: Coins