This study was designed to determine the origin, processing methods and the structural and compositional properties of the iron used in the construction of the Palacio Real, Madrid, in the form of reinforcement bars and ties used for the support and anchorage of stone elements. These iron structures, analysed by XRD, ICP-AES, ICP-MS, SEM+EDX, EPMA, magnetometry and ultrasounds velocity were found to be of high-quality iron. The iron, worked in an air furnace at a temperature below 690°C followed by puddling, is composed of ferrite and cohenite. Two types of slag were detected as streaks in the iron: friedelite-vivianite and friedelite-titanomagnetite. These bands of impurities in the iron favour corrosion processes that give rise to iron hydroxides (lepidocrocite followed by goethite) causing an 83% increase in volume. This volume increase exerts a pressure of some 196 MPa leading to fissuring and disaggregation of the stone.