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How is arsenious oxide formed during the roasting of metal ores?

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A labeled diagram of the periodic table entry for arsenic, showing its atomic number 33, symbol As, atomic weight 74.921595, electron configuration, and classification as an other nonmetal. Additional icons indicate arsenic is solid at room temperature, has a rhombohedral crystal structure, and weakly acidic oxides. — arsenic Properties of arsenic.

Arsenious oxide, also known as white arsenic ( $A s_{4} O_{6}$ ), is formed when arsenic present in metal ores—such as copper, lead, and other specific metals—unites with oxygen during the roasting process. This reaction also occurs when roasting arsenopyrite or arsenic sulfide ores.1

When these ores are heated in the presence of air, the arsenic content reacts to form the oxide, which is easily sublimed. The resulting vapor is collected and condensed in a series of brick chambers to produce the final arsenious oxide dust. This substance serves as the primary starting material for the production of most other arsenic compounds.1