Conserving South Foreland Lighthouse

The structural conservation project

During the mid-2000s, we noticed that the internal plaster and paint in the lighthouse tower was flaking, with white crystaline deposits on the exposed surfaces. These crystals, called efflorescence, are usually associated with water migrating through brick or stone structures, collecting chemical salts as it does so. When the water evaporates, it leaves the salts behind as white crystals on the surface.

Although surface repairs to the plaster had been attempted, these were ineffective, and the situation gradually worsened with further paint and plaster deterioration. The National Trust started an investigation in the winter of 2010/11.

Where was the water coming from, and why was it appearing throughout the tower?

Investigation

The investigation covered the lighthouse structure and sought records of past work and surface treatments. Unfortunately, few records existed, and information had to be taken from the structure itself. The Trust employed a team of specialists to perform invasive research into the source of the dampness. Core samples were taken of the tower structure. These confirmed a brick structure throughout, which had been rendered on both the inside and outside surfaces, then painted.

Analysis

Analysis showed that the external render was an impervious Portland cement-based coating and was unlikely to be original. This was probably applied at some point during the twentieth century, but no record existed of this work. The internal plaster was found to be Roman cement, a traditional finish, typical of the period of construction, which on its own would allow the brickwork to ‘breath’. However, this had been heavily overcoated with several layers of paint, some of which were also impervious.

Any moisture entering the brickwork would thus become trapped between the two impervious inner and outer layers. Water would migrate through the porous internal plaster, causing this to deteriorate and the paint layer to lose adhesion and detach from the plaster.

Long-term moisture measurements

To further investigate the source of the dampness, holes were drilled on the inside of the tower and special timber dowels, some 250 mm long were inserted. Over a period of time, the dowels would assume the level of moisture present in the render and brickwork allowing it to be measured and recorded to reveal whether the moisture levels were increasing or decreasing. Our volunteers have monitored these since 2012 and they have helped us to gauge the effectiveness of our work.

External works

The investigation concluded that it would not be possible to remove the external render without seriously damaging the underlying brickwork. The recommendation was to strip the paint coats on the outside back to the render by careful sandblasting. This would allow any underlying cracks to be treated with a special filler, following which, several coats of a highly breathable silica-based paint would be applied.

Monitoring after this treatment showed a gradual decrease in moisture content in all but one area of the tower structure. In the service room, which is directly beneath the balcony, the ceiling and walls remained stubbornly damp, and this was steadily getting worse.

Where was the water coming from?

The moisture source

Further investigation showed that the plastic-based membrane, used to coat the balcony deck, which sits immediately above the thick wall structure was dilapidated, allowing moisture to permeate through to the brickwork beneath.

At this point, work had to cease due to the pandemic lockdown.

When work was able to recommence, our contractors removed the existing balcony membrane, revealing a stone deck, which effectively formed a cap on top of the brickwork. Once contractors had laid a new asphalt layer and this had been sealed to the building, monitoring showed the situation steadily improving, particularly around the service room directly beneath the balcony door.

Internal works

Unlike Portland cement, the Roman cement, on which the internal plaster is based, is a natural material made from limestone imported from Italy. It combines a good compressive strength with the ability to breath.

Following sealing of the balcony, the core monitoring showed that parts of the brick structure still contained high levels of moisture. To allow the brickwork to dry out all the moisture-damaged plaster was removed. We installed a temporary dehumidifier, controlled by a humidistat, which maintained the relative humidity inside the tower at around 60%. This allowed the drying out process to continue steadily, despite the outside conditions. The drying-out process lasted over one year, extending through much of the Covid pandemic.

The tower was rewired, with the new wiring run secretly behind the old, decommissioned wiring, and a new lightning conductor system was installed. The old-style storage heaters, installed in the 1960s, were internally stripped to remove the thermal storage bricks. The heater casings were retained with new heater modules installed inside, maintaining the original visual appearance, but now repurposed as convection heaters. These heaters are now primarily controlled by humidity and not temperature, and form a conservation heating system. This was completed just before lockdown started and assisted the drying process.

The improvement

The moisture tests continued when permitted during the pandemic, and showed the advantage of having both controlled heating and dehumidification at the same time. Moisture levels were steadily improving.

Putting it back together

Following the easing of the pandemic lockdowns, parts of the roof of East Cottage were repaired with new slates and leadwork. This, combined with the new decking on the tower balcony, and the correct treatment of the outside of the tower, meant that the structure was, once again, weatherproof.

But the inside of the tower still looked a complete mess. Many of us wondered if our much-loved lighthouse would ever be the same again?

Restoring the inside of the tower

Following the closure at the end of the 2022 visitor season, the tower was again internally scaffolded. The remaining plaster was checked for adhesion and, where necessary, stripped back to sound material. Old impervious paint was removed. Finally, the walls were coated with 3 layers of breathable plaster of similar composition to the original. Plastering was completed in November 2022, and this was allowed to dry naturally, by limiting direct heat and dehumidification.

