The ancients distinguished between eras, from the first age of the world to the last, into golden, silver, copper, and iron ages, to which they also added an age of clay. The golden age they called the times when there was innocence and integrity (See Most Ancient Church). But they the times 'the silver age' when there was no longer innocence, but still a type of integrity. The copper and iron ages were the names for the times which were still lower. They gave these names on account of correspondences, not comparison.

(References: Arcana Coelestia 5658)