Reveal the stone. Don't strip it.

London's stone

Portland stone is the capital's principal building stone. It leaves the quarry creamy white and weathers to a grey-buff, with the sheltered parts, under cornices and in porches, going darker where rain never washes them. On the worst-affected fronts that soiling hardens into a black crust of sulphates and carbon, which is not just dirt but a chemical change at the surface. Bath stone is softer and more golden; York stone is the hard sandstone under your feet on steps and paving.

Why "clean" is not the same as "white"

Conservation officers in London routinely attach a condition that cleaning must not leave the building looking "as new". The aim is to remove the superficial soiling and airborne grime and reveal the stone's natural weathered colour, not to strip it back to quarry white. Over-cleaning removes the surface the stone has formed over a century, and that surface is what protects it.

The methods, in plain words

Superheated water

Hot water at about 150°C, delivered gently with low water volume. Takes off carbon soiling, green algae and most biological growth and kills the spores, so biocide is often unnecessary. The stone dries quickly. The usual first choice for limestone fronts.

Poultices and nebulous water

For fine carved detail and heavy crusts: a clay or paper poultice draws out the soiling over hours; a fine water mist softens crusts so they can be lifted without abrasion. Slow, careful and the right answer for cornices, capitals and sculpture.

Low-pressure abrasive

Fine inert powder in a swirl of air and water, for the hardest crusts on robust stone. Needs a skilled operator and a trial patch; the wrong pressure on Bath stone can be seen from the street for decades.

What cleaning uncovers

Cleaning shows what the dirt was hiding: open joints, cracked sills, rusting cramps, failed earlier repairs. On one public building cleaned recently, repointing to around a tenth of the joints followed the clean. Ask for an inspection after the trial patch and a separate price for any repair.

Cost

Stone cleaning prices depend more on access and detail than on area, so published per-square-metre guides are a poor guide for a Georgian front with a portico. Expect a firm to price from the trial patch and an access plan. For ornamental work (cornices, balusters, carved detail) the daywork rate matters more than the square-metre rate.