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era (2) f. (Noun) "era"
13th cent. From Late Latin æra, plural of æs "quantity," but originally "bronze," "copper." From Proto-Italic *ajes- 'id.' From Proto-Indo-European *h2ei̯-es- "bronze." From a root *h2ei̯- meaning "to burn."
Indo-European
Italic
Oscan αιζνιω (aiznio) "bronzen" (< *ajes-n-ejo-), Umbrian ahesnes "by the bronzen" (< *ajes-no-)
Germanic
Gothic ais "ore," "brass," Old Norse eir "brass," Old Saxon ēr "ore," Old High German ēr 'id.,' Old English ǣr 'id.' (English ore)
Indo-Iranian
Sanskrit áyas- "metal," Avestan aiiah- 'id.'

"The basic word for ‘metal’ in Proto-Indo-European is *haey-es- (e.g. Lat aes ‘copper, bronze’, NE ore, Av ayah- ‘metal (probably bronze)’, Skt áyas- [earlier] ‘copper’, [later] ‘iron’) and it is generally presumed to mean ‘copper’ or the copper-tin alloy of ‘bronze’ although it has come to mean ‘iron’ in some of the Indo-European languages, e.g. Indo-Iranian; however, there is clear evidence that it earlier meant ‘copper’ or ‘bronze’. In the Germanic languages it tends to mean ‘ore’ and it is possible it simply meant ‘metal’ rather than a specific type of metal." ~ Mallory & Adams, The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World (2006)