London has some impressive mausoleums. One of them is a Neo-Byzantine structure in St Mary’s Roman Catholic cemetery in Kensal Green. Impressive, but dilapidated. At least, it was until it was carefully restored by Cliveden Conservation.

The project to preserve the mausoleum of the family of John Campbell, which has recently been upgraded to Grade II* listed building and was on Historic England’s Heritage at Risk Register, was funded by Historic England with support from The Pilgrim Trust.

The Campbell Family mausoleum was built in 1904 to a design by CHB Quennell. It is of great architectural interest and a high-quality example of an Edwardian mausoleum in the striking neo-Byzantine style.

But water had penetrated the roof and major structural damage had been caused by a jungle of buddleia roots. Both the structure and architectural features of the mausoleum had seriously deteriorated.

Specialist conservation skills were required to repair the external structure and stabilise the micro-climate of the interior.

Restoration of the decorative elements – the gold mosaic ceiling, stained glass windows, marble-clad walls and stone floor – were also needed to preserve this building.

Cliveden Conservation began by removing the vegetation and repairing the damaged roof. Other external works included resetting the two stone arches and repairing cracks and open joints with appropriate mortars.

Layers of sulphation covering the red brick and Portland limestone of the structure were removed with specialist poultices and all the elevations were cleaned.

Inside, Cliveden Conservation repaired and cleaned the mosaic ceiling created from golden tesserae (gold leaf embedded between layers of glass). All the loose pieces that had fallen to the floor were collected and carefully reset. Conservators also recovered broken pieces of marble and spent days identifying them.

Kris Zykubek, one of Cliveden’s most experienced conservators, explains the challenge: “This project required highly skilled conservators and stonemasons every step of the way. Each section of marble we found was assessed for repair using stainless steel dowels, resin and modified plaster. Those that could be repaired were reset on the walls but the main hurdle was sourcing replacement marble. Ashburton and Belge Rouge marble are no longer quarried but luckily we found an antiques and ancient marble expert who had a limited supply.”

With the cladding complete, Cliveden Conservation carried out repairs to the damaged floor, replacing sections with new stone slabs where required. The whole floor was established on mortar bedding. Other works included restoration work to the stained glass windows and the copper clad doors.

Verena McCaig, Heritage at Risk Project Officer for Historic England reflects on the renovation. “This has been a very special project, not only in practical conservation terms but also in what we have learned about the construction of the mausoleum and the social history of the family.

“Campbell was a major figure in the nitrates industry in Peru and many of his extensive family are interred here. We were thrilled to meet a direct descendant just before works began. And Cliveden Conservation enthusiastically welcomed his visits to see work in progress.

“We are delighted that the mausoleum has now been repaired and conserved – it is certainly worthy of the care that has been taken and we hope now that a wider audience will be able to appreciate its intricacies.”