Recently I had to dig out an old box of .357Mag Gas Checked Bullets.
Some Lyman #358156, these were cast from 35 - 40yrs ago. And Checked with a combination of old Lyman brass slip on and Hornady copper crimp on.
The brass gas checks were as bright, shiny and clean as the day they came out of the box. The copper checks were heavily tarnished and discolored.
Can't figure out why the copper were tarnished and the brass were not.
They were stored in a cardboard box, just dumped in. Bottom, back shelf. Untouched for 30yrs.