Fire Extinguisher Powder Found to Corrode Ancient Brass Replica Coins

A fire extinguisher mounted on a wall is one of the most familiar objects in any museum storeroom, archive, or heritage building, and for decades it has been regarded as an unambiguous safety asset. A new study published in the journal Heritage now asks an uncomfortable follow-up question: what happens to metal artefacts when that extinguisher is actually discharged? The research, which examined the physicochemical damage inflicted on ancient-style brass replica coins during fire suppression with ABC dry chemical powder, suggests that the very equipment intended to protect collections from fire can, under certain circumstances, become an agent of chemical attack on the objects themselves. The findings carry weight far beyond the coin cabinet, because brass and related copper alloys are among the most common materials in museum holdings, spanning archaeological bronzes, historic medals, scientific instruments, and decorative arts.

ABC dry powder is the workhorse of portable fire suppression across the world. The designation ABC refers to its effectiveness against Class A fires involving ordinary combustibles, Class B fires involving flammable liquids, and Class C fires involving energised electrical equipment, a versatility that explains its ubiquity in public buildings and heritage institutions alike. The dominant active ingredient in most formulations is monoammonium phosphate, typically blended with ammonium sulphate, and the powder is carried on a nitrogen propellant and formulated with silicone-treated additives to keep it free-flowing and water-repellent. When directed at flames, the phosphate salt works partly by melting and smothering the burning surface, and partly by interrupting the radical chain reactions that sustain combustion. This chemical aggressiveness, so valuable against fire, is precisely what raises concerns when the powder settles on delicate cultural materials.

The experimental core of the study involved subjecting ancient-style brass replica coins to controlled fire suppression events using standard ABC dry powder, and then characterising the resulting changes to the coin surfaces with an analytical toolkit drawn from conservation science. Replica coins are a standard and ethically sound stand-in for authentic artefacts in destructive testing, allowing researchers to quantify corrosion behaviour without sacrificing genuine heritage objects. Following exposure, the researchers documented visible alterations in colour, texture and lustre, and then interrogated the surfaces to identify the chemical species responsible for the damage and to understand how deeply the alteration penetrated the metal substrate.

The results paint a picture of rapid and chemically specific attack rather than generic soiling. The ammonium salts deposited by the extinguisher are hygroscopic, meaning they pull moisture from the ambient air and form thin films of acidic electrolyte on the brass surface. Monoammonium phosphate in particular dissolves to give acidic solutions that can react with the zinc component of the alloy, a process related to the well-known dezincification behaviour of brasses, in which zinc is selectively leached from the copper-zinc matrix and leaves behind a porous, structurally weakened, copper-rich layer. The corrosion products identified on the treated coins, including zinc phosphate phases and sulphate-bearing compounds derived from the ammonium sulphate fraction, represent a permanent chemical transformation of the surface rather than a simple deposit that could be brushed away.

The analytical characterisation behind these observations combined techniques that conservation scientists routinely deploy when assessing corroded metalwork. Spectroscopic methods were used to identify the molecular fingerprints of the corrosion products, distinguishing phosphate and sulphate phases from the copper and zinc oxides and carbonates that normally constitute the natural patina of aged brass. Microscopic examination revealed changes in surface morphology, with the powder treatment producing roughened, pitted textures where the protective patina had been chemically consumed. The distinction between deposit and corrosion proved to be central to the study: while loose powder can, in principle, be removed by cleaning, the acidic electrolyte it generates acts quickly, and once phosphate and sulphate species have reacted with the alloy, the damage is embedded in the object’s chemistry and cannot be reversed by dry brushing or gentle washing.

One of the most consequential findings concerns the interaction between the extinguisher powder and pre-existing patinas. Museum objects are rarely bare metal; centuries of slow oxidation produce patina layers that are both chemically protective and historically and aesthetically significant, often forming part of an object’s documented condition and its value to scholarship. The study indicates that acidic and ammonium-rich residues from ABC powder do not respect these layers. Instead, the hygroscopic salts can penetrate micro-cracks and pores in the patina, attack the underlying alloy, and cause localised undermining and detachment of the patina itself. For a genuine archaeological bronze or an antique brass instrument, this means that a single discharge event could strip away surface material that took centuries to form, destroying diagnostic information about manufacture, burial environment, or previous conservation treatments.

The time dimension of the damage is equally important. The experiments suggest that the worst effects are not necessarily instantaneous. Once powder residues remain on a surface, the hygroscopic salts continue to cycle with humidity, absorbing moisture in damp conditions and releasing it when the air dries, each cycle delivering a fresh pulse of acidic electrolyte to the metal. This means that a fire suppression event followed by inadequate or delayed cleaning can leave an object in a state of ongoing, self-perpetuating corrosion, sometimes described in conservation literature as an active corrosion state. Objects that appear superficially intact in the immediate aftermath of a discharge may therefore deteriorate progressively over the following weeks and months, a delayed-onset failure mode that is easy to miss in routine condition checks unless staff know precisely what to look for.

For museums, historic houses, libraries and archives, the study lands at a genuine dilemma, because fire remains one of the most catastrophic risks facing cultural heritage, and modern fire codes rightly mandate suppression systems. The research does not argue that heritage buildings should abandon fire protection; rather, it argues that the choice of extinguishing agent and the response to any discharge must account for the chemical sensitivity of collections. Alternatives exist. Water mist systems, inert gases such as nitrogen or argon, and clean agent halocarbons suppress fire without leaving corrosive solid residues, although each carries its own cost, space and engineering considerations. In spaces where ABC dry powder extinguishers remain the only practical option, the findings underline the importance of rapid post-discharge response: documented removal of residues by trained conservators, prioritisation of metal objects, and monitoring of treated surfaces for delayed corrosion activity.

The study also contributes to a broader and growing literature on collateral damage in emergency response. Previous investigations have examined the effects of water-based sprinklers on paper and textiles, smoke damage on paintings, and the chemical legacy of firefighting foams, but portable dry powder extinguishers have received comparatively little attention despite being the most likely suppressant to come into direct, close-range contact with objects during a small or incipient fire. By quantifying the specific corrosion chemistry of monoammonium phosphate and ammonium sulphate on a copper-zinc alloy, the research converts a vague institutional anxiety into a concrete, characterisable risk with testable mitigation strategies. That conversion from anecdote to measurement is what allows heritage institutions to negotiate with fire safety engineers from a position of evidence.

Ultimately, the message for the heritage sector is one of informed vigilance rather than alarm. Brass and bronze collections are not doomed by the presence of an extinguisher in the room, but the chemistry documented here shows that the aftermath of a discharge is not the end of the incident; it is the beginning of a second, slower episode of damage that unfolds on the surfaces of the objects themselves. The researchers’ characterisation of phosphate and sulphate corrosion products on powder-treated brass gives conservators a diagnostic target, gives facility managers a rationale for reviewing suppression choices in collection areas, and gives the field a reminder that fire safety and object preservation are not automatically aligned. Protecting heritage from fire, the study implies, must include protecting heritage from the means of that protection.

Subject of Research: Physicochemical corrosion damage to brass artefacts caused by ABC dry powder fire suppression

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

Article References: Physicochemical damage to ancient brass replica coins during fire suppression with ABC dry powder. (n.d.). https://doi.org/10.1038/s40494-026-03013-5

Image Credits: AI Generated

DOI: 10.1038/s40494-026-03013-5

Keywords: ABC dry powder, fire suppression, brass, corrosion, cultural heritage, conservation science, monoammonium phosphate, dezincification, patina, museum collections, Physicochemical, damage