Sancho Panza’s “por negros que sean, los he de volver blancos o amarillos” (DQ 1.29) and Juan de Mariana’s De moneta of 1605

The scientific reason for this is that copper reacts with oxygen in the air to form copper oxide, which is black. [...]the more copper content a coin has relative to its gold or silver content, the blacker it will become over time. Since copper is more plentiful and easier to mine than the precious metals, the blackness of a coin indicates its lack of value. [...]we should note that in his Tesoro de la lengua castellana o española (1611), Sebastián de Covarrubias, in his entry for the term prieto, refers to the same history of Alfonso X that had caught the attention of Mariana: Color que tira a negra [...] [...]an arbitrista recommends that the state mandate a day of fasting every month and collect the unspent money as revenue.