Repurposed Buckle

The nickels, in order for them to be heated and soldered, how old do they need to be? Since the quality and metal content of our coin has changed over the years, I know now some of it just melts, maybe all of it. I have a collection of old pennies to use as copper. I’m trapped under the cat and can’t check but I think the year for pennies is something like 1981 and earlier, with a partial year later that may or may not be copper. Anyway you can check them by simply heating with a torch, they’ll either melt into a scary piece of nothing or behave like copper and get hot and change color. Curious about the nickels though as I’ve never used those….