The building
A Victorian commercial building in the City of London, carrying an ornate Portland stone frieze at second-floor level. Carved circular medallions and cresting run the width of the elevation — the kind of civic-scale detail that was standard on City buildings of this period, and easy to lose entirely under enough soiling.
The problem
Decades of atmospheric soot had settled into a near-uniform black layer across the frieze, sitting on top of a stable sulphate crust. From ground level the carving read as a flat, dark band rather than the sculpted detail it actually was. Beneath the soiling, the stone itself was sound — this was a cleaning problem, not a repair problem.
The method
DOFF superheated steam at 150°C, calibrated through a 1m² trial panel before any full works began. The trial panel and the method statement were submitted to and pre-approved by the City of London Conservation Officer. The full facade clean was carried out over four days from a hanging scaffold, working the carved sections by hand at close range.
DOFF removes soiling with heat, not force. The water pressure is low enough to be safe on friable carved detail — the kind of ornament that a pressure washer would round off within minutes.
The result
The depth and precision of the carved medallions and cresting were restored. Arrises that had been invisible under the soiling layer were visible again within hours of the stone drying — no abrasion, no substrate loss, and a written record of the method held on file for the building.
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