Short answers, pulled from the story.
Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a base metal. Carbon monoxide, produced by incomplete combustion of coke or charcoal, reacts with the ore and pulls oxygen atoms away from the metal, leaving the purified element behind. Flux materials such as lime are added to bind impurities, which separate as slag.
The earliest current evidence of copper smelting has been found at Plocnik and Belovode in Serbia, dated to between 5500 BC and 5000 BC. A mace head from Turkey once considered the oldest example was later determined to be hammered native copper rather than smelted metal.
The earliest iron-making evidence consists of fragments found in the Proto-Hittite layers at Kaman-Kalehoyuk, dated to between 2200 and 2000 BC. Iron implements were produced in Central Anatolia in limited quantities around 1800 BC and were in general elite use during the New Hittite Empire, roughly between 1400 and 1200 BC.
Smelting releases toxic metals including copper, silver, iron, cobalt, and selenium into the atmosphere, and emits sulfur dioxide gas, which contributes to acid rain. The smelter in Flin Flon, Canada was one of the largest point sources of mercury in North America in the 20th century, and lakes near that site will likely receive mercury contamination for decades from both re-emissions and soil leaching.
Flash smelters account for more than 50% of the world's copper smelters. Flash smelting is part of a newer generation of technologies that replaced the older reverberatory furnaces, which were relatively energy-inefficient and released sulfur dioxide at concentrations too low to capture effectively.
Pre-Inca civilizations of the central Andes in Peru mastered the smelting of copper and silver at least six centuries before the first Europeans arrived in the 16th century. They never, however, mastered the smelting of iron for use in weapons.