Laser precision on Lincoln Cathedral conservation

Temporary scaffolding has been erected on the cathedral’s west front to enable stonework conservation

Digital scans, laser guns and CNC machines are among the technology being used on a major conservation works programme at Lincoln Cathedral. Will Mann reports.

Twenty metres above ground, on a giant temporary scaffold structure on the west front of Lincoln Cathedral, Alex Blades is pointing a laser gun at the masonry. A stone conservator, she is clad from head to toe in PPE and her work zone is completely contained within black sheeting. Blades squeezes the trigger, a beam of light emits from the gun, then there is a crackling sound and a smell like burning hair.

“That’s hundreds of years of industrial pollution – a build-up of general grime – being burnt away by the laser,” explains Jane Cowan, the cathedral’s head of conservation.

As if by magic, the sooty covering on the masonry disappears, revealing the rich, golden hue of the limestone underneath. “It’s a very efficient way of cleaning the stonework,” says Cowan. “No other technique cleans better – the finish is superb.”

The laser cleaning is part of a major programme of conservation works currently underway at Lincoln Cathedral. The National Lottery Heritage Fund (NLHF) agreed in 2017 to provide £16m to finance the programme, called Cathedral Connected. It includes conservation work to masonry and decorative sculptures, structural repairs, a newbuild visitor centre and landscaping.

Leading the west front work is Michael Sheppard, who joined the Cathedral’s property team in July as “clericus fabricae”. “Essentially it means clerk of works,” he explains, “though my official job title is director of works and property. My line manager is the subdean, one of our residential canons.”

West front works

Prior to Sheppard’s arrival, the project was led by Rebecca Thompson, past president of the CIOB. who runs her own heritage consultancy. She also helped recruit Sheppard.

“Cathedrals are complex places to work, with the primary purpose being the welcome of people of all faiths and none to worship, visit and enjoy the cathedral,” she says. “The role of the works department is to enable this function by providing a safe and well-maintained building.

“One of my main tasks was to ensure the works to the west end of the cathedral were on programme for the Connected project.  The restoration and conservation of the west end includes the very special southern frieze that has been covered over for 30 years.”

The west front works required assembly of an enormous temporary works structure (see box, p24), which extends to the top of the gable, a height of some 25m.

Above: A laser burns through the soot on the masonry Below: Part cleaned stone shows the effect of the laser

This elevation includes the famous 12th century Romanesque frieze. The 21 panels, each one about 1m square and weighing 0.5 tonnes, are split into two sections: the northern run, the New Testament sculptures, were removed from the building and replaced with “copy carvings” between 1989 and 2001; the southern run, the Old Testament, is being worked on currently.

“There are nine Old Testament panels and we are currently assessing their condition,” says Cowan. “Our policy was developed on the northern run, where the approach was to leave every panel in situ unless in very poor condition. We have the same presumption for the southern run.”

In the event, all the northern run panels were taken out, some in over 60 pieces. After a full condition assessment of the southern run panels, it was decided to remove only one, which was in poor condition – a delicate operation.

“Initially, we protect the face with a build-up of latex, clays, release materials and paper, all applied in situ – about six weeks’ work,” explains Sheppard. “Then we put sliding straps around the panel, release it from its mortars, and slowly remove it from its bed, manually. It’s a very slow process – three to four weeks – and it came out in dozens of pieces.” Propping was fitted into the void left behind.

Sheppard explains that the condition of the stone is partly down to the quarry it comes from. Lincoln Cathedral, unusually, is built from the limestone rock it stands on, and it owns a quarry a mile away where it sources stone for maintenance and repairs.

“When you extract stone from the ground, the bed height determines the height of the stones you can work with,” Sheppard says. “With limestone, these restrictions are especially tight. And where the stone was not cut completely within the bed, you can see the impact on the relief carvings, where they have been delaminated by weathering, freeze-thaw and water ingress.”

Lincoln’s 14th century Gallery of Kings, on the west front, is currently being conserved

The removed panel has been taken to the workshop behind the cathedral. “A variety of techniques will be used, including doweling, grouting and mortar repairs, but they all have to be reversible,” explains Sheppard. “We can’t make permanent changes, and everything we do will be recorded in detail.”

Eventually the panel may be returned to the frieze, but Sheppard says it’s difficult to say when. “Compared to programming on a conventional construction project, you have to build in a huge contingency on conservation work,” he says.

Another issue with the frieze is structural. “Some of the panels are load-bearing, which has meant compression and further damage to the face of the panels,” explains Sheppard. “Because we have decided on minimal intervention with our work on the southern run panels, we will have to keep a careful eye on the impact of the compression.”

He adds that a structural engineering assessment currently underway will report on the compression’s likely impact.

Copy carvings are being made of all the panels, some of which may be used on the building, depending on the outcome of the survey work. The sculptures include depictions of Noah and the ark and Daniel in the lion’s den.

“Four of the country’s leading copy carvers were chosen for this work and each panel will be an exact copy of the original but with weathered or missing elements intact,” explains Cowan. “CNC copies were made out of foam as an aid for our carvers, who were based off site, along with regular site visits and 1:1 photography.

