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Sutherland Lyall talks to Julian Harrap of Julian Harrap Architects about the restoration of stonework to the outer vestibule and tower of St Mary-le-Bow Church, east London.

According to Pevsner, of all Wren's City churches St Mary-le-Bow on London's Cheapside has 'the proudest steeple'. The sound of its bells defines who is Cockney and who isn't. And its relationship with the church attached is pragmatic rather than principled. The vestibule nestling between the four great corner piers of its base is actually a semi-open ante-chamber to the church's real vestibule.

Inspecting architect to St Mary-le-Bow, Julian Harrap, explains: 'Wren was rebuilding an 11th-century church after the Fire of London and he found that just to the north of the old tower there was the line of the old Roman road which provided a wonderful foundation for his new tower.' During the blitz the church was severely damaged again, but the 3,000-or-sotonne tower remained intact, partly because of the solid Roman foundation and partly because of the quality of Wren's masons, Thomas Cartwright and J Thompson. You hope Cartwright's and Thompson's client was as appreciative when they found their steeple cost only a little less than the church itself.

PROJECT BRIEF The new rector, George R Bush, wanted to revitalise the dialogue between City and church. With Harrap, who he knew from his Shoreditch ministry at the 19th-century St Anne's Church, he embarked on a campaign to improve the image and purpose of the church. Harrap's task was to clean up the Portland stone exterior, resuscitate the entrances and make stone repairs, with similar care to the interior. New lighting was needed to address a concern about security. Harrap says: 'The street has always been a fairly wild place through the history of this building.'

REVITALISING THE VESTIBULE The first thing was to do something about the ramshackle quality of the street/church interface. Harrap says of the vestibule: 'It is the room which links the inner vestibule of the church and Cheapside.

It is the public's transiting space between entering the church and leaving the business of the commercial street.' On plan, the vestibule is a squat symmetrical cross with several steps at the junctions of the four short arms. The stepping is a variant of that old Gothic mason's trick of maximising the bearing area of the stone disguised by a succession of vertical folds in the fabric. The entrance to the church's internal vestibule is on the south and there are elaborate street entrances to the north and west. Overhead, in the middle of the space, is a dome springing off a heavy cornice with what would have been an open oculus at its apex. This is actually a large hole for bell-ringing ropes and for hoisting the bells into place. Closed, the street doors have a concave semicircular head leaving, in combination with the stone arch overhead, a big circular opening. There are spikes around the top of these doors to discourage climbers.

EXISTING CONDITIONS Some time early in the vestibule's history, its interior had been given a harmonising lime wash, followed by its first overall coat of paint. Over the following centuries it was re-painted in whites, creams, water paint, oil paint and a bituminous black at the lower levels: the 20-second coat was a modern paint which, not surprisingly, had some difficulty adhering. Harrap explains: 'It's something that people did in order to brighten up a tired surface.

It was economical and straightforward and something that they did without thinking about the consequences of sealing the surface of stone - which has to breathe. There were indications that there was entrapped damp behind it forcing the paint off - and creating a damaging salt layer in the top surface of the stone. The cultural surface was being eroded.' Outside there was a build-up of jumbled fixings, wires and notices on the Portland stone. There was also encrusted dirt on the complex detailing of the two entrances, each with its rusticated niches enclosing engaged Doric columns and entablatures surmounted by putti on either side of garlanded oculi. Harrap says dryly: 'This did not really present the appropriate architectural expression of a great piece of architecture which the client was keen to establish.'

QUANTIFIED SPECIFICATION Harrap explains: 'We used the JCT Minor Works contract. We were very keen to use what we describe as a quantified specification.

We annotate the drawings with words and then that is converted into a fuller description, enabling a builder to give a fixed price for the works. So that, even for quite complex installations, we are able to judge fairly and accurately between tendering contractors and we can use this as a document for changes and variations.

'I know it is a popularly held view that this kind of work is susceptible to variations. But if you are able to analyse what you are intending to do and are given time to do the analysis, it is not. We would usually have a pre-contract phase during which we would explore the areas of difficulty and confine them to small areas of the contract so that the risk to everybody is minimised.' At St Mary-le-Bow, this involved a close visual inspection of all the surfaces and detailing.

PAINT ANALYSIS Paint analysis specialist Crick Smith Conservation was brought in at this stage to take 6mm square samples of the whole paint layer. Their edges were polished and examined under a microscope and dust layers were analysed to give an idea of the time interval between the coats. Here Harrap was particularly interested to know about the first dirt layer: would this indicate that the Bath stone had initially been left bare and then over-coated with a thin lime wash? His suspicions proved correct.

