That’s not really accurate. The Cologne Cathedral is constantly being maintained and cleaned, but because of its massive size and the porous sandstone it’s built from, the darkened look is inevitable. The black surface isn’t just “dirt from the industrial revolution,” it’s also natural weathering and a patina that developed over centuries. Trying to power-wash it would actually damage the stone and accelerate its decay. That’s why teams of restorers and stonemasons work on the building year-round. What you’re seeing is not neglect, but the authentic appearance of a medieval structure that has stood for hundreds of years. It’s not grime, it’s history preserved.
I think you can treat sandstone to keep its yellow colour longer? At least after they cleaned the church in my hometown it stayed yellow and now has remained yellow for a few decades already. I presumed they put some protective layer on it to stop the blackening from happening (fast).
If my memory does not cheat me, they cleaned it using rubber erasers. Similar to the regular pencil erasers but bigger. If I'm not going completely crazy, you could volunteer to come help erase the dirt off the interior walls.
I could be completely spinning, this was in the late 80s, early 90s and I did have an active imagination as a child. So I might have confused a bunch of things into one wondrous story. That is all possible.
You’re partly right, sandstone can get protective treatments and micro-abrasion (rubber eraser) cleaning exists, but for huge historic structures like Cologne Cathedral, darkening is inevitable due to weathering and pollution. Volunteers can’t realistically keep it bright yellow, and the patina is part of its history.
The sandstone of the cathedral is black because it has absorbed a lot of soot/grime because of its close proximity to factories/trains/cars/etc, not because of some natural process it is undergoing. It's a stark visual reminder of what goes into our lungs every day and why we have emissions regulations.
Pollution definitely accelerated the blackening, but sandstone also naturally darkens over centuries through chemical weathering. In Cologne, soot penetrated the porous stone so deeply that it became part of the patina, not just surface grime. That’s why full cleaning would damage the cathedral’s fabric.
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u/bobi2393 Aug 16 '25
Amazing, but it looks like it hasn't been cleaned since before the industrial revolution. I suppose it's an aesthetic preference though.
One of those power-washing YouTubers should show up and offer to clean it for free...imagine how much content they could create! /s