“The copies not used on the building will go into an exhibition area in the new building, along with the frieze panels removed during the northern run work.

“Some of the panels have had repairs carried out during the 19th century, using Roman cement, and in the 20th century, with Portland cement. If not detrimental to the performance of the panel we leave them as they are,” she adds.

“It’s important to note the difference between ‘conservation’ and ‘restoration’. We only ‘restore’ the masonry if it’s past the point of conservation, though it’s a different consideration if it has a structural or functional role.”

The main issue with the west front conservation is cleaning, Cowan says. This is where the laser comes in. It’s currently being used on another of the sculptures on the elevation, the 14th century Gallery of Kings.

Lincoln was the first cathedral to use lasers on site in 1998 and has its own unit. They typically cost £25,000. The portable laser units are about 1m across and 0.5m high. The operative connects the gun to the unit and fires the beam on to the blackened surface.

Above: Deputy head glazier Dan Beal at work. Below: Cathedral head of conservation Jane Cowan and clericus fabricae Michael Sheppard

“The laser is only attracted to dark areas and because the sulphation carbonation is black, it means we don’t overclean,” says Cowan. “There is also less room for human error as they are ‘self-limiting’. Lasers are a very controlled and measured way of cleaning the dirt.

“The laser is mainly used for our finest carvings, because the width of the beam is small, and it is not practical to use on a large scale.

“In some areas the build-up of crust can be two inches thick – a legacy of the Trent Valley industry.”

Safety protocols are strict. “Laser beams can cause blindness up to a mile away,” says Cowan. Operatives are contained in the black sheeted unit, wearing body suit, face mask, eye protection, with an extraction unit running throughout.

Other cleaning techniques are not as controlled and can be subject to human error. These usually involve mixing water, sometimes superheated, with aggregate or solvent and spraying it at the masonry. Nebulous systems create a fine mist which dissolves the dirt and pollutants.

“All work is done by hand and only occasionally would we use a pneumatic tool, such as a chisel, to work at the face,” adds Sheppard.

The masonry conservation works are being executed by the cathedral’s team of nine: three apprentice bursary stonemasons, three fixers (based on site) and three bankers (based in the workshop), with eight stone conservators. The cathedral, along with Lincoln Castle, is running training and apprenticeships with Historic Environment Skills (HES), funded by NLHF, which aims to encourage young people into the built heritage sector.

Other conservation work in the Connected programme will include the stunning stained-glass windows in the south and north-east transepts, some of which date from the 13th century, plus masonry repairs to the East Gate and North Cloister. The work will run alongside the cathedral’s ongoing programme of works and reactive maintenance, which has a budget of up to £2m a year.

Re-roofing and repointing

While Sheppard’s team manages any work involving the fabric of the cathedral, some of the Connected project will be delivered by heritage specialist William Birch. The contractor is carrying out re-roofing, repointing and structural repairs to the 14th century Exchequergate Arch, which frames the walkway leading to the cathedral’s west front, and is currently encased in a temporary works structure.

William Birch will also renovate the Old Deanery, landscape external areas and build the new visitor centre which will house the cathedral’s store of medieval stone and sculpture, as well as the new copy carvings of the Romanesque frieze.

As part of the Connected project, the whole west front will be laser scanned. “We would like to do 360-degree scans of the Gallery of Kings as well,” says Sheppard. “This is part of our ongoing programme of recording works carried out on the cathedral. This informs our disaster management, which obviously has been brought into sharp focus since the Notre Dame fire.”

The 83m-high tower has already been laser scanned, commissioned by Buttress, the cathedral’s architect, and Sheppard says the aspiration is a complete digital asset model of the whole cathedral and the 87 other buildings in its estate portfolio.

The Lincoln Cathedral Connected project is to complete in March 2022.

Temporary works

A ‘workshop in the sky’ enables conservation to continue in a very sensitive location.

After the NLHF grant for the Lincoln Cathedral Connect project was confirmed, much of 2017 was spent designing the west front’s temporary works structure.

This “workshop in the sky” was engineered by Elliott Wood and erection of the structure, by scaffolding contractor PMC, began at the start of 2018.

The self-supported steel gantry is raised roughly 3m off the ground, which keeps access open for visitors into the cathedral. It occupies around one third of the west front, extending as high as the gable.

“It’s a very sensitive location as we are right next to the main entrance and have to coordinate around cathedral operations and public tours,” says Sheppard.

The structure is not “actively” tied into the cathedral, but instead is counterbalanced internally. Four penetrations were made by removing glazing panels from the vast, arched Norman window in the west front.

Sturdy 203mm-thick steel beams, to a bespoke design, were then threaded through and fixed to the counterbalance. The structure is also braced off the 3.5m-deep Norman niche.

Above, the gantry supports 11 scaffold lifts, which are being used to execute the NHLF programme of works.

The cathedral has decided to carry out further renovation work above the niche in house, as part of its ongoing works programme, so has added a further five lifts, taking the structure to the top of the gable. A single hoist serves the structure.