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With the grain

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Our conservation feature begins with a review of work on the twelfth- century ceiling of Peterborough Cathedral. A sophisticated and meticulous programme is making it secure for the future

In any cathedral there is an elaborate world of staircases and passageways concealed from its congregation and visitors. At Peterborough, one such staircase at the west end leads to a chamber dominated by a wooden windlass more than 3m tall. Dating from c1200, when it was used to haul up stone, it is a vivid reminder of the way these huge medieval works were made. Climb another claustrophobic spiral and you emerge eventually at clerestory level, breathtakingly high in the building. There, on a vertiginous ledge, you are as close as you can get to one of the cathedral's greatest treasures - the late-twelfth-century timber ceiling that stretches down the length of the nave.

Conservation of this ceiling, the only one of its kind that survives in situ in the uk, is central to a £7.3 million programme of works which the Peterborough Cathedral Development and Preservation Trust hopes to execute within and around the building. Phase one, confined to the ceiling in the easternmost of the nave's ten bays, is now complete, after painstaking investigation and treatment of both structure and decoration.

Many different disciplines have been involved. The dendrochronology laboratory at the University of Sheffield undertook a tree-ring analysis of selected timbers and boards; Helen Howard of the Courtauld Institute of Art examined paint samples; and a team from English Heritage conducted a photogrammetric survey. Barry Knight of eh was responsible for environmental monitoring, while cathedral archaeologist Donald Mackreth and art historian Paul Binski of Cambridge University added expertise. The subsequent treatment - intended to forestall any further intervention for at least 50 years - was carried out by Hugh Harrison (the structure) and the Perry Lithgow Partnership (the decoration), in collaboration with cathedral architect Julian Limentani of Marshall Sisson.

It was Limentani who, in 1994, set the process in motion: 'I was looking at the ceiling on one of my regular inspection visits to the cathedral and its condition began to worry me,' he says. The Dean and Chapter were persuaded to share his concern and a team duly assembled to investigate: 'It was very important that I got the best brains to come in and advise me, and vital too that we spent the length of time that we did - two years or more - in research and monitoring. Without that we wouldn't have understood very much.'

Peterborough's ceiling, 62m long and 11m wide, and canted on either side, is made of oak boards of varying section arranged to form a series of lozenges. The borders of each lozenge are ornamented with key, wave, chevron and scroll patterns and at the centre is a figure of some sort - human or animal. With its mingling of saints, angels, kings, and archbishops, the iconographic scheme is believed to symbolise the powers through which God created and rules the world.

Representations of the liberal arts appear - Music, Astronomy, Grammar and Geometry - as do such pagan survivals as the 'green man', familiar from medieval bench-ends and misericords. One of the two dominant lozenges in the conserved easternmost bay depicts Janus, the Roman god of time, his head looking back and forwards (to past and future). The other shows four lions with fish between their feet, signifying earth and water in a semblance of physical creation.

To better understand the structure and history of the ceiling you must follow more of those hidden staircases and passageways until you arrive in the space between the top of the ceiling and the pitched roof above. In the dim light a dense grove of timbers gradually resolves itself into a row of scissor-brace trusses, which were largely reconstructed during restorations in 1834-5 and 1924. Deathwatch beetle infestation was the problem in 1924, when new pine members were attached to the truncated collars and braces of the original roof.

Along with this major intervention had come more localised, less conspicuous ones over the years, as the multidisciplinary research revealed. For example, a considerable number of softwood boards had been inserted among the oak ones since the sixteenth century; there, and elsewhere on the ceiling, the original nails to secure the boards had often been supplemented or replaced. Such pragmatic remedial work was sometimes injurious, as happened during the 1924 restoration when screws inserted from above pierced the underside of the ceiling boards and splintered their painted surface. In addition, when examined minutely from below, the boards were split in places (as the wood contracted and dried after completion of the ceiling), and there were scattered instances of wet rot, infestation and wood loss. There was even some damage by lead shot, which had presumably been aimed at an irritating bird.

As for the painted decoration, which dates back to c1220, Helen Howard's survey established the original pigments that were used but also revealed occasional crude overpainting that obscured former subtleties. This overpainting included nineteenth-century layers that were high in both calcium sulphate and clay-rich minerals, and so very susceptible to moisture - a clear pointer to what should (and should not) be done when it came to conservation. Other problems that would have to be addressed included flaking and powdering paint, bloom and efflorescence, surface staining, and the residue of microbiological growths.

Remedial action

Conservation work at the altitude of Peterborough's ceiling, some 25m above the nave floor, must be a daunting prospect. 'To begin with, it takes about a month to erect the scaffolding,' says Limentani. 'Everything has to be hauled up and put together so carefully.'

The structural repairs required access from both above and below. This, in the case of the offending screws, had to be carefully co-ordinated. During the 1924 restoration the top of the ceiling had been covered in hessian, in which windows were now cut to reveal the screws. They were then carefully loosened while, linked by a walkie-talkie, someone watched from beneath so that the operation could be halted if any splinters were coming loose. The displaced splinters were then repositioned and secured with a solution of Plextol B500, while the gaps in the hessian were patched with sailcloth.

Individual treatment specifications had been drawn up for more than 70 of the oak boards, depending on how secure each of them was. Some of the loose pieces were fastened with 3mm threaded stainless steel studding (which was bent over to form an angle and fixed with nuts and washers above); others, with 25mm stainless steel screws and washers, using old screw holes if possible. Where small areas of boarding were unstable because of decay or infestation, exposed wood was consolidated with Paraloid B72.

One further structural issue involved the wrought-iron hanging bolts that were installed in an 1830s restoration to carry the flat part of the ceiling. As they were liable to corrode, and anyway uneven in the support they were giving, it was decided to remove them all and re-insert them with stainless steel spring washers, adjusting slackness or over- tightness where necessary - a complex process that called for Science Museum advice.

A major task in conservation of the decorative elements was to re-attach flaking paint while minimising any contact with water. Each flake was first made sufficiently flexible with a five-minute jet of warm, dry air from a preservation pencil set at 40degreesC. Then the surface behind it was injected with industrial methylated spirits followed by drops of an adhesive solution (Plextol B500 again, in de-ionised water). Finally the flake was eased back into place with a small pad of dry cotton wool through Japanese tissue, with the cotton wool absorbing any excess adhesive.

A moisture-free method for cleaning the painted surface, to deal with bloom and efflorescence as well as dirt, required Wishab sponges which - made of synthetic rubber granules - self-abrade as they are used. Loose particles of dust were first brushed softly from the surface and sucked into a nearby vacuum-cleaner nozzle, and then small pieces of these sponges were applied in gentle circular strokes. More intractable stains were reduced by means of acetone swabs. Glue which had seeped between the boards after the hessian was attached in 1924 caused complications in places, and where warm de-ionised water wasn't a suitable remedy, the preservation pencil was again brought into play.

Funding the future

Limentani is confident that, when such treatment has been carried out to the remaining nine bays, the ceiling is unlikely to need attention for a century or more. 'One doesn't want to go up there too often,' he says. 'Every intervention brings disturbance with it, no matter how careful you are.'

The superficial benefits of conservation are already clear, not just close to the ceiling at clerestory level, but back on the ground in the nave. This easternmost bay is perceptibly lighter in tone, giving its decoration greater immediacy. For aesthetic impact, the ceiling must always have profited from the architectural character of the cathedral, which, almost entirely Norman, is quite austere in detail, so offsetting the embellishment above.

This first phase of work was funded by English Heritage and individual donors; with the second, which tackles the next two bays of the nave and is just starting on site, comes an international dimension. While English Heritage is again contributing, the cathedral has received £10,000 from Japan's Sumitomo Foundation and over £80,000 from the European Community's raphael programme, which funds projects to preserve the Community's cultural heritage. Peterborough's bid had to be made with European partners who were undertaking similar work, and these were eventually found in Norway, at the church of Vestre Slidre, 100 miles north of Oslo, and in Sweden - at the Museum of National Antiquities in Stockholm, which plans to conserve comparable panels from a demolished thirteenth-century church at Bjorsater in the south of the country.

Already scheduled as part of the £7.3 million programme at Peterborough (for which some £6 million has now been raised) is a £400,000 installation of new lighting in the cathedral, which should enhance the presentation of the ceiling. To appreciate the scope of the other conservation works, however, you need to leave the cathedral by its west door for the calm precinct that surrounds it, just off the city's market square. There, Anthony Richardson & Partners is converting the chancel of a demolished church, a Victorian dwelling, and an early Georgian terrace into a cafe, shop and visitor centre - for the last of which, ingeniously, the architect has dropped the floor-level by 1.5m to make habitable space out of former vaulted storage areas for the three Georgian houses. In this centre the history of the cathedral will be presented to its visitors: doubtless the ceiling, that product of the twelfth-century made fit for the twenty- first, will play a prominent part.

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