The building
A free-standing Portland stone Ionic column, Grade I listed, in an exposed position in the City of London. The capital, shaft, and base form a single carved composition — the kind of set-piece where every part of the treatment has to be considered together.
The problem
Soiling across the shaft and biological growth on the capital and base were the visible issues. Less visible, but more significant: repeated wet-dry cycling over many decades had left the north-facing flank of the shaft surface-friable. Cleaning that section at normal parameters risked taking the weakened surface with it.
The method
DOFF cleaning was carried out first, at reduced temperature to account for the friable condition. Once the stone had dried thoroughly, a lime-based consolidant was applied to the affected section by brush, in multiple thin coats, allowed to carbonate between applications. The consolidant does not change the stone's appearance and does not form a film on the surface. Given the Grade I listing, Historic England was consulted directly on the consolidant specification, and the full method statement and product data sheets were submitted with the Listed Building Consent application.
Replacing a friable section with new stone is faster, but it removes original fabric that a Grade I listing exists to protect. Consolidation stabilises what's there. Slower, and the right call on a building like this.
The result
The friable section is stabilised without any visible change to the stone. The column reads exactly as it did before — which, on a Grade I listed structure, is precisely the point. A written record of the consolidant used and the areas treated is held on file for future reference.
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