Physicochemical damage to ancient brass replica coins during fire suppression with ABC dry powder

Abstract Fire poses a substantial threat to metallic cultural heritage not only through high-temperature exposure but also through the potential secondary effects of fire-suppression agents. In this study, artificially patinated brass replica coins were subjected to fire exposure followed by either natural cooling or extinguishment with a commercial ABC dry powder containing 75% NH₄H₂PO₄ and 15% (NH₄)₂SO₄. Surface morphology, phase composition, and organic residues were characterized by SEM, XRD, and GC–MS. Compared with natural cooling, ABC dry powder substantially shortened high-temperature exposure; however, the powder-extinguished specimens exhibited more pronounced cracking and the highest surface porosity of 10.9%. The semi-quantitative SnO₂ content was also higher after ABC extinguishment than after natural cooling (1.2 versus 0.6 wt.%). These results are consistent with coupled rapid-cooling-induced thermal stress and powder-associated chemical alteration, supporting cautious evaluation of ABC dry powder use around metallic heritage collections.