Cologne Cathedral's Cleaning History: When And Why?

The Cologne Cathedral is a Gothic cathedral located in Cologne, Germany. It is one of the country's most visited landmarks, attracting up to 7 million visitors annually. Over the centuries, impurities, including coal heating systems, factory chimneys, and steam locomotives, have accumulated on the cathedral's stone walls, resulting in black discoloration. While some visitors appreciate the character and history of the sooty exterior, others perceive it as a sign of neglect. In October 2019, parts of the cathedral's facade, including the medieval choir chapel wreath and areas of the southern transept, underwent professional cleaning using a low-pressure microparticle blasting technique. This process involved using fine aluminium silicate to dissolve black crust deposits without damaging the original substance of the cathedral's facade. The cleaning revealed traces of over 770 years of history, including microorganisms such as bacteria, algae, and moss.

The cost of cleaning Cologne Cathedral

Cologne Cathedral is one of the most visited landmarks in Cologne, Germany, attracting up to seven million visitors a year. The Gothic cathedral, whose foundation stone was laid in 1248, has stood for over 700 years. Over the centuries, impurities, including coal soot, have accumulated on its stone walls, causing black discolouration.

The cost of cleaning and maintaining a historic building like Cologne Cathedral can be incredibly high. The cathedral is constructed from various types of stone, and the intricate architecture features numerous small figurines and towers. This complexity makes cleaning a challenging and costly endeavour.

In 2019, Kärcher, a cleaning company, supported the cleaning and restoration of the cathedral's outer walls. They utilised a low-pressure microparticle blasting technique with fine aluminium silicate as the blasting abrasive. This method effectively dissolved the black crust deposits without damaging the original substance of the facade. Kärcher provided their expertise, technology, and equipment free of charge for this project.

However, the cost of cleaning and maintaining the cathedral is an ongoing concern. The maintenance costs for the building alone are approximately €19,000 per day. The vast size of the cathedral means that by the time cleaning is completed at the top, the bottom sections would need to be cleaned again, making it a seemingly endless task.

Additionally, the black soot that has accumulated on the stone gives the building a consistent colour, and cleaning it could result in a dappled appearance. To address this, some have suggested replacing stones of minority colours with stones that match the majority colour. However, this proposal would be extremely time-consuming and expensive, and it might not be permitted by the office for monument protection.

The cathedral's construction and maintenance team, the Cologne Cathedral Builders' Works, consists of specialists in various trades, including stone, glass, metal, and woodworking. They are dedicated to preserving the cathedral for future generations, despite the challenges posed by its immense size and intricate design.

The different types of stone used in the cathedral's construction

The Cologne Cathedral is constructed from a variety of stone types, including sandstone, carbonate, and volcanic rocks. The specific types of stone used in the cathedral's construction include:

• Drachenfels trachyte: A light grey, partially pale yellowish or reddish, porous, porphyritic trachyte with phenocrysts of sanidine. It was the primary stone used in the cathedral's construction, particularly in the medieval portions, and was sourced from quarries in the Siebengebirge.
• Montemerlo trachyte: A replacement stone for the Drachenfels trachyte, sourced from Montemerlo in Italy. It has a similar trachytic matrix but is slightly more brownish in colour.
• Stenzelberg latite: A medium grey, porphyritic, and partially porous latite. It is very resistant to weathering and has low capillary water absorption.
• Obernkirchner sandstone: A medium-grained, orange-coloured sandstone. It is resistant to weathering and has a high resistance to water vapour diffusion.
• Schlaitdorfer sandstone: A whitish to yellowish, coarse-grained sandstone. It is prone to weathering and has high capillary water absorption.
• Bozanov sandstone: A coarse-grained to medium-grained arkosic sandstone. It is not very resistant to weathering and has high capillary water absorption.
• Krensheimer Muschelkalk: A brownish-greyish fine porous limestone consisting of shell fragments. It is resistant to weathering but is susceptible to acid rain, which can cause microkarst formation.
• Londorf basalt lava: A brownish to bluish-grey basalt. It is very resistant to weathering and has low capillary water absorption.

The use of different types of stone in the cathedral's construction is due to its long building history, spanning several centuries. The varying degrees of weathering exhibited by these stones are also a result of their distinct characteristics, such as porosity and moisture behaviour.

