St. Paul’s Cathedral Restoration, London

So, it’s not surprising when work began on restoring this landmark, plenty of questions arose concerning the treatment of its stone … but the final results should answer any critics.
St Paul’s Cathedral in London is not only a superior example of classical Baroque architecture. According to a recent poll, it’s the most popular building in England and the greatest landmark in the capital.
St Paul’s is also regarded as the supreme achievement of Sir Christopher Wren (probably the only architect known to every schoolchild in the United Kingdom) and has hosted some of the most important commemorative events in British history including the funeral of Winston Churchill and the wedding of Prince Charles and Lady Diana Spencer.
But it is the image of the cathedral as a symbol of resistance during World War II, when Churchill declared it must be preserved “at all costs,” that has imprinted itself on the British national psyche. Under such a bright spotlight, embarking on an extensive restoration project was never going to be an easy task, especially since so much of the job involved the controversial issue of stone cleaning.

HOW TO CLEAN?
As elsewhere in Europe, the stone-cleaning debate rages in the United Kingdom. Scottish conservationists rarely agree to the cleaning of historic stone buildings, and they document numerous cases of long-term damage (especially to sandstone) caused by cleaning agents. In England, decisions are made on a case-by-case basis. Where conservation officials grant approval, it is usually only after considerable investigation and deliberation by specialists and heritage organizations.
Damage to Wren’s masterpiece (constructed between 1675-1708) was out of the question. But it was obvious that the cathedral needed sprucing up.
Nearly three centuries of London grime took their toll on the building, which is built from 70,000 tons of Portland Stone. The south side, in particular, was severely blackened after decades of exposure to soot drifting from the former Bankside power station (now the Tate Modern art gallery).
For Martin Stancliffe, whose job as cathedral architect carries the delightful official title of Surveyor to the Fabric of St Paul’s, cleaning both outside and inside became a major element of his long-term plan for its upkeep.
“I noted very early in my tenure just how much the cathedral interior needed cleaning,” he says. “And, of course, there were huge concerns about whether cleaning was the right thing to do. We had so many questions. Was it appropriate? Did we know enough about Wren’s intentions for the interior? Should we see the dirt and disfigurement as the historic patina of passing years, and cherish it as such?
“Could the many different elements of the internal decorations – the painted dome, the mosaics, the stone carvings – be successfully unified? And, could it be achieved technically?”
There were also logistical questions: What cleaning technique would both achieve the desired result without damage, and allow the cathedral to stay open as church and tourist attraction? Clearly, any technique creating dust or using excessive water would create problems while the Cathedral was open. And the removal of waste materials was an issue to be considered with any techniques using chemical processes.
Five different techniques were tested. These included steam cleaning; Mora poultice (sodium bicarbonate, ammonium bicarbonate and ethylene diamine tetra-acetic acid (EDTA)); a solution of de-ionised water and white spirit with 5% synperonic N; micro air abrasives; and a latex paste called Arte Mundit®, developed by FTB Restoration of Grobbendonk, Belgium.
The first three produced little effect; The surface deposits, up to 1” thick in places, remained. The air abrasive was effective but noisy, and required high levels of protection to control dust. It was also slow because of the level of care required to avoid damage.
The Mora poultice, used extensively elsewhere in Europe, was effective but required excessive water for rinsing. Quite apart from the logistical problems of water in the Cathedral, Portland Stone can stain if it becomes saturated.
After five years of trials, the Arte Mundit latex pack proved to be the pick. The latex encapsulates the active chemicals (EDTA and organic additives), trapping them once they have softened the dirt on the surface. This ensures the chemicals cannot be absorbed into the stone and cause reactions at a later time. Another major advantage is that is it contains only a very small proportion of water.