Colour matching

To retain the original appearance as much as possible, paint colours were obtained from Trinity House. Analysis of the existing paint showed that the cast iron weights column had originally been white, then in the early-twentieth century had been painted mid-green. This was a lighter green to the paint colour we see today, which was applied in the mid-twentieth century.

It was decided to keep the colour scheme that existed on the Trust’s acquisition, rather than to try to revert to former colours. The paint for the walls would be a breathable conservation grade coating. Wall and floor painting started in December 2022, finishing in late January 2023. The remainder of the internal surfaces were re-decorated during February 2023.

Reopening

The staff and volunteers spent late February and early March 2023 carefully cleaning the artefacts that had been in storage for over three years, and replacing them in their familiar positions. The static and moving parts of the optic were cleaned and retouched with matching paint, and its mechanism thoroughly cleaned and maintained. The speaking tubes were restored and the brass parts once again gleam.

On 31 March 2023, we welcomed our visitors into a lighthouse we are proud of, at the start of the 2023 season. The culmination of many years of determination, expertise and sheer hard work. The completion of a project that shows the true spirit and dedication of the National Trust – for every one for ever.

The Future

Conserving a structure in such an exposed position as South Foreland Lighthouse will always be challenging and ongoing. Our newly acquired knowledge of the building, combined with the recent conservation work, will mean that this historic icon can be properly maintained and will continue to grace the White Cliffs of Dover for many generations to come. We've replaced one of the cracked panes of glass in the lantern and our next challenge is to replace the remaining damaged glass panes.

South Foreland Lighthouse relit?

We're often asked if South Foreland Lighthouse will ever be re-lit? Although Trinity House intended that the lighthouse should never show a light again after it was decommissioned, permission was given to re-light the lighthouse, as a beacon, for the Diamond Jubilee celebration in 2012. Permission was again granted in 2018 to mark the centenary of the Armistice, in 2020 to celebrate the life of Dame Vera Lynn and in 2022 for Her Late Majesty Queen Elizabeth II’s Platinum Jubilee.

The relighting does not impact the historic parts of the lighthouse. The original lamp cannot be used as it has a non-standard voltage and is itself a historic artefact. The lamp is carefully removed, wrapped and stored, and a modern 2,200 watt metal halide lamp is temporarily clamped to the existing historic lamp turret. The lamp element is carefully aligned with the 'bullseye' lenses in the optic using the original alignment tool - an exacting process that can take well over an hour. A second, smaller 400 watt lamp and a modern generator are also installed to ensure that the light continues to shine if either the main lamp or the mains power fails as, once we have permission to show the light, it must not go out.

As dusk falls, the lamp is lit and the original hand-wound clockwork mechanism is used to rotate the lens. The effect is stunning. A brilliant, almost pure white light is emitted with the original precise flash signature and is clearly visible right across the Dover Strait. A small team ensures that the lamp stays lit and the mechanism wound every hour throughout the night, as it would have been before automation. The tower steps start to prove challenging at 4 am!

In the morning, at the specified time, the lamp is turned off and allowed to cool. All the modern equipment is removed and the historic lamp returned to the turret, ready for the first visitors at 11 am, as if nothing had happened. Some very tired volunteers and members of staff take a well-earned rest.

Other works

While the major works were taking place, attention was also being given to some more minor, but equally important features.

• The speaking tubes to allow communication between the lantern room and the base of the tower were found to be blocked during the conservation work. These have been cleared, repaired and the brass mouthpieces restored.
• The weathervane was re-gilded and reinstalled while the scaffolding was in place for the external redecoration in 2018.
• The weights cable, driving the rotating lens mechanism failed in late 2022 and was replaced.
• Replacement glass for one of the cracked lantern panes was delivered in December 2022 and fitted in February 2023.
• The brackets supporting the rose casting in the ceiling of the service room were badly corroded. New brackets to support the casting were made from marine grade 316 stainless steel and were installed prior to redecoration.
• New transparent panels have been fitted as safety shields for the mechanism under the lantern.
• New glass panels have been fitted to the internal balcony in the lantern room which will allow an interesting new aspect to visits.

Thanks

Our thanks go to:

• Trinity House for their invaluable help and collaboration, particularly with paint specifications and colours, techniques for conserving the glass in the lantern windows, and many other smaller details.
• Our National Trust staff and volunteers for their help with clearing and carefully packing the artefacts prior to work starting, for their tireless cleaning efforts, and for going up in the lighthouse in all weathers for the past 11 years to monitor the moisture probes, as well as their research work.
• The contractors and consultants involved with the work, for their diligence and resilience, in all weathers, especially during the uncertainties of the pandemic and breaks to the planned programme.
• Our generous donors, and finally to you - our loyal visitors who have continued to come to the lighthouse and support us, despite the appearance and lack of artefacts.

To you all - a heartfelt 'thank-you'.