Portland stone comes from the Isle of Portland in Dorset. It's an oolitic limestone — formed from tiny spherical grains of calcium carbonate cemented together — and it has been quarried for London building since the 1620s. Inigo Jones used it. Christopher Wren specified it for St Paul's and for fifty city churches after the Great Fire. The Bank of England, the National Gallery, the Tate Britain, Somerset House. Much of Whitehall. The terrace houses of Mayfair and Belgravia. Portland stone is the material that London's formal architecture is built from.
It is also the stone we work with most. Understanding how it weathers, why it soils the way it does, and how it responds to different cleaning methods is a large part of what we do.
How Portland stone weathers
Portland stone has two distinct weathering behaviours depending on whether the surface is exposed to rainfall or sheltered from it.
Rain-washed surfaces — the top faces of cornices, horizontal string courses, the upper parts of facade panels — are cleaned continuously by rain. The calcium carbonate at the surface dissolves slightly in acidic rain, taking soiling with it. These surfaces tend to stay pale, sometimes with calcium carbonate runoff (white streaking) over darker surfaces below.
Sheltered surfaces — the undersides of cornices, reveals, recessed panels, sheltered bays — accumulate soiling rather than shedding it. Atmospheric sulphur dioxide from combustion reacts with the calcium carbonate at the surface to form gypsum (calcium sulphate). Carbon particles from combustion become trapped in this gypsum crust. The result is the characteristic black deposit visible on the underside of any Portland stone cornice in London. This is not biological growth — it's a chemical reaction product from atmospheric pollution.
The differential between washed and sheltered areas is part of Portland stone's visual character in the city. Whether it needs addressing depends on the building's use, its listing status, and the degree of soiling.
How Portland stone responds to cleaning
Portland stone is a robust material and tolerates the correct cleaning methods well. The key word is correct.
DOFF superheated steam works effectively on Portland stone with moderate soiling. The 150°C steam kills biological growth and loosens atmospheric soiling without introducing significant water into the substrate. For carved Portland stone — columns, capitals, friezes — DOFF is the only appropriate method.
TORC vortex cleaning is effective on heavily calcified soiling on flat Portland stone surfaces. The gypsum crust on a sheltered area that has accumulated over decades can resist DOFF alone; TORC's aggregate component shifts it where steam cannot. We specify TORC on flat panel work when DOFF test cleaning shows insufficient result.
Poultice cleaning is appropriate for localised staining — iron rust staining from fixings, oil contamination, mineral deposits — rather than general atmospheric soiling.
What damages Portland stone during cleaning
High-pressure water washing erodes the surface texture, opens the oolitic structure, and introduces large volumes of water into the masonry. It also damages lime mortar joints. A common outcome of high-pressure washing is visible surface roughening and accelerated biological recolonisation — the opened surface provides better substrate for algae and lichen than the original.
Abrasive grit blasting removes the soiling along with the stone surface itself. Tooling marks, fine detail, and the characteristic texture of the Portland stone face are permanently lost. This method should not be used on historic Portland stone.
Acid cleaning is inappropriate on Portland stone. The stone is calcium carbonate — acid dissolves it. Hydrochloric acid, used on modern brick, will etch Portland stone and should never be applied.
Portland stone on listed buildings
Most significant Portland stone buildings in London are listed. Grade I listing (exceptional interest) covers buildings like St Paul's Cathedral and the Bank of England. Grade II* and Grade II cover the vast majority of Georgian and Victorian commercial and residential buildings with Portland stone facades. All of these require conservation officer liaison before any external cleaning.
DOFF is the specified method in most conservation officer method statements for Portland stone listed buildings. Test cleans are required. Written approval is required before works commence. We manage this process as part of our standard service.