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| RESEARCH: Cultural Heritage: MULTI-ASSESS maps pollution risk to EU monuments

Air- and rain-borne pollution attacks all man-made objects, but Europe’s historic structures, such as churches and statues, are particularly vulnerable due to fragile building materials that are often located in busy urban settings. Devising tools to assess today’s complex mix of pollutants and their effects were the goals of MULTI-ASSESS, whose results will help cultural specialists to determine risks and find the balance between the costs and benefits of preservation.

Air pollution inflicts huge costs – both material and economic – to Europe’s rich cultural heritage of buildings, structures and monuments. This is obvious to anyone who has gazed at the blackened façade of medieval towers or damaged statues near areas of heavy street traffic.

The composition of airborne pollution is changing, however. For example, scientific effort has focused for years on analysing the corrosive effects of sulphur dioxide (SO2), which comes largely from factory and power plant emissions. But these have been declining due to tighter EU and national pollution controls. The greater threat now comes from Europe’s ever-rising number of cars and lorries. The compounds and particulates they produce are creating a complex multi-pollutant situation that requires new analytical and monitoring tools, as well as a more inflected response from policy-makers. That is where MULTI-ASSESS entered the picture.

“SO2 pollution levels have plummeted quite substantially in the last ten to 15 years. You now have to take into account the pollutants that have been ‘hidden’ by the focus on SO2, such as nitric acid, ozone and particulate matter from vehicles,” said MULTI-ASSESS’ coordinator Dr Vladimir Kucera of the Swedish Corrosion Institute (SCI).

New models

A 40-month project supported by the EU’s Fifth Framework Programme, MULTI-ASSESS (Model for multi-pollutant impact and assessment of threshold levels for cultural heritage) brought together 16 scientific establishments in 14 European countries. Ending in April 2005, its main objectives were to:

  • develop new models for predicting the rate of multi-pollutant deterioration of materials and the soiling of cultural objects via new dose-response functions;
  • exploit the dose-response functions to define ‘tolerable’ pollution levels;
  • produce maps showing areas of increased risk for Europe’s cultural sites;
  • develop passive samplers to collect and measure atmospheric concentrations of nitric acid and particles at specific sites;
  • produce a simple and low-cost test kit for preservationists to assess the deterioration risk to their cultural objects and buildings in order to devise a cost-effective response.

Passive and active data collection

To achieve these objectives, Kucera and participants gathered data on spatial and temporal variations at 30 places in Europe, Canada and the United States. These belonged to the existing network of sites created in 1987 by the UN/ECE Convention on Long-Range Transboundary Air Pollution.

The team measured corrosion and soiling involving the kinds of materials typically found in historical buildings and objects: copper, zinc, cast bronze, potash-lime glass, stone materials, concrete and steel. For example, potash-lime-silica glass was commonly used for church and cathedral windows in Europe, north of the Alps, for about 500 years well into the 16th century. While beautiful when lit by sunlight, the glass is unstable due to its high potassium content and vulnerable to deterioration caused by pollution. Portland limestone from England is another highly vulnerable material.

To focus on local and micro-scale conditions, pollution data was collected from sites in Athens, Cracow, London, Prague and Rome, where information on site-specific soiling rates was collected via specially designed sheltered ‘racks’ placed in half a dozen different positions along the façades and roofs of historic buildings. A special study was also carried out in the five cities, plus two sites in Latvia, to compare pollution-induced deterioration rates on Portland limestone and other calcareous building materials frequently used in Europe.

The researchers relied on a mixture of passive sampler devices and active sampling equipment. “In many cases, it is not possible to use large or bulky equipment at historic buildings, so you have to use smaller passive samplers. If you place these and selected materials specimens on and around buildings for a year, you get a good picture of the aggressiveness of local pollution,” explained Kucera.

Maps and indicators

While collecting their data was difficult enough in many cases, the project’s real challenge was to provide practical advice and guidance to those who need it most: the people directly in charge of preserving cultural sites and the policy-makers who must address Europe’s pollution levels.

The on-site monitoring and test methods developed by MULTI-ASSESS go far beyond merely indicating whether pollution levels at a given locality are high or low, according to Johan Tidblad, Kucera’s colleague at SCI and a main participant in the project. “The Portland limestone analysis techniques enable us to tell preservationists how many microns of a limestone’s surface has been lost to corrosion over a certain period and how much is likely to be lost in the future without corrective action,” observed Tidblad. “If a preservationist knows the corrosion rate, then they have a better idea of what their maintenance schedule should be. It’s not just a matter of timing and money. There’s a trade-off between tolerating a minimum level of acceptable soiling and the frequency of a façade’s cleaning. After all, different cleaning techniques can also have their negative effects on historic materials.”

Test kit to the rescue

To help cultural specialists make such decisions, the MULTI-ASSESS team developed a simple-to-use prototype analysis kit they can use to quickly and cheaply assess the corrosion risk posed by local pollution conditions to their cultural site. The kit includes three of the materials that, together, span the earliest medieval period up to modernity: Portland limestone, zinc and carbon steel. Managers responsible for monuments registered on UNESCO’s list of world heritage sites must meet special monitoring requirements, for example, and some 350 of these sites are located in Europe.

Another significant achievement of the project is its contribution to the UN/ECE Convention on Long-Range Transboundary Air Pollution monitoring network. As Kucera explained: “The UN’s pollution- monitoring capability has been strengthened. MULTI-ASSESS’ work on nitric acid and particulate matter analysis has led to the addition of these two variables to the pollutants that the network monitors. This was a big step towards a more effective cultural preservation strategy because these are two of the important pollution threats our monuments face today.” 

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graphic element
  • Project coordinator
    Vladimir Kucera
    Swedish Corrosion Institute
    Tel: +46 8 674 17 25