This story was originally published in Dwell’s October 2007 issue.

People often introduce Bertus Mulder by talking about his extraordinary pedigree. He is one of the last surviving links to the glory days of the Dutch avant-garde—a one-time pupil of the radical architect/artist Max Bill, and a longtime assistant to Gerrit Rietveld, whose architecture and design were seminal influences on what became known as the International Style. The next thing you might hear about Mulder is that he is remarkably busy these days, engaged in a host of modernist restoration projects and occasional new buildings at an age (78) when reminiscence and even retirement would be perfectly understandable pursuits. Both of these descriptions are important, but both miss the most interesting thing about Mulder, and the source of his own considerable influence: He approaches architecture as an interdisciplinary collaboration, and he is most at home with his creativity when he is sharing it.

“I’m the one they ask to do the impossible,” says Mulder, with a wry smile. He’s referring to the difficulties inherent in one of his current projects—a complex plan to rescue the Anne Frank House from the wear and tear caused by the roughly 900,000 people who come to visit it each year. But extreme technical challenges characterize a lot of Mulder’s work. Take, for example, the 2005 exhibition space he created in collaboration with the conceptual artist Stanley Brouwn, on a new housing estate outside Utrecht. The Building, as it’s called, is a dreamlike minimalist structure composed of two white, glass-punctuated rectangles, one balancing with seeming weightlessness on top of the other. Or the design scheme that Mulder carefully superimposed on the Aula, a ceremonial building Rietveld designed in the 1960s, near the end of his life (when, coincidentally, Brouwn was beginning to win recognition for his works featuring measured distances).

Then there is the ne plus ultra of restorations—or collaborations, since that is closer to what happened when Mulder agreed to revitalize the Rietveld Schröder House, a job that was completed in the mid-1980s. Built in 1924 in what was then the outskirts of Utrecht, the house grew out of a set of ideas concerning abstraction and purity in art and architecture, known as De Stijl, that are associated