Yeah, that's why I think there will be a "part 2" to this thing...buikding a simple little trim frame around the penny field and pouring in a 1/8" or 1/4" layer of clear epoxy. That would freeze things in place, shouldn't it?
What causes a new, shiny penny to darken/tarnish? I assumed it would be constant use and handling, but would just simple exposure to air do it as well?
Was thinking about dome 3/8" square dowel, or maybe something with a bit of fluting or coving (or just some simple quarter circle trim. Miter the ends to form a "pool wall" around the pennies to hold the epoxy.
Would give it a little dimension/visual interest as well, another plane rising up from backing board.