Your stone likely turned black due to a chemical reaction with air pollution, specifically sulphur dioxide, which trapped dirt in areas not washed clean by rain.
The primary culprit for blackening on outdoor stone is often air pollution. Specifically, gases like sulphur dioxide, commonly released from burning fossil fuels, react with the stone itself.
Based on the provided reference:
- Sulphur dioxide reacts with the stone to form calcium sulphate. This reaction changes the composition of the stone's surface.
Why a Black Crust Forms
The calcium sulphate crystals formed by the reaction act like tiny traps for other pollutants present in the air, such as soot and dirt particles.
- In protected areas of stone (which are not washed by rainwater), these crystals trap pollution causing a black crust to form. These "protected areas" might be found under architectural overhangs, within carved details, or on surfaces sheltered from direct rainfall. Rain naturally washes exposed surfaces, preventing this build-up.
Impact on the Stone's Health
This black crust isn't just a cosmetic issue; it can affect the stone's structure and longevity:
- This causes the pores of the stone to become clogged and movement of water in and out of the stone is affected.
- When pores are blocked, the stone cannot "breathe" or dry out properly after getting wet. This can lead to moisture being trapped inside, potentially causing damage, especially in freezing temperatures or if salts are present within the stone.
In summary, the blackening is a result of pollution (sulphur dioxide) reacting with the stone to create a substance (calcium sulphate) that then traps dirt, particularly in spots not cleansed by rain.
