The Effects of Bird Excrements on Copper and Bronze

This study was aimed at understanding the effects of pigeon excrements on copper and bronze. Specimens of pure copper sheets, copper sheets with a naturally developed patina, and bronze sheets were exposed to six different contaminants representing bird dropping components. The contaminated metals were aged in two climatic chambers. Both chambers were set at room temperature and a relative humidity of 100%, but one chamber was dosed with SO₂ gas in order to simulate atmospheric pollution. Samples were monitored at various intervals over a span of four weeks. Ion exchange chromatography results showed that the bird excrements consisted of phosphorous, chloride, sulfate, ammonium, potassium, and sodium. The x-ray diffraction tests gave a composition of quartz, weddellite, aphthitalite, and magnesium ammonium phosphate hydrate. The surface contaminant pH levels were around a pH of 5. Digital microscopy showed changes in the metal surface colour and texture. After four weeks in climatic chambers, minimal differences could be seen between specimens exposed and not exposed to SO₂. Scanning electron microscopy with energy-dispersive X-ray spectrometry exhibited the change in surface texture and identified the presence of fungi and dust particles. Bird excrements do promote the corrosion process of metals. The presence of patina can act as a protective layer towards corrosion for a short time.