Amsterdam is, quite literally, a city built on borrowed time.
Its gabled canal houses stand on wooden piles driven into soft peat, its 17th-century warehouses lean at photogenic angles, and hundreds of kilometres of brick quay wall hold back the water that gives the city its shape.
Keeping all of it standing is one of the largest conservation and restoration challenges in Europe — and a growing body of research suggests the answer may lie not in more scaffolding, but in stone that can heal itself.
The idea sounds like science fiction: masonry and concrete that mend their own cracks the way skin closes a wound.
But it is already being tested on some of Europe’s most treasured monuments, and the Netherlands is at the centre of the field.
A trial run at Tintern Abbey
Some of the most striking evidence comes from Wales.
During the two-year GEOHEAL project, which ended in early 2020, researchers collaborated with the Welsh government to see whether a bacterial “immune system” could help preserve the historic Tintern Abbey in Monmouthshire, Wales.
The ruined Cistercian abbey, founded in 1131, is exactly the kind of weathered, porous sandstone structure that conventional treatments struggle to protect.
Tests conducted on stone samples taken from the abbey showed that the bacteria could improve the microstructure of the masonry — and, crucially, without changing how the stone looked or behaved.
The GEOHEAL team, led from Cardiff and Newcastle universities, brushed and sprayed the stone with a liquid carrying naturally occurring soil bacteria (Sporosarcina pasteurii and Sporosarcina ureae) together with calcium and nutrients.
As the microbes settle into the rock’s pores, they produce calcium carbonate that hardens into calcite, effectively knitting damage back together from within.
“No colour alterations were found that could be detectable by the human eye,” reported Dr Michael Harbottle, the project’s coordinator.
Just as importantly, the treatment did not compromise the stone’s “breathability” — the ability to let moisture escape.
Many conventional sealants trap salts behind the surface, causing the outer layer to flake away and accelerating the very decay they were meant to prevent.
For conservators bound by charters that demand minimal, reversible intervention, a living treatment that leaves a monument looking untouched is close to an ideal.
A Dutch speciality
If self-healing stone has a spiritual home, it is the Netherlands.
At Delft University of Technology, microbiologist Henk Jonkers pioneered bacteria-based “bio-concrete”, embedding dormant Bacillus spores and calcium lactate directly into the mix.
When a crack lets in air and water, the bacteria wake, feed and seal the gap with limestone.
The spores can lie dormant in the material for up to two centuries, and the technology has since been commercialised by the Dutch firm Basilisk — even earning a nomination for the European Inventor Award.
That Dutch expertise matters, because few cities have as much immovable cultural property to look after as Amsterdam.
Amsterdam’s restoration mountain
The city counts roughly 8,000 protected monuments, the vast majority in private hands, and its seventeenth-century Canal Ring has been a UNESCO World Heritage Site since 2010, taking in around 1,550 historic buildings inside the Singelgracht.
Guarding that fabric is the job of the municipal heritage office, Monumenten en Archeologie, which polices everything from cornice profiles to window proportions.
The most urgent front, though, is beneath the waterline.
Amsterdam is responsible for some 600 kilometres of historic quay walls — many more than a century old, some standing for 300 years — along with hundreds of monumental bridges.
Under its Bridges and Quay Walls programme, the city is working to renovate or replace roughly 850 bridges and 200 kilometres of quay wall, at a cost running into the billions of euros.
Engineers have already had to close streets and canals where brick walls, undermined by ageing timber foundations and heavier modern traffic, threatened to collapse into the water.
Here the appeal of self-healing materials becomes obvious.
The GEOHEAL and follow-on GEOBACTICON researchers argue that bacterial repair is especially valuable for structures that are hard to reach or partly buried — bridge abutments, tunnels and, yes, canal-side retaining walls.
Self-healing concrete alone could save Europe up to €120 million a year in tunnel and retaining-wall maintenance.
For a city that must inspect and shore up 600 kilometres of quay largely from the water, a wall that quietly repairs its own hairline cracks would be transformative.
From ruin to living monument
None of this replaces the craft of traditional restoration — the bricklayers, stonemasons and foundation specialists who keep Amsterdam upright.
Bacterial treatments are still moving from the laboratory to full-scale use, and heritage bodies will rightly test them slowly on non-critical fabric first.
But the direction of travel is clear.
The same principle proven on a Welsh abbey’s sandstone could one day be brushed onto a Herengracht facade or mixed into the concrete core of a rebuilt quay.
Conservation and restoration of immovable cultural property has always been a race against water, weather and time.
In Amsterdam, a city that has fought all three for four centuries, the prospect of stone with a built-in immune system is more than a curiosity — it may be part of how the canal ring survives its fifth.
Reference: The European Commission’s Horizon Magazine feature “Buildings, tunnels and bridges could soon repair themselves” by Richard Gray.
Featured image: brick quay reconstruction on the Nieuwe Herengracht, Amsterdam — photo by Fons Heijnsbroek, 2022 (CC BY-SA 4.0).
