Here’s some more technical details for those who want to attempt this.
The coins were 14-15 gauge, rimmed brass stamping blanks, and the etchant was copper etching solution (ferric chloride) from Dick Blick.
I made a total of seven coins, with the image on both sides, using the same etchant bath. Some of them came out significantly better than others.
In the picture above, the coins are arranged from the least time in the bath to the longest time in the bath.
The first two coins were placed in the bath first, for 30 minutes. They resulted in a super faint design.
Along with the first two coins, I had also etched a copper disk, for comparison.
The copper disk didn’t have significant difference from the two brass ones.
However, I believe it did pollute the bath.
All successive coins I had a rust-colored coating, that may have been the result of the eroded copper in the bath.
The next two coin were left in the bath for 2 hours, and the remaining three were left in the bath for 3 hours. There were not much difference between them.
As for the rust-colored coating, it only happened on one side: the side that was facing down in the bath. As seen here, the two sides of the same coin came out drastically different.
Hope this helps some crafty folks out there!