The descriptions of iron metallurgy by the 18th century scientists are exposed and discussed through cast iron production and refining into wrought iron or steel. This first part shows (i) the conditions of white or gray cast iron production as a function of the furnace working parameters, (ii) the descriptions of cast iron microstructures with the presence of a 'vitreous heterogeneous part' in the metal, in order (iii) to discuss the definitions of purity and iron output, without neglecting (iv) the famous question of oxygen in cast iron. To cite this article; J. Le Coze, C R. Chimie 11 (2008). (C) 2008 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.