Inevitably, building materials are damaged by their environment. Often this is from climate, the weathering by frost, the abrasion by wind blown sand, saturation with water and many other factors. Also, biological growth as well as the effects of air pollution affect buildings. Such pollution acid rain as has come to special prominence—and received increasing attention and concern—during the late 20th century. Such processes of change were recognized in classical times by such writers as Herodotus and Vitruvius. This historical context reminds us that degradation of built structures is ever present. Combined with the idea that great buildings are meant to survive many centuries it places our consideration of damage in terms of hundred or thousand year timescales.
Such long timescales are difficult to approach experimentally, but the use of models and early descriptions by architects gives some insight to the pressures our buildings face. This long term perspective is easiest for some of the great European cities as records are particularly long. The buildings of ancient Rome—the poet Horace tells us—were much blackened by smoke. The cities of Northern Europe, for example, London and Paris used large amounts of fuel for heating and in the case of London from the late 13th century coal burning released large amounts of corrosive sulfur dioxide—enhancing the formation of damaging gypsum (calcium sulfate) crusts on facades.
The story of long term damage is especially clear in London. The traditional view of its climate has been one of a warm medieval period, followed by oscillations in long term climate trough to a cooler period in the middle of the second millennium, known as the Little Ice Age and a gradual warming into the present century—enhanced now by the rising carbon dioxide concentrations in the atmosphere. Although modern researchers see the Little Ice Age as poorly defined, it has remained a useful indicator in terms of buildings damage as frost damage to stone has declined across the last few centuries. Frost was a very important cause of damage to porous stone in the past. When the stone became saturated with water a period of low temperature can cause the water to freeze. Ice had a larger volume than liquid water, so the expansion exerts stress on the outer layers of the stone which is degraded in a process sometimes called frost shattering.
The pollutant concentrations in London were low in the medieval period even though there were frequent complaints about the odor of burning sea-coal. It was only with the widespread adoption of coal as a domestic fuel in the Elizabethan period (late 1500’s) that significant amounts of coal were burnt. By the mid-1600s such writers as the diarist John Evelyn began to describe smoke damage—both in terms of the blackening of the buildings and the enhanced rate of corrosion. In 1713, the architect Sir Christopher Wren observed sulfate crusts some four inches thick and another early description of St Paul’s Cathedral suggests:
"… you rather behold the skeleton of a church than any great comeliness in her appearance, being so shrivelled and parcht by the continual blasts of the northern winds, to which she stands exposed, as also the continual smoaks of the sea-coal which are of a corroding and fretting quality…" Keepe (1682)
The concentrations of such aggressive pollutants as sulfur dioxide and coal smoke rose in concentration through to the 20th century and then, as London grew in size and used less coal, concentrations, declined. Continued pressures to improve the air of the city in the 21st century seem likely to ensure that these pollutants remain at low concentrations. Recently, a number of other pollutants, for example, nitric acid and diesel smoke have threatened buildings, but their impact seems much less than the corrosive pollutants of the past.
Computer models can be used to predict past pollutant concentrations on the basis of fuel imported to the city and climate can be derived from early measurements and qualitative date from diaries and other records. Climate and pollution estimates for the past and predictions of these into the future can be used in dose-response (damage functions) to predict the rate of damage to building materials over time (Brimblecombe and Grossi. 2009. [Fig. 1]). We see here a very dramatic period with rapid damage to stone that agrees with the limited number of observations of stone damage available from the late 1700s. Also, early observations suggest the importance of pollutants and recognise damage to metals also:
“that pernicious Smoake which sullyes all her Glory, superinducing a sooty Crust or furr upon all that it lights, spoyling the moveables, tarnishing the Plate Gildings and Furniture, and corroding the very Iron-bars and hardest stones with those piercing and acrimonious Spirits which accompany its Sulphure” - John Evelyn (1661)
The estimated rate of damage to metal, in this case copper, is also shown in Brimblecombe and Grossi. 2009. (Fig. 1) and this follows a somewhat similar path to that of stone emphasising the importance of air pollutants in recent centuries. There is a sharp peak in rate of damage in the late 20th century, that arises because of the sensitivity of copper to ozone in combination with sulphur dioxide. Historical measurements of damage to metals and not available until the 1930s, but the corrosion at this time was rapid. Similar estimates can be made of the soiling of buildings and the increasing haze on glass in London and Paris again suggesting that the worst may now be over.
We are thus left with the impression of the striking nature of the corrosive attack on some building materials over the two to three centuries that appears anomalous compared with the earlier periods or potentially the future. The controlling influence of the major acidic pollutants is clear and because these are now controlled their impact has been much reduced. The rise of pollution in the early stage of economic growth followed by a decline is often called a Kuznets Curve. This relationship is not fundamental to the way pollution changes over time, but where local pollutants have local impacts and can be locally controlled, economic growth may ultimately lower these pollutants and their impact. The Kuznets Curve is apparent in terms of building damage, which has been a localised process. In countries, such as China where pollution is a more recent issue, a rapid increase in concentrations followed by a more rapid decline is likely.
We should also be aware that new pollutants have arisen over the last fifty years, especially ozone. It affects polymers and rubber and damage to these contemporary materials might increase through the 21st century. There is also the chance that as the atmosphere blackens buildings less than in the past, rain will remove the older dark material in undesirable rain streaked patterns. The oxidizing nature of the atmosphere in the 21st century could also enhance the warmer tones of buildings. This is already seen at the Tower of London where some light coloured stone appears to have an increased yellow hue. The changes in damage over time remind us that buildings and their construction materials show subtle responses to their environment.
- Peter Brimblecombe, Carlota M. Grossi. Millennium-long damage to building materials in London, Science of the Total Environment 407, 1354-1361 (2009).