## 5 Discussion  
Analysis of the elemental composition and microstructure of metals and alloys may assist in differentiating between diverse manufacturing technologies and is a valuable tool for understanding the period, and in some cases, the origin, of ancient metal objects [9]. The metallurgical methodology used in the current research contributes to the understanding of the manufacturing technologies of four ferrous objects, categorised as rigging elements, which were retrieved from the nineteenth-century Akko Tower shipweck. Based on the current analysis results, the type of alloy and manufacturing process of these objects is divided into two main groups following engineering considerations: (1) the winch drum and the stud-link chain controller (Figure 1a,d) were manufactured from type B grey cast iron (formed by the casting), which is a relatively brittle and hard material with low tensile strength since these two objects were not designed to withstand tensile stresses; while (2) the winch shaft, shackle, and deadeye strap were manufactured from wrought iron (heated and worked by plastic deformation), which is a highly malleable and ductile material with high tensile strength (Supplementary Material Figure S1a and Figure 1b,d). This difference in material selection was apparently based on different requirements: the grey cast iron was chosen for its good compressive strength and its low cost, while the wrought iron was chosen for its excellent plasticity, good tensile strength, impact resistance, and weldability [41]. A minimally destructive FMM method was applied here for the first time on ancient ferrous objects (Figures 3, 10 and 15 and Supplementary Material Figures S3 and S7). This method was previously applied on a bronze powder chamber retrieved from an underwater excavation and demonstrated that the FMM method agrees well with conventional metallography [42]. The FMM method is an improvement in field metallurgical replication (FMR) metallography. The FMR method involves the preparation of a curved or irregular surface by locally grinding, polishing, and then etching the external surface of the object by swabing without cross-sectioning it. Next, a thin plastic film is pressed against the etched surface, producing a 'negative' replication of the microstructure, which is then observed by LM. The minimally destructive testing (MDT) FMR metallography does not always produce the high-quality micrographs attained by conventional destructive metallography. The FMM is a powerful MDT method that involves the preparation of a curved or irregular surface in a similar way as the FMR method, and then observing the surface by a multi-focal light microscope instead of a standard optical light microscope, omitting the plastic film replication step used in the FMR method and directly observing the microstructure of the surface, as well as identifying microscopic defects related to the manufacturing process on-site after proper preparation, producing a high-quality image without having to cut samples and destroy unique archaeological objects [42]. The current observation of the objects by using the novel FMM method produced metallographic images comparable in quality to the images obtained by conventional metallography of destructively obtained samples set in Bakelite. In a future study, it could be useful to perform Raman spectroscopy on the four ferrous objects retrieved from the Akko Tower shipw (on the surface areas of the objects that were not ground and polished). However, this is beyond the scope of the current study. Incorporating Raman spectroscopy into the research of ancient ferrous objects can offer several advantages. For example, it can provide information about the organic and inorganic components present on the surface. It can also assist in identifying long-term corrosion products, processes, and coatings, providing insights into the historical use and preservation of the objects.