With this inspection and analysis to hand, the main contract was left to specialist contractor, David Ball Restoration.

Harrap challenges the concern that traditional construction skills are disappearing, saying: 'There has been an enormous growth in the conservation industry which has been very welcome, with young people wishing to train in traditional trades or take degrees in conservation. All of this has brought about a treasury of skills for tackling conservation projects.'

REWORKING THE EXTERIOR Work on the exterior comprised three main activities: the first two involved straightforward mason's work; replacing small pieces of damage and the odd stress cracking and repairing the moulded stonework of the columns, the enriched frieze and the cornice.

Here there was damage from previous scaffolding and impact, plus some stress cracking caused by settlement and slight spreading of the whole structure. Harrap says: 'In some cases you need to support a structure but here you were taking out a neatly cut pocket and replacing it with a piece of stone of exactly the same size - you should see a puff of stone dust when it goes in. We would not accept the BS standard which allows tolerances of a few millimetres. In the curved work we have to get properly qualified stonemasons to mould them on site. If you have a lot to do, you will pre-mould and fit on site. But with small elements of repair you try to avoid pre-run mouldings.' The third element is the carved stonework to each of the two street doorways with their pairs of putti and garlands of fruit. Harrap says: 'We were faced with a difficult dilemma - one not infrequently met - that in a 1956 restoration the contractors, working within the knowledge and practice of the time, had applied hard sand-and-cement weathering to these details and had embedded copper nails to discourage pigeons. The copper nails had worked so there was little in the way of pigeon droppings. But we looked at the dangers of taking off this capping. If the details were in a museum you might want to take the capping off. But here you are in the open and coating protects the stone for the time being - maybe 20 years. So we decided to pack, fill and repair the surface of the carved stone, then over-coated it all with several lime washes to draw together the colour of the cement and the Portland stone. Now the carvings look very handsome. I know where the cement and Portland stone meet but I don't think anyone else will.'

STONE MATCHING Harrap's team knew what kinds of stone they wanted - they had chips from unobtrusive locations looked at under a microscope for grain and colour. But the original quarries were long worked out and they had to decide on available stones which would match - not easy since the Bath stone involved is not constant in colour and has imperfections such as shell and stone beds. Eventually, Stoke Ground Base, Top Bed and Guiting stone were selected as the nearest equivalents.

PAINT STRIPPING Getting the 22 layers of paint off the vestibule interiors involved some pre-contract rehearsals. The outer and most glutinous surfaces were removed using poultices of caustic material. Deeper, water-based layers were removed with pressure water jets. Harrap explains: 'Once we were down to the first lime-wash surface we could then use a mild vortex abrasive system, using fine silver sand to get through to the stone surface without damaging it.' With the stone surface gently cleaned, Harrap had several lime washes applied, as on the exterior, to harmonise the slightly different colours of the replacement stone.

LIME WASH Protection methods have their fashions. Thirty years ago penetrating silane treatments seemed to offer a solution but, Harrap says: 'We now take the view that lime wash is the safest.

It is reversible and when you use various additives such as gluten, egg white or pozzolanic admixes like earth colours or brick dust you provide yourself with a coating material which can be matched to the colour of the substrate.' The subject of lime is quite complex because it can be used in so many different ways, from super-strength hydraulic mortar to the very soft sacrificial mortar used in the St Mary-le-Bow crypt to protect the brick from erosion.

LIGHTING There is a tendency to light old buildings from below - which is wrong because classical mouldings and details are designed with daylight (which comes from above) in mind. Inside the vestibule, Harrap decided to use Aktiva strip lights concealed along the top of the main cornice to wash the dome with light. Research had shown that Aktiva fittings were best at providing a continuous wash, rather than spots of light.

IRONMONGERY Harrap says: 'The great oak gates are restrained open by iron hooks fixed in the stonework and they were not all present. So we had to replicate new ones and we chose to make them in steel instead of recycled cast iron. It was all straightforward and robust: steel 1.5m long, an inch in diameter and a hook at one end.

'Up until the 1960s there used to be firms like Gibbons who made ironmongery. Now it is either too architecturally styled or false antique which you must avoid. And so I decided to use recycled ironmongery from Drummonds. It is a posher firm than I would usually go to but they have a very good warehouse and I found some 1930s bronze handles and got two pairs. They are for the rector. He opens the doors every morning and I hope he has a daily tactile pleasure.'

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