While researching a report as part of the MRes, I recently stumbled across a paper discussing the contribution of pigeon droppings to the deterioration of our cultural heritage. The course had taken us on a trip to a historic house to carry out a week-long field research project. I was looking at salts and their movement through porous solids, such as building stones, bricks and mortar and subsequently using the literature in trying to find out what salts I had discovered. Unfortunately, my results were inconclusive, as I suspect my FTIR ATR spectra were not clear enough and due to lack of time and sufficient sample size I was unable to chemically test all of the samples for sulphate, nitrate, ammonia, chloride or phosphate. It was however, likely that my samples contained primarily sulphate and/or nitrate, indicating a probably ground water source of moisture in the building.
The study was based in Madrid on the Panteón de Hombres Ilustres where there was a 30cm thick layer of poo! The sampling site was chosen due to the lack of exposure to rainfall, and the building material was a porous limestone which is a common building material worldwide. Pigeon droppings as a source of soluble salts had already been established, but it was not considered a major factor in building deterioration, leading to the further detailed study carried out in this paper.
When leached by water, pigeon droppings were found to form salt solutions with high acidity. The salts identified in the solutions included sulphates, chlorides, oxalates and the previously known phosphates and nitrates. Salts cause damage to buildings through crystallisation within the pores of the building materials when the water they were dissolved in evaporates. The cycles of evaporation and dissolution of the salts cause stress within the pores (they will expand when the salts crystallise) and eventually loose cohesion and can cause eventual loss of structural integrity of the building if left untreated. The water containing dissolved salts will either be from the ground or rainwater ingress due to problems with the roofing or foundations of the building. When the salts attack the mortars between stones and bricks you can generally see crumbling and flaking plaster and depending on the salt visible white efflorescence’s on indoor wall surfaces.
The paper discussed how sulphates can form as a result of reaction between atmospheric sulphur dioxide and building materials containing calcium and oxalates can be formed through lichen metabolism or applied organic patinas on the surface of the building material. The study demonstrated that these are not the only ways that these types of salts can be formed on the outside of buildings, with dissolved pigeon excrement possibly a much larger deterioration factor than previously thought. The presence of the salts as mentioned above was also accompanied by acidic solution, meaning that further deterioration of the limestone was apparent with etching of the surface of rock forming minerals such as calcite.
This leaves us with the question of how do we get rid of the damn pigeons? Not only are they unsightly and a health hazard but their poo is actively destroying out precious heritage! With a count of 5,000 pigeons in Trafalgar Square in London back in 2004 how do we manage this pest? Thankfully, this is not a question that is going completely unanswered, there have been many initiatives to help lower the pigeon count in London and undoubtedly elsewhere in the world (such as Edinburgh!), just Google ‘pigeon population London’ and you will find a whole host of articles relating to this. And if you have a chance to read, you have to love the Pigeon Blog.