The Royal Saltworks of Arc-et-Senans (25-Doubs), built between 1775 and 1779 by the visionary Enlightenment architect Claude-Nicolas Ledoux, who conceived the idea of an ideal industrial town centred on a perfect semicircle, was originally intended for salt production until the end of the 19th century.

Subsequently taken over by the Department of Doubs, it was renovated and maintained until its inscription on the UNESCO World Heritage List in 1982.

Now managed by Hubert Tassy, the Royal Saltworks houses a museum dedicated to the architectural work of C.-N. Ledoux, as well as a hotel, a restaurant, a conference centre/performance hall and music recording studios.

In 2022, the Royal Saltworks were enhanced by the addition of a 5-hectare semi-circular garden facing the original site, forming a 'Cercle immense'. A garden festival is held there every summer.

It has been described as "a tiara discreetly crowning the Royal Saltworks garden".

To supply the Royal Saltworks restaurant's kitchens whilst prioritising a short supply chain, a large vegetable garden has been established in this new space, from seed to plate. The construction of a greenhouse quickly became essential to enable the cultivation of fruit and vegetables all year round, and architect Yann Rocher was commissioned to design this structure. He details his thought process:

'Several challenges arose during the project.

- Firstly, we had to design a project based on market gardening techniques and the principles and constraints inherent to bioclimatic design. The aim was to address issues such as sunlight exposure, heat accumulation, thermal inertia, ventilation, irrigation and the ergonomics of handling the growing trays where the plants are cultivated, as well as the cyclical use of water. Calculations carried out by bioclimatologist Jacques-Yves Baumann enabled various project parameters to be optimised, including the incorporation of a Canadian well system.

- Taking into account the site was also important, as the greenhouse is situated within the prestigious ‘Cercle immense’ laboratory garden. This is further complicated by the fact that Claude-Nicolas Ledoux, the architect of the Royal Saltworks, was a master of spherical forms.

- Embrace the geodesic theme suggested by the Saline Royale. The project is based on the geometry of a polar rhombohedron of revolution – a polyhedron with double-helix rotational symmetry around an axis composed of diamond-shaped faces, also known as a 'zome'.’

The historic nature of the site on which the greenhouse is built, as well as the project's sustainable approach, called for the selection of high-quality, durable, and recyclable materials.

‘Ultimately, stone, steel and glass were chosen to form a setting, a skeleton and a separate cladding respectively, all of which are distinctive in that they are visible.’

Thus, Yann Rocher concludes that this modern, multifaceted dome takes the form of a 'tiara discreetly crowning the Royal Saltworks garden, which can be glimpsed in the distance'.

To fulfil its primary function of protecting the market garden crops from adverse weather conditions such as temperature fluctuations, frost and hail, 72 single pane laminated glass panels in a diamond shape are stapled to the metal structure.

The complex comprises low-carbon Pilkington Mirai™ glass combined with Pilkington K Glass™ N low-emissivity hard-coated glass, with EVASAFE films used for their moisture-resistant properties.

Pilkington Mirai™ glass has 52% less carbon content than traditional float glass, which is fully in line with the environmental approach of this project. The manufacture of the 8 mm thick glass used here prevented the emission of nearly 3 tonnes more CO₂ than if conventional glass of the same thickness had been used to build the bioclimatic greenhouse.

In July 2025, the Royal Saltworks geodesic greenhouse won the People’s Choice Award in the Infrastructure category at the 2024–2025 Green Solutions Awards.

This competition, organised by the sustainable building and urban development media outlet Construction21, recognises innovative and sustainable projects.

Photography: Valentin Reigneau (first 4 photos) and K. Khalfi