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Restoration drama

5 Nov 21 Building conservators have just completed a pilot contract paving the way towards the complete restoration of the West Front of Wells Cathedral. David Taylor paid them a visit

Historic building restoration isn’t quite like normal repair and maintenance. It often goes beyond simply using standard materials and following standard procedure. In its purest form, it is more akin to archaeology or even fine-art preservation – with a generous helping of science thrown in.

On a Grade I-listed architectural gem like Wells Cathedral, in deepest Somerset, authenticity and attention to detail are paramount.

Specialist contractor Cliveden Conservation has recently concluded a pilot contract to carry out repairs to part of the ornate West Front of the 12th century cathedral. Its work here is the latest in a catalogue of pioneering restoration projects at Wells going back decades, says Cliveden’s senior project manager Berenice Humphreys:

“Over the years a Hollywood A-list of conservation specialists has worked on this cathedral,” she says. “This is the sort of job conservation firms would fight tooth and nail to win.” The ultimate aim is to restore the whole West Front.

This article was first published in the September 2021 Top 100 Contractors issue of The Construction Index magazine.  Sign up online

In fact, Wells Cathedral played a central role in the rediscovery of traditional building techniques and the development of the ‘lime method’ by Professor Robert Baker during the 1970s and ‘80s.

Baker is widely credited with developing the methods used by today’s conservators following the wholesale loss of traditional craft skills in the early 20th century. Construction in the post-WW1 era rapidly embraced the new materials and methods that came to characterise the modernist era: cement was in; lime was out.

Baker’s work highlighted the damage done to old buildings through the misguided use of modern materials and methods in repair and maintenance. And Wells Cathedral was the focus of his work.

The aim of Cliveden Conservation’s pilot project at Wells is to get an understanding of the repairs that have already been done, find out what’s causing the deterioration of the stonework and decide on the best methods for repairing it.

Some repairs are so good that they are difficult to see and some detective work is required to identify them. “Knowing that a repair is there isn’t important; but knowing it’s faulty is,” says Humphreys.

The West Front of Wells Cathedral is widely acknowledged as the finest of its type in the UK and one of the best examples of early English Gothic architecture. Extending to a height of around 30m and approximately 45m in width, it is ornamented with deep relief carvings and rich iconography occupying niches in the stonework.

Cliveden Conservations’ contract is confined to the very top tiers of the central gable. These comprise four levels featuring (from the top) Christ in Majesty, The Apostles, The Order of Angels and the Resurrection. With the exception of the larger-than-life statue of Christ, which was replaced in the late 20th century, all the statues and associated carvings date from the early to mid-13th century.

Access to the gable is provided by a very large scaffold structure obscuring most the façade. And while it is difficult to appreciate the full extent of the stonework’s deterioration from ground level, this becomes very clear when you get up close.

Centuries of exposure to the elements have caused deep erosion of the locally-quarried Doulting limestone that comprises most of the masonry. Many of the statues lack details such as fingers and facial features – some are so badly eroded that only a rough outline of the figure remains.

Perhaps surprisingly, the restoration work does not aspire to replace these damaged statues or restore them to their original appearance – which in most cases would necessarily involve a degree of guesswork.

Instead the focus is on cleaning the façade and arresting the deterioration.

“We first have to understand what’s causing the deterioration and how it’s affecting the stone,” explains Humphreys. “As well as damage to the exposed parts, the wind creates turbulence in and around the sculptures so that there’s erosion behind them as well.”

The methods used, and the extent of each repair, have to be carefully considered to determine the most appropriate treatment. “We’re conservators, not builders,” explains Humphreys. “We ask ourselves does it have structure? Will it survive? And we consider the aesthetics – for example, is it worth restoration or is it just a blob of stone?”.

Some details – mostly those with a structural role to play such as plinths, quoins and capitals – are replaced with new masonry carved by Cliveden’s own masons. But most of the repairs to the statues are done using lime mortar to fill voids and build up the stone where erosion is most severe.

First, though, there is the cleaning. The stonework is heavily contaminated with moss, algae, pigeon droppings and general detritus and this has to be carefully, but thoroughly, removed. More difficult to treat is the build-up of calcium sulphate adhering to the surface of the stone.

