How did Antoine Lavoisier explain the increase in weight when metals rust?

Antoine Lavoisier explained the weight increase observed during the rusting of metals—a process known as calcination—by demonstrating that it resulted from the fixation of oxygen from the air, rather than the loss of a hypothetical substance called phlogiston.12

While earlier theorists believed that metals lost phlogiston when turning into metallic ash (calx), they struggled to explain why the resulting calx weighed more than the original metal. Lavoisier hypothesized that the weight gain was caused by the metal combining with air. Through rigorous experimentation, he proved that when metals like tin or lead were heated, they reacted with a specific component of the atmosphere, which he identified as oxygen.12

By carefully weighing all substances involved in these reactions, Lavoisier provided experimental evidence for the conservation of mass, establishing that matter is neither created nor destroyed during chemical reactions. This work effectively replaced the phlogiston theory with his oxygen theory of combustion.312