The building
A late seventeenth-century Portland stone facade in Westminster, centred on a baroque broken pediment with acanthus scroll ornament and a dentil cornice. This is the kind of high-relief carving where soiling doesn't behave uniformly — it settles differently on an exposed cornice than it does in a sheltered scroll.
The problem
The soiling was two-layered. General atmospheric deposit covered the exposed surfaces evenly. In the sheltered recesses of the acanthus ornament — areas rainwater never reaches — a dense sulphate crust had built up instead, harder and more stubborn than the surface grime around it. One method was never going to shift both.
The method
DOFF addressed the atmospheric soiling on the open surfaces. TORC vortex cleaning, using a 60-micron calcium carbonate aggregate, removed the sulphate crust from the sheltered detail without abrading the carved surface beneath it. The two methods were used in sequence, not simultaneously, and the full method statement was pre-approved by the Westminster Conservation Officer before works began.
Running DOFF and TORC at the same time on one elevation risks over-treating the areas where they overlap. Cleaning the open surfaces first, then the recesses, keeps the two methods doing only the job each is suited to.
The result
A damaged section of the acanthus scroll was repaired with lime mortar on completion, colour-matched to the adjacent stone and not visible at normal viewing distance. The pediment reads as a single, coherent piece of carving again — not two different states of clean and dirty.
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