This is the result of pollution from fossil fuel combustion that reacts with the calcium carbonate in the limestone masonry. The result is a film of hard calcium sulphate and carbon particles that clings to the surface of the stone, particularly in sheltered locations which are, of course, the most difficult to access.

Sulphation makes the stone look dirty, but it can also cause a lot of damage, explains Humphreys: “It forms a hard film and actually halts the erosion of the surface layer, but the stone behind it carries on deteriorating.” Eventually blisters form over the eroded stone which then crumbles away.

Badly sulphated stone can sometimes be repaired successfully by injecting a thin lime-based grout into the void behind a blister – a method which Humphreys used recently on a restoration contract at Tewkesbury Abbey.

Mortar repairs are mostly carried out by building up eroded stonework with several thin layers of lime mortar blended carefully to match the natural stone. When the mortar has cured, it is virtually indistinguishable from the substrate.

Numerous trial mixes were made up before Humphreys and her team felt they had perfected the optimum solution for the four types of stone on the façade.

Some repairs consist of simply plugging holes and filling cracks, but more extensive repairs require more complex solutions. Where mortar repairs are considered excessively deep, ceramic armatures – essentially fired twigs of clay – are used to provide a framework to build on.

Armatures made of stone or stainless steel wire can also be used, but Humphreys favours ceramic: “Ceramic is better than wire because it holds moisture, bonds with the mortar and doesn’t have the expansion-contraction cycle of metal,” she explains.

Most modern building materials are produced in large quantities and – current shortages notwithstanding – are usually easy to acquire. Not so with some of the materials needed for this project:

“Because this is a pilot, part of our job is to find sustainable and reliable sources of material in sufficient quantity for the whole West Front,” says Humphreys.

“We’ve been scouring the market for well-aged lime putties. We found some 15-year-old stuff that was really fantastic but there was not enough. We need a consistent, reliable source that won’t run out,” she explains.

While lime putty mortars are used for most repairs, other products are also employed. Hydraulic lime – hydrated lime in powder form, gauged with small amounts of pozzolanic additive, such as burnt clay, to make it set – is used on some string courses, coping stones and other architectural details where greater strength is needed. These mixes set harder and more quickly than the soft putty mortars.

Finished repairs are then given a ‘shelter coat’ of thick limewash – usually coloured to match the existing stone – which is then rubbed back with hessian. This helps to bond the loose surface of the stone but it doesn’t leave a skin.  A shelter coat is essentially sacrificial, so when weathering occurs it erodes the shelter coat rather than the stone.

Various other techniques have been considered, although not adopted, for use on this project, including the use of consolidants and hot-mix lime mortar.

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Consolidants are chemical solutions which (rather like the wet-rot wood hardener sold in many DIY stores) soak into friable stone and set hard to reinforce the fragile matrix. Most are water-based and the most common is limewater, which soaks into the stone, depositing crystals of calcium carbonate.

However a problem with limewater is that large volumes of solution are needed and this can have harmful side-effects such as the dissolution, mobilisation and recrystallisation of salts. For this reason, limewater is not being used at Wells.

Neither is the more recent innovation known as ‘nanolime’. This is a solution of synthesised calcium hydroxide in alcohol which deposits crystals of calcium less than 100 nanometres in size within the stone.

Hot-mix lime mortar is a much less scientific and far more traditional product. This is mortar made by mixing quicklime with aggregate and slaking it on site immediately before use, rather than using pre-slaked lime putty and mixing the mortar in the usual way. The slaking process is highly exothermic, generating temperatures in excess of 100oC.

“It’s the latest thing,” says Humphreys – by which she means it is becoming increasingly popular among conservators. “It certainly would have been used here by the original builders,” she adds. During the construction of medieval buildings, the limestone would have been burnt in the local lime kiln and the resulting quicklime delivered directly to site along with all the other materials.

Here, it would have been mixed with water and sand and the resulting hot mortar used straightaway. There are several advantages to this – one being that hot-mix mortar can be used in very cold weather without any risk of failure.

Another is that the material is used as soon as it arrives and isn’t stored on site – just-in-time delivery is not such a new idea after all. However, hot-mix mortar is in every sense a ‘quick and dirty’ method – fine for new-build projects but lacking the precision and finesse required for delicate restoration work.

