Microscopic observation and SEM-EDX analysis revealed traces of organic residues preserved on the cutting edges, blades, and hafting plates or tangs of the daggers. Using Picro-Sirius Red (PSR) solution as a staining material allowed us to identify micro-residues of collagen and associated bone, muscle, and bundle fibers of tendon, suggesting that the daggers had come into contact with multiple animal tissues. SEM observation showed the residues to be clustered along the cutting edges and at the junction between dagger blade and hafting plate/tang. The residues were mostly trapped within metal corrosion products and striations sited on cutting edges, which we interpret as use marks (10, 11). A similar association of organic residues and striations was observed on replica daggers used for experimental butchering and the working of hard and soft animal tissues (SI Appendix, S2). The interpretation presented here is supported by SEM-EDX analysis of the residues extracted from the archaeological daggers. The analysis revealed abundant hydroxyapatite (HA), a calcium phosphate present in the mineral fraction of the bone (31).
Previous research clarifies the conditions – such as contact between organic matter and the copper metal – that prevent bacterial decomposition while preserving animal tissue intact. Langejans (32) demonstrates that combined salt and metals can inhibit the activity of microbes and enzymes, enabling protein-muscle tissue to survive. Likewise, Grömer (33) argues that the acids and tannin contained in soil sediments (e.g., peat bogs) enable preservation of protein-rich organic matter (e.g., wool, skin, hair, and horn), while Janaway (34, 35) makes a similar argument for textiles preserved within metal corrosion products.
3.1. Organic residues
Organic residues were searched for on all ten daggers. Areas observed included blade face, cutting edge, point, and hafting plate or tang (both sides). We identified organic residues on eight specimens (SI Appendix Tab. S2, S3). The residues belonged to the following categories: 1) type I and III collagen (36), mainly clustering on the cutting edges; these are interpreted as use-derived residues (Fig. 2); 2) mineralized residues of fibers belonging to (a) several species of plants, one of which appears to be cfr. Alnus (SI Appendix, Section S1); these are interpreted as remnants of dagger sheaths an artefact types known from the Alpine Iceman’s lime tree bast dagger sheath (37); and (b) worked non-determined animal fur, which is also interpreted as a dagger sheath residue (Fig. 3; SI Appendix, S1); 3) bone residues clustering on the hafting plates or tangs, which are interpreted as handles or handle plates (SI Appendix, section S1); and 4) soil contamination including starch grains of Triticeae, feathers, raphides (i.e., calcium oxalate crystals), and an animal hair. Examination of a soil sample associated with one of the daggers supports our view that these are environmental contaminants, not use-derived residues (SI Appendix, section S1-Fig.S3).
Micro-residues were sampled from the eight daggers using the PSR staining procedure described below (see Methods). The residues were mostly embedded in the metal corrosion patina coating the objects. Corrosion is a natural alteration process affecting most types of metal and alloy, which is caused by a wide variety of factors, most commonly electrochemical reactions and oxidation. Depending on taphonomic conditions (e.g., temperature, pH, and soil composition), minerals and crystals may grow on the metal itself; their structures depend on the type of organic matter they have been in contact with (38). Dagger corrosion showed under the microscope as irregular bands or spots ranging in color from green to orange/red, associated with inter-granular attack and pit formation (SI Appendix, section S1). Similar corrosion structures developed on the replica daggers used in carcass-processing experiments (SI Appendix, section S2).
Overall, results of microscopic observation and analysis of use-derived organic residues (point 1 above) can be summarized thus: A) three types of micro-residues are interpreted as bone, namely: A1) clumps of bone flakes with an angular outline, characterized by a high birefringence and appearing predominantly red in cross-polarized light, with rare yellow/orange spots (Fig. 2, a-b); A2) amorphous compact residues with a rough or cratered surface and peripheral crystalline fragments; under cross-polarized light, these residues appear red with medium-high birefringence (Fig. 2, c-f); A3) bone tissue with longitudinal grooves (Fig. 2, g-h). B) Fibers, namely B1) bundles of fiber interpreted as tendons, appearing red in cross-polarized light with medium-high birefringence (Fig. 2, i-p); B2) parallel reticula of fibers that are interpreted as muscle; they appear orange and green in cross-polarized light with a medium-low birefringence (Fig. 2, q-r). C) Unidentifiable amorphous residues (Fig. 2, s-t). All types of residue are described in Tables (SI Appendix Tab. S2, S3).
3.2. SEM-EDX analysis
Scanning Electron Microscopy coupled with Energy-Dispersive X-Ray analysis (SEM-EDX) was performed on five daggers from the sample. High-power observation and microprobe analysis were carried out on multiple spots (4 ≤ p ≥ 8) on both faces of the blades and hafting plates or tangs. The analysis highlighted mineralized areas rich in organic residues, as well asbetter preserved areas that were selected for alloy composition analysis (Fig. 4, a). All specimens analyzed displayed varying amounts of Sn with traces of Fe, as is common for Middle-Late Bronze Age metals from northern Italy; no As or Pb were detected (39, 40). Corrosion structures including cracks and flaked surfaces were observed under the SEM. As one would expect, these were concentrated along blade edges where the metal is thinnest and most susceptible to degrading (Fig. 4, f-h).
Specimen no 1707 returned some of the best-preserved organic residues in the sample. SEM-EDX analysis revealed traces of fur fibers, which we interpret as remnants of a dagger sheath. The fibers were associated with Cu and a C-O-Si-Al compound likely resulting from the sheath’s mineralized fur/hair remains (Fig. 4, c-d). Residues consisting of dark amorphous matter were also observed (Fig. 4, e). These are located along the cutting edge and trapped within use-generated striations. Their analysis returned an association of Ca/P in a reasonable ratio (specimen no 1707, Fig. 4, Atomic% ratio Ca/P point b =1.43; Ca/P point e=1.35; Weight % ratio Ca/P point b=1.85 and point e=1.75). Specimen no 1798 yielded similar results (Atomic%=1.33 and Weight%=1.73). These parameters are indicative of hydroxyapatite Ca10 (PO4)6 (OH)2, a calcium phosphate present in the mineral fraction of the bone (31).
An important feature of hydroxyapatite is its stability relative to other calcium phosphate compounds under physiological conditions including temperature, pH, and composition (41). Published experiments show that heated bone has elemental atomic percentage values between 1.07-1.67 for defleshed bone and between 1.42-1.93 for fleshed bone. Weight percentage values of defleshed bone lie between 1.66-2.35, while the bracket is 1.84-2.50 for fleshed bone (41). These values are broadly in line with those observed on specimens 1707 and 1798, as is their Ca/P atomic ratio (as determined by EDX analysis) vis-à-vis the literature (17, 22, 42,43;). Overall, these studies support our view that the daggers from Pragatto had been in contact with bone tissue.