Interesting Science experiment using post 1982 US Cents. I just hope that somehow they don't go into circulation or else we will be seeing threads asking if it's a legitimate error I want to share this webpage with you - edited - safety (this image is not mine. it's from the webpage I provided which I am sharing with you)
Neat! Why stop at hollowing them out, though. You should be able to refill them with something else. If you can do it under pressure, would you be able to distort the cent into a ball or would it tear apart? When rinsing, use water with baking soda in it that will neutralize any remaining acid. Also, pour the acid into a large bucket or jar of water. It's safer than putting water into the acid.
It's not the tool, but how you use it. If I had a dollar for every time I heard "but I had anti-virus software installed and it still got through" I'd be very well off. And... I am.
It's safer than putting water into the acid. No harm in it with other acids , but only sulphuric acid, highly concentrated, has a violent exothermic reaction on dilution with water that can cause steam to spit droplets of water and acid at you.
When in doubt, to handle coins, glue a wood clothespin (remember those?) on the end of a yardstick when you fiddle with dangerous chemicals, OUTDOORS, which puts you quite a ways from the action; wear a swim mask to protect your eyes. Be prepared to explain all this to the SWAT team.
Cool experiment! Actually, I know someone who did this in high school. He didn't make a necklace, but he said it was fun to dissolve the inner zinc.