The historic azulejo tiles of Porto, Portugal, are under threat from decay, developers, and thieves. A unique public works project collects the city’s lost tiles—not just for conservation, but to give them to building owners free of charge as part of a restoration project.

Near the statues and black-and-white stone pavement of the Praça Carlos Alberto sits the Banco de Materiais, or Bank of Materials. It looks a lot like a museum, and once inside visitors can see plenty of azulejos, the hand-painted tiles that cover the exteriors of buildings across Portugal. But there’s a reason it’s called a bank: Tiles can be deposited there, and they can also be withdrawn.

Azulejos have existed for centuries, but they gained widespread popularity in Porto during a 19th-century building boom that defined the city’s visual heritage. Unfortunately, years of neglect meant many of those original tiles got lost and damaged as buildings collapsed, or as thieves stole them for sale to private collectors.

Today, the Bank of Materials stores tens of thousands of those tiles in its collection. Nearly all of them are available for building owners to take, free of charge, to restore a façade. If the bank doesn’t have a matching original piece, it will help owners find a suitable reproduction.

Building owners can also bring fallen or damaged tiles into the bank, and the fire department will even assist in collecting azulejos if a building is at risk of collapse. Tiles recovered by law enforcement go to the bank as well.

The bank also works with developers to teach restoration techniques, which is currently an important task. As tourists flock to Porto’s historic center, once-neglected buildings are undergoing a massive revitalization effort. The Bank ensures that those restorations take place with original materials whenever possible.