Why Does Vinegar /Nick-A-Date Work?

When metal is displaced during striking it changes at a microscopic level, work-hardening it. The greater the degree of displacement, the greater the hardening. It's like if you have a metal rod, and you repeatedly bend it. Each time you bend it, it's harder to bend in the exact same spot because the bent part hardens. It becomes very difficult to make straight and becomes increasingly wavy each time you bend it. Eventually it becomes very hard and brittle and breaks instead of bends.

The hardened metal is also less susceptible to chemical attack, so higher points on devices that were more displaced react less to the chemical used to restore the date.

I could not have explained it better myself!

interesting, thanks for the Q&A

There are acids that can be used on virtually every type of coin to get the same effect depending on the metal, zinc lincons would probably be the exception.