Cologne Cathedral's Cleaning Project: Before And After

The Cologne Cathedral is one of the most visited landmarks in Germany, attracting up to 7 million visitors a year. It is a UNESCO World Heritage Site and has stood for over 700 years. Over time, the outer walls of the cathedral have accumulated impurities, with black crust deposits caused by coal heating systems, factory chimneys, and steam locomotives. In 2019, parts of the cathedral's facade underwent professional cleaning using a low-pressure microparticle blasting technique. This process involved blasting the walls with a fine aluminium silicate that dissolved the deposits without damaging the original substance of the facade. While the cleaning process has improved the appearance of the cathedral, it remains a constant challenge to protect the structure from the effects of climate change, erosion, and air pollution.

Cleaning techniques

Cologne Cathedral is one of the most visited landmarks in Germany. Over its 770-year history, impurities, soot, and microorganisms such as bacteria, algae, and moss have accumulated on its stone walls, causing black discolouration.

In 2019, Kärcher, a cleaning company, provided the Cologne Cathedral Workshop with several machines to clean the outer walls of the medieval choir chapel wreath and areas of the southern transept. The cleaning technique used was the low-pressure microparticle blasting method with a fine aluminium silicate (40-90 μm) as the blasting abrasive. This method effectively dissolved the black crust deposits without damaging the original substance of the cathedral facade.

The cleaning process began with the creation of test surfaces to ensure the best possible results. Then, the sensitive rock was carefully cleaned using low-pressure blasting guns provided by Kärcher. The blasting abrasive was applied at a pressure of about 2 bar, gently removing the deposits.

The cleaning process required expertise in restorative cleaning. Thorsten Möwes, a Kärcher specialist, trained the restorers and stonemasons of Cologne Cathedral in the professional use of the provided technology.

In addition to the cleaning efforts, the team also works to preserve the cathedral and protect it from the damaging effects of climate change, erosion, and air pollution. The north side of the cathedral, where the choir chapel is located, is currently the largest construction site due to the crumbling trachyte masonry from the 13th century. To repair this section, the stone must be carefully sandblasted to preserve as much of the surface as possible.

Cost of cleaning

The cost of cleaning the Cologne Cathedral is a complex and challenging issue. The cathedral is a massive structure with intricate details, and its construction from various types of stones with different colour gradations further complicates the process. The accumulation of impurities, including black soot, on the outer walls, has resulted in a "dappled" appearance that would be expensive to rectify.

The cathedral's master builder has expressed concerns about the high cost of cleaning the entire structure due to its numerous nooks, crannies, small figurines, and towers. The use of different types of stones in the construction also presents a challenge, as cleaning could disrupt the building's colour consistency.

One suggestion to address the issue is to remove and replace stones of minority colours with stones of the majority colour. However, this would be an extremely costly and time-consuming process. The sandstone used in the cathedral, for example, has suffered immensely from environmental influences, and a harsh clean-down could cause further damage.

An alternative suggestion is to use lasers or chemical processes to clean the stone without causing degradation. However, these techniques may not be suitable for all types of stone, and the cost of developing and implementing these processes could be significant.

In addition to the direct costs of cleaning, there are also maintenance costs associated with the cathedral. The daily maintenance costs for the building alone are substantial, amounting to 19,000 euros per day. This includes the cost of specialists in various trades, such as stone, glass, metal, and woodworking, who are employed by the Cologne Cathedral Builders' Works for the cathedral's construction and maintenance.

The complexity and cost of cleaning the Cologne Cathedral have led to discussions and debates among experts and the public. While some suggest auctioning off the old stones to defray replacement costs, others propose painting the stones to maintain consistency. However, the use of paint is not a feasible long-term solution due to its degradation over time.

Environmental damage

The Cologne Cathedral, a UNESCO World Heritage Site, has been affected by environmental damage over the centuries. The stone walls of the cathedral have accumulated impurities, with coal heating systems, factory chimneys, and steam locomotives causing black discoloration. This is evidenced by the black crust deposits found on the outer walls of the medieval choir chapel wreath and the southern transept.

The environmental damage to the cathedral is not just limited to the exterior. The interior of the cathedral has also been affected, with microorganisms such as bacteria, algae, and moss making the stone appear very dark. The combination of these factors has resulted in the need for professional cleaning and restoration techniques to preserve the valuable original substance of the cathedral facade.

One of the main contributors to the environmental damage is air pollution. In the past, sulphur in the air clogged the walls and turned them black. While the air quality has improved, the cleaner air has led to increased vegetation growth on the cathedral, with moss, grass, and small trees transforming it into a large biotope.

