Question

In sixteenth-century Andean silver metallurgy, the transition from traditional native smelting furnaces (*huayras*) to cold-mercury amalgamation—introduced to Potosí in 1571 under Viceroy Francisco de Toledo—dramatically increased the extraction yield of low-grade argentiferous ores. However, the cold amalgamation process suffered from substantial mercury loss through the formation of insoluble calomel ($Hg_2Cl_2$) when processing ores rich in copper sulfides. To mitigate this inefficiency, metallurgist Álvaro Alonso Barba developed the hot-amalgamation or *cazo* process in the early seventeenth century. Barba's innovation involved heating crushed ore slimes with mercury, common salt, and water in copper-bottomed cauldrons rather than open patio beds.

Historians long assumed that the primary thermodynamic advantage of Barba's *cazo* technique lay solely in elevated reaction temperatures accelerating amalgamation kinetics. Recent chemical analyses of seventeenth-century slag and tailings at Tarabuco, however, demonstrate a more complex chemical mechanism. The metallic copper walls of the cauldron acted as an active reducing agent, undergoing preferential oxidation to donate electrons to ionic mercury species. This metallic copper involvement suppressed calomel precipitate formation by reducing mercurous ions back to metallic mercury before insoluble compounds could form. Furthermore, Barba’s requirement of adding concentrated brine served not merely as a chloridizing agent for silver sulfosalts, but generated copper-chloride complexes that catalyzed the reduction of silver sulfide ($Ag_2S$) without consuming elemental mercury. Consequently, while the cold patio process lost up to two kilograms of mercury per kilogram of refined silver when processing complex pyritic ores, the *cazo* method reduced mercury consumption to less than three hundred grams, rendering sub-marginal sulfide deposits economically viable.

According to the passage, the metallic copper bottom of the cauldrons used in the *cazo* process contributed to reducing mercury loss by doing which of the following?