INSIDE INTRICACIES
A special formulation of Arte Mundit was produced for the unique problems encountered at St Paul’s. These problems date right back to 1709 when, as the Cathedral’s archives explain, Wren arranged for the interior to be painted “three times in oyle.” Although the ochre-colored paint soaked into the stone, the resulting sticky surface absorbed the coal smoke and filth of the surrounding city.
By the mid-1800s, the Portland surfaces flaked and darkened with age. An 1870s restoration project removed the flaking; the archives record this as having been done with “an immense amount of labour, elbow grease and caustic paste.” Some 60 years later, an attempt to clean the exposed stonework failed.
The 21st Century solution required the Arte Mundit to be sprayed onto the stonework and left to cure for 24-48 hours; FTB Restoration performed the work in the evening, when the Cathedral closed.
After curing, staffers from Nimbus Conservation Ltd. of Frome, England, peeled off the latex in small sections and bagged it for disposal. A light mist of water was then sprayed onto the already softened deposits, the stone was brush-scrubbed, and the dirt sponged off. Toothbrushes were used to scrub decorative elements such as capitals. A final rinse completed the task.
This procedure was carried out over 15,500m² (166,840 ft²) of stonework. Only one small section, close to the West Front door, was left untreated, so that visitors can see the level of dirt removed.
Cleaning the interior stonework – the paintings, mosaics, gildings, tombs and other sculptures within the Cathedral – cost £10.8 million (or, $19.1 million, paid for by a Scottish banking family) and took four years. The result, unwrapped for the public last June, is heralded a great success. The interior is light and airy for the first time since Wren laid the final stone in 1708.
“Some people considered the use of the latex compound highly controversial,” says Stancliffe. “All I can say is that we thought about it very, very carefully, and at all stages of the trial program we set out exactly what we wanted to achieve. People get into a panic about cleaning; but, in my opinion, any tool is only as good as the people who use it.”
For this reason, he insisted a specialist conservation company should be employed rather than stone masons or a general stone-cleaning company. “Conservators bring a different attitude, a different approach. Using the right people and the right tools is what it’s about. I think the interior works have been enormously successful.”

ON THE OUTSIDE
The external works have been hailed a success, too. Although this part of the restoration project isn’t expected to finish until the end of next year (in time for the tricentennnial anniversary of the cathedral’s first religious service), the two most-prominent faces are complete. With both the East End and the West Front phases of cleaning, the aim was not to achieve a pristine appearance, but rather to remove harmful sulphate deposits and dirt and allow structural flaws to be identified.
Both ends of the cathedral were cleaned using a system of fine air abrasives from Hodge Clemco Ltd. of Sheffield, England. This method uses just enough water to control dust, and is a standard method of cleaning in England. It is hand-controlled and gentle, so there is no risk of scarring to the limestone surface.
When cleaning was completed on the East End, stonework conservation began. This included the selective removal of corroded iron cramps, which caused cracking in the stones, and replacement with stainless steel. Cracked asphalt cappings to the cornice at middle and upper levels were renewed, too, and lead drips inserted to throw rain clear of the stonework façade. Inappropriate cement pointing was replaced, and fresh Portland Stone replaced eroded stones.
However, marks caused by shrapnel from bombs dropped in World War II have been left, so the fabric can tell its own story to future generations.
The other end of the Cathedral – the West Front – is an iconic image with great national significance. This has been the route of entry to St Paul’s, and its £5 million ($8.8 million) restoration was funded by the late Sir John Paul Getty II.
Wren’s original grand flight of steps, built of a black, hard limestone, weathered badly and was replaced in the 1800s by granite from Guernsey (an island in the English Channel). Stancliffe says the stone is still in fine condition, but the steps had settled on their foundations and needed resetting.
“However, the top step of the flight, the one which runs under the columns, was still the original black limestone,” he says. “It was in very poor condition, so we decided to replace it. The trouble was, we didn’t know its source. Wren’s records said it arrived from Dublin but we couldn’t identify a similar Irish limestone.”
Further investigation revealed that the limestone came from the Poyll Vaaish quarry on the Isle of Man in the Irish Sea, halfway between Wales and the Republic of Ireland. “Supply was difficult,” Stancliffe says. “The quarry is at sea level, so quarrying can only be carried out at low tide!”
With the scaffolding removed from both the interior and the West Front, visitors and parishioners can once again enjoy Wren’s wonderful cathedral, and the restoration programme, which started with trials in 1994, is beginning to wind down. The South side is currently being cleaned, but plans for the North face may be postponed if funds are exhausted before the scaffolding reaches that side of the building.
So far, the restoration total for St Paul’s Cathedral is £40 million ($70.9 million) – some £39.2 million ($69.5 million) more than it cost Wren to build.
Claire Santry is a free-lance journailist based in the United Kingdom, specializing in architecture and natural-stone coverage for several international publicaitions.

This article first appeared in the January 2006 print edition of Stone Business. © 2006 Western Business Media Inc.