Climate change is another significant concern for the restorers of the cathedral. The structure was not built to withstand the extreme weather conditions brought about by climate change, including heavy rain, storms, and drought. To monitor the impact of the changing climate, a climate station has been set up to measure wind and weather conditions around the cathedral. If the winds are too strong, the police will close the cathedral square as a safety precaution.

The type of stone used in the construction of the cathedral has also contributed to its vulnerability to environmental damage. Trachyte, a volcanic rock used in the masonry, is susceptible to temperature fluctuations, which cause tension and cracks. This allows salt and frost to penetrate, leading to further deterioration. Additionally, the use of sandstone in some parts of the cathedral has suffered immensely from environmental influences, requiring careful restoration techniques to prevent further damage.

The environmental damage to the Cologne Cathedral highlights the ongoing challenges faced by restorers in preserving historical structures. The combination of external influences, such as air pollution and climate change, along with the delicate nature of the materials used, requires specialized knowledge and craftsmanship to ensure the cathedral's longevity for future generations.

The cathedral's construction

Construction of the Cologne Cathedral began in 1248, but it was not completed until 1880—632 years later. The cathedral is a Gothic masterpiece, and its construction took place in several stages over seven centuries. The builders were inspired by their faith and a spirit of absolute fidelity to the original plans.

The foundation stone was laid on 15 August 1248, by Archbishop Konrad von Hochstaden. The eastern arm was completed under the direction of Master Gerhard and was consecrated in 1322. This section was sealed off by a temporary wall so that construction could continue. Work on the structure of the nave between the west front and the eastern arm was intermittent and eventually stopped in the 16th century.

The project stalled for centuries, and during this time, a large wooden crane stood some 184 feet (56 meters) above the ground at the top of the south tower. In the 1790s, French Revolutionary troops occupied Cologne and used the cathedral as a stable and hay barn. Restoration work began in the 1820s, spurred on by Sulpiz Boisserée, a German proponent of the Gothic Revival movement.

In 1842, a new cornerstone was laid by King Frederick William IV of Prussia, and work to complete the cathedral resumed. Architects Ernst Friedrich Zwirner and Richard Voigtel carried out the construction, guided by architectural drawings made around 1300. The nave was completed, and the towers were added. The bells were installed in the 1870s, and the completion of Germany's largest cathedral was celebrated as a national event on 15 October 1880.

The cathedral is the tallest twin-spired church in the world and the second tallest church in Europe. It is built from different types of rock, which weather differently due to their characteristics. The filigree buttresses and arches are exposed to the elements and are susceptible to water damage, air pollution, and bird droppings. The sandstone used from 1842 onwards for the transept facades and the upper parts of the nave and transept shows the most intensive weathering.

The cathedral's history

The Cathedrals History

Cologne Cathedral, officially Hohe Domkirche Sankt Petrus, is a Roman Catholic cathedral church in Cologne, Germany. It is Germany's most visited landmark, attracting between 6 and 7 million visitors per year. The cathedral is the seat of the Archbishop of Cologne and the administration of the Archdiocese of Cologne. It is a renowned monument of German Catholicism and Gothic architecture and was declared a UNESCO World Heritage Site in 1996.

The site of Cologne Cathedral has been occupied by Christian churches since the 4th century. An older cathedral was destroyed by fire in 1248, and work began on the present cathedral immediately after. It was designed in the Gothic style, emulating French church architecture. The foundation stone was laid in 1248 by Archbishop Konrad von Hochstaden, and the eastern arm was completed under the direction of Master Gerhard, the first master builder of Cologne Cathedral. The choir was consecrated in 1322, but construction continued until the 16th century when work stalled for several hundred years. During this period, a large wooden crane was left standing about 184 feet (56 metres) above the ground at the top of the south tower.

In the 1790s, troops of the French Revolution occupied Cologne and used the cathedral as a stable and hay barn. Restoration work began in the 1820s, spurred on by Sulpiz Boisserée, a German proponent of the Gothic Revival movement. In 1842, a new cornerstone was laid by King Frederick William IV of Prussia, and work to complete the cathedral resumed in earnest under the architects Ernst Friedrich Zwirner and Richard Voigtel. Using the original medieval plans and drawings, alongside more modern construction techniques, the nave was completed and the towers were added. The completion of Germany's largest cathedral was celebrated as a national event on 15 October 1880, 632 years after construction had begun.

Cologne Cathedral suffered fourteen hits by aerial bombs during World War II but remained standing in an otherwise flattened city. Repairs to the war damage were completed in 1956. The cathedral has suffered damage from wind, rain, pollution, and acid rain, which has severely affected the stonework. In 1996, it was placed on the "World Heritage in Danger" list due to plans to construct high-rise buildings nearby. It was removed from the list in 2006 after the authorities decided to limit the heights of buildings constructed near the cathedral.

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