Gary Gygax respondeu a essa pergunta em uma entrevista da EN World, e mais em Guia do Slayer para Dragões .
Originally there were the five chromatic and evil dragons, each with a color that suited their breath weapon, and a sixth good dragon patterned on the Oriental model of that imaginary creature. As it was both or different origination and alignment I decided to empower the gold dragon so as to more closely resemble the potent Oriental sort. So it got more of everything, including two breath weapons.
There came a time thereafter when more metallic dragons were desirable so as to expand the roster of good, Oriental-type ones. Thus all of them were modeled on the gold dragon template, had two breath weapons.
Logically, with metal value being used as the basis for potency, platunum (Bahamut) being the highest, then gold and silver, the sequence should have been platinum-gold-electrum-silver-copper-bronze. However, I thought bronze looked more potent than copper, and skipped then to brass—that metal conveying some not-so-benign connotations.
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