The impact of climate change on the cathedral

The Cologne Cathedral, a UNESCO World Heritage Site, is a testament to the architectural prowess of generations past. However, it now faces a new challenge: the impacts of climate change.

The climate crisis poses a significant threat to this 700-year-old structure, which has endured wars, bombings, and the passage of time. The cathedral's delicate stonework, crafted from a variety of stones, is particularly vulnerable to the changing climate.

One of the main concerns is the impact of extreme weather events. Heavy rain, storms, and drought pose new challenges that the cathedral was not designed to withstand. The volatile weather conditions cause tension and cracks in the trachyte masonry, leading to stone chipping and potential structural instability.

Additionally, climate change has indirectly contributed to the growth of vegetation on the cathedral. While the air is cleaner now, with reduced sulphur content, the increased greenery poses its own set of challenges. Moss, grass, and even small trees are turning the cathedral into a large biotope, which can trap moisture and contribute to the deterioration of the stonework.

The restorers and stonemasons of the Cologne Cathedral are in a race against time to preserve this Gothic masterpiece. Their work involves carefully sandblasting and restoring the oldest parts of the cathedral, a process that is expected to take around 15 years to complete. They utilise a low-pressure microparticle blasting technique with fine aluminium silicate to dissolve deposits and clean the stone without causing damage.

The impacts of climate change on the Cologne Cathedral highlight the urgent need for ambitious, long-term initiatives to protect our cultural heritage. As we face an ecological crisis, drawing on the past can inspire us to build a sustainable future, just as the medieval cathedral builders constructed their "symphony in stone."

The use of scaffolding during cleaning and restoration

Scaffolding is a common sight on the Cologne Cathedral, a necessity for the ongoing cleaning and restoration of this Gothic masterpiece. The cathedral is one of Cologne's most visited landmarks, attracting up to 7 million visitors annually. However, its popularity comes at a cost, as the constant flow of tourists, in addition to external factors like climate change, pollution, and vegetation, contribute to its deterioration.

In October 2019, parts of the cathedral underwent professional cleaning using a low-pressure microparticle blasting technique. This method, employing fine aluminium silicate as the blasting abrasive, effectively removed black crust deposits caused by centuries of coal heating systems, factory chimneys, and steam locomotives. The gentle approach ensured the original substance of the cathedral's façade remained intact.

Scaffolding plays a crucial role in the ongoing restoration work, particularly on the north side of the cathedral, which is currently the largest construction site. The trachyte masonry from the 13th century has begun to crumble and requires careful sandblasting to preserve as much of the surface as possible. Restorers and stonemasons climb the towering scaffolding to perform their meticulous work, ensuring the cathedral's longevity for future generations.

The role of the Cologne Cathedral Builders' Works in maintaining the cathedral

The Cologne Cathedral Builders' Works, or the Dombauhütte, is the organisation responsible for preserving the Cologne Cathedral, a Gothic cathedral that is the largest in Northern Europe. The cathedral is Germany's most visited landmark, attracting an average of 6 million visitors per year.

The cathedral has stood for 700 years, defying the Thirty Years' War, World War II, and the effects of climate change. It was designated a UNESCO World Heritage Site in 1996 and has required constant maintenance and renovation to keep it standing.

The Cologne Cathedral Builders' Works was established to build the cathedral and keep it in repair. It employs skilled stonemasons and specialists in every trade, including stone, glass, metal, and woodworking. The team has around 100 employees, including the head of the stone restoration workshop, Tanja Pinkale, who is in charge of the restoration of the cathedral's stonework.

The cathedral's stonework has been affected by impurities and black crust deposits over the centuries due to coal-heating systems, factory chimneys, and steam locomotives. In 2019, parts of the church facade were professionally cleaned using a low-pressure microparticle blasting technique that removed the deposits without damaging the original substance of the cathedral facade.

In addition to cleaning and restoring the stonework, the Cologne Cathedral Builders' Works is also responsible for other maintenance tasks, such as replacing iron anchors and dowels that hold the architectural decorations and are beginning to rust. They also work to protect the cathedral from the effects of climate change, such as heavy rain, storms, and drought, which are causing the stone to deteriorate.

The role of the Cologne Cathedral Builders' Works is crucial in maintaining the cathedral and ensuring its preservation for future generations. Their work includes cleaning and restoring the stonework, protecting the cathedral from climate change, and performing regular maintenance tasks to keep the structure in good condition.

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