The evolution of knowledge on steel and iron, with the first quantitative chemical analysis by Lavoisier, is described along the 18th century and the first years of the 19th. After the classification by Reaumur of wrought iron, steel and cast iron, based on an increasing content of 'sulphurs and salts', replaced by phlogiston and plumbago, the publication by Lavoisier of a high oxygen content in cast iron darkened the understanding of iron metallurgy. It was necessary to wait until the beginning of the 19th century, when Berzelius restored the situation.