PCA of portable XRF data uncovers compositional trends in Illyrian bronze weaponry from Albania

The archaeometallurgy of Illyrian bronze weapons remains poorly documented despite the wealth of typological evidence available for the western Balkans. This study applies Principal Component Analysis to portable X-ray fluorescence data obtained from 84 bronze weapons, spanning spearheads, swords, daggers, knives, arrowheads, and armor components, dated from the Middle Bronze Age through to the 3rd-2nd c. BC. Four principal components collectively accounting for 87.4% of the total compositional variance were extracted from seven elements (Cu, Sn, Pb, Sb, Zn, Fe, Ni), whose concentrations were log-ratio transformed following Aitchison's compositional framework (Cu retained despite closure effects given its interpretive significance across the assemblage), with below-detection-limit values substituted by half the minimum detected value and all variables standardized to unit variance to prevent major elements from dominating trace constituents. PC1 contrasts near-pure Cu objects against Pb bronzes, driven by opposing loadings of Cu and Pb, while PC2 captures secondary variation linked to Sn and Fe content. PC3 isolates Zn-bearing outliers potentially associated with sphalerite-bearing ore inputs or polymetallic sulphide mineralization, and PC4 distinguishes Sb-rich from Ni-bearing samples, both consistent with Albanian sulphide ore geochemistry. Chronologically, Middle and Late Bronze Age weapons show broad compositional dispersion across the score plots, whereas Iron Age and 3rd-2nd c. BC objects form tighter clusters, reflecting a trend toward greater technological consolidation through time. When examined by weapon type, all categories display extensive compositional overlap, demonstrating that alloy selection was driven by shared raw material access and regional metallurgical conventions rather than by functional requirements. Together, the multivariate results document long-term continuity in Illyrian bronze-working traditions while revealing subtle chronological shifts in compositional variability that complement the typological record.