Throughout the project, Cliveden Conservation has kept detailed records of all the defects encountered and all the repairs carried out. This has required Humphreys, in consultation with cathedral architect Nick Cox and clerk of works Jez Fry, to develop a bespoke system of recording the condition of the stonework and its treatment (see ‘For the record’). “It took a long time,” admits Humphreys. “We even had discussions over the colour of the pens we’d be using.”

Work began on site in late spring with a strict deadline for completion in late August. All the scaffolding had to be dismantled and cleared away by the 23rd August in time for a ceremony to mark the installation of one of sculptor Anthony Gormley’s cast-iron sculptures in an empty niche below the cathedral’s south-west tower.

This article was first published in the September 2021 Top 100 Contractors issue of The Construction Index magazine.  Sign up online

For the record...

Records of previous repairs to Wells Cathedral cannot be traced, so Cliveden Conservation has had to start again from scratch.

High-resolution images – including a 3D scan taken in 2015 – were however available and provided the team with a template from which to start.

“Digital recording has its place, but is less use on a messy working scaffold, so we had printed copies, coloured pens, and notebooks corresponding to each element for additional notes,” says project manager Berenice Humphreys. 

“The condition of each was marked up in hatching to show decay processes, and again marked up with a proposed repair schedule – we used the tender specification as a guide but they only had cherry-picker access so relied on our further investigations following cleaning. 

“We will then collate that information into a set of marked up images.”

This article was first published in the September 2021 Top 100 Contractors issue of The Construction Index magazine.  Sign up online

The lime cycle

The mortars used at Wells are all based on non-hydraulic lime putty mixed with carefully-chosen sands and fine aggregates. Lime putty is made by slaking calcium oxide – quicklime – with water to produce calcium hydroxide.

This slaked lime is left to mature in water and over time consolidates, becoming sticky and ‘fat’ as all the quicklime is converted. This mature putty mixes well with the aggregate, is easy to work and produces better, more stable, results. The longer it is left, the better the quality.

Unlike cement, which sets rock hard even under water, non-hydraulic lime needs to dry out and will only cure by absorbing carbon dioxide from the atmosphere. This changes the calcium hydroxide to calcium carbonate – the same substance, chemically as the limestone that was burnt to create the quicklime at the start of the process.

This cycle of chemical transformation – limestone-quicklime-limestone – means that, besides being essential for authentic and non-destructive repairs to old buildings, non-hydraulic lime has a much smaller carbon footprint than cement, which dumps huge quantities of carbon dioxide into the atmosphere but reabsorbs none of it.

This article was first published in the September 2021 Top 100 Contractors issue of The Construction Index magazine.  Sign up online

Special skills

Building conservation is a specialised sector requiring a diverse set of skills, says Cliveden Conservation’s Berenice Humphreys. “There’s a lot of science in it; but there’s also a lot of history and art,” she says.

Training, therefore, is not as straightforward in this sector as in many other disciplines.

“It can be difficult to provide the range of projects an apprentice needs to fulfil their apprenticeship…and we need to have the right project for the candidate to get the best out of it.” 

Getting a CSCS card is even harder: “Yes, you can get a card in some skills such as carving, masonry or plastering, but some of our team have obtained these through ‘grandfather rights’ – historic apprenticeship schemes that are no longer recognised by CSCS,” she says.

“For conservators there is no Heritage card as the ‘skill’ is too varied.  The only way is to be accredited through the Institute of Conservation,” explains Humphreys.

“For most new graduates, it is natural that they would need to gain experience in the field prior to applying to become an Accredited Conservator, and yet they will struggle to get that experience as they cannot get a CSCS card to allow them on site.  Some degree qualifications allow you to be an ‘academically qualified person’ as a CSCS cardholder but, like the Heritage cards, not all courses count.”

The CSCS was originally intended to show competency on site but now it focuses very much on NVQ qualifications, says Humphreys: “Most conservators come either through degree-level qualifications in a directly-related subject or something allied - such as Fine Art, or even cross-over from something entirely different.  This makes it very tricky for the team.”

Humphreys herself has a degree in combined arts from Leicester University and a master’s in building conservation from Bournemouth. The two conservators working on site with her at Wells – Andrea Walker and Jenna Burrell – both have first-class honours degrees in conservation from Lincoln University, one of the few universities to offer a dedicated heritage conservation degree course.

This article was first published in the September 2021 Top 100 Contractors issue of The Construction Index magazine.  Sign up online

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