Report on the extraction of Portland stone

They don't say the 'R' word in Portland. Such is the island's connection to quarrying that to mention the small mammals is considered bad luck. Locals say they would see them emerge from their burrows immediately before a rock fall and blame them for increasing the risk of deadly landslides. Rabbits are referred to only as 'bunnies' or 'long ears'.

Portland is a central part of the Jurassic coast and is, in effect, a huge tilting table of limestone, 4.5 miles long and 1.75 miles wide, rising to over 150m, before sloping gently southwards to Portland Bill. Limestone has been quarried on the island for 700 years, but it was only when Sir Christopher Wren rebuilt St Paul's Cathedral with the material in the late 17th century that its use really took off.

What added to its appeal was its ability to be manipulated. As it is composed of minute egg-like structures called 'ooliths', it can be cut or sculpted in any direction without peeling or flaking. This, together with the pure white colour and durability, produce the qualities of a fine building stone. These were the features that convinced Wren to specify what he described as "a local stone for London". At the time, there was no such element as cladding so it had to be strong enough to be load bearing yet soft and malleable enough to be curved.

Landmark buildings

It is now probably the best-known stone in the UK, having been used on landmark buildings such as Buckingham Palace and Westminster Abbey, and throughout the city of London.

"Portland is a light-coloured stone so using it on the narrow streets of London means they aren't too overbearing," explains Alan Gale of stonework and restoration contractor Szerelmey. "Extraction has always been high, but now it's used more for restoration work. Although there are many high-profile buildings constructed using it."

Szerelmey is at the larger end of stonework and restoration contractors. The firm has worked on some major new builds such as 291 Bishopsgate and the Broadgate Tower alongside main contractor Bovis Lend Lease, and Kings Place, also in London, with Sir Robert McAlpine.

"The stone industry has benefited from the reversion to natural products," Gale adds. "The natural stone market is now significantly bigger than it's ever been and also more affordable.

"Our expertise is in procurement, design and fixing. Around 60% of our work is with Portland stone. Portland is historically most of the stone work available in London, but now we are working throughout the UK, our use of stone is diversifying."

The contractor sources the majority of its Portland stone from Albion Stone, a mineral extractor with long extraction leases covering all of the Crown Estate's reserves on Portland. Its mines and quarries yield the full range of Portland stone: Roach, Whitbed and Basebed, and variations in between.

The difference between the varieties is the amount of shell in the stone, says quarry and mine manager Mark Godden. "The basebed, which has the least shell, is very consistent throughout the island," he adds. "For carving and intricate cornices, this is the kind of stone you want. Whitbed features more crustaceans and has recently been used at Paternoster Square in London, while Roach has the most fossils but it is gaining in popularity."

Diamond saw

The stone is removed by cutting vertically with a diamond saw. This creates a fault 3m deep and 30-mm wide, into which hydro bags are placed. These 'bags', which resemble pillows, are made from high-tensile steel. They are pumped full of water and, once inflated, will cleave the stone from the bedding plane.

"The hydro bags were designed for the marble industry, but we were the first to use them in the UK," says Godden. "The bottom of a bed of stone is a weakness and the force of these bags will cleanly shear the stone from the bed. This is an efficient way of mining as you don't get loose chunks of rock as you would with gunpowder. In fact, since we stopped using explosives production has doubled."

Once the blocks are out of the ground, they are sliced up by a wire saw, resembling a giant cheese wire, to remove the 'chirt'. "This means you can see all the flaws and will have a flat surface to keep the stone safely on the trucks," Godden adds.

Some 90 blocks a week are removed and any waste stone is used locally for piers, groynes and, more recently, used to create the Weymouth and Portland National Sailing Academy that will host the sailing events for the London 2012 Olympics.

The blocks are taken to the nearby factory, where they are cut and polished to the exact specifications for a particular job. Neville Mort, factory manager at Albion Stone, says the firm goes to great lengths to meet the required aesthetic. "We can make sure the stone is delivered so that it's erected in a consistent finish or, for some jobs, architects want a random mix of clean face and heavy shell," he adds. "In these cases, we make sure it's delivered in a random fashion."

Architect's drawings

In Szerelmey's case, the contractor provides all the information itself, translated from the architect's drawings to make it easier for its workers on site to erect the stone.

"We take the architect's drawings and detail up every single piece," adds Gale. "It's then a massive jigsaw puzzle that Albion cuts and we refit on site. If we have a tight range in terms of geological features, we will work to a longer lead-in period to make sure all the detail is exact. Although you have to bear in mind geology is never an exact science."

Portland superstitions

Bunnies/long ears: Introduced to the island in the 12th century, the creatures were believed to be omens of bad luck blamed for rock falls on quarries. It is still believed by Portlanders that calling them by their proper name is bad luck.

Courting: This custom carried on well into the 19th century. A couple would court and get married only once the girl was pregnant. If she did not fall pregnant they would part, believing they were not suited.

Wedding: On his wedding day, a groom would jump three times over an iron rod used for drilling while being helped by married men at one end and bachelors at the other, presumably for good luck.

Famous buildings constructed with Portland Stone

• Broadcasting House
• The British Museum
• New Scotland Yard
• The Monument
• The Whitehall Cenotaph
• The United Nations HQ in New York
• The government buildings in New Delhi
• Brighton Civic Centre

Portland Stone was also selected for the gravestones of the war dead and by 1957 over 800,000 had been placed around the world.