Microstructural and corrosion studies of an archaeological iron knife: implications for conservation

Abstract This research presents a metallographic study of an archaeological iron knife from the Nubian civilization. The study aims to characterize the structural and chemical composition of the metal, analyze the morphology and formation mechanisms of corrosion products, and, finally, conduct a treatment and conservation plan based on the obtained results. Several analytical and investigative techniques were employed, including scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDX), reflected-light microscopy (RLM), portable X-ray fluorescence (pXRF), micro-Raman spectroscopy, and USB digital microscopy. The results revealed that the knife was made of bloomery wrought iron, predominantly consisting of the ferrite phase, with slag inclusions dispersed throughout the structure. Furthermore, the investigation revealed the presence of an incomplete and irregular pearlite phase and the twinning phenomenon, which is uncommon feature of BCC structures in general and ferrite iron in particular. Despite being buried in soil for a long time, the artifact exhibited an unusually good state of preservation, lacking typical corrosion morphologies. The corrosion appeared as a powdery superficial layer on a well-preserved original surface. Iron oxides and hydroxides, typically accompanying excavated iron artifacts, were identified. The knife’s exceptional preservation is largely attributed to the dry environmental conditions of Nubia, which limited the electrochemical activity necessary for severe corrosion. These findings provide valuable insights into the preservation mechanisms of iron artifacts in arid burial environments. Finally, the obtained results were used to carry out conservation and protection procedures for the artifact.