Short answers, pulled from the story.
The earliest current evidence of copper smelting dates from between 5500 BC and 5000 BC. This discovery occurred in Pločnik and Belovode, Serbia. The process likely happened in pottery kilns rather than open campfires.
A reducing agent such as carbon monoxide from incomplete combustion binds with oxygen in the ore at high temperatures. This chemical reaction liberates the metal from its rocky prison. The carbon source acts as a chemical reactant to remove oxygen from the ore.
The earliest evidence for iron-making consists of iron fragments found in the Proto-Hittite layers at Kaman-Kalehöyük. These fragments are dated to 2200 to 2000 BC. Archaeologists also found indications of iron working in Ancient Egypt between 1100 BC and 750 BC.
China introduced the blast furnace to Europe during the 13th century during the High Middle Ages. China had been using the blast furnace since as early as 200 BC during the Qin dynasty. This technology replaced the direct reduction methods used in bloomeries.
Smelters release gaseous sulfur dioxide which contributes to acid rain and toxic metals such as copper, silver, iron, cobalt, and selenium. Air pollutants from aluminium smelters include carbonyl sulfide, hydrogen fluoride, and mercury. Wastewater pollutants from iron and steel mills include benzene, naphthalene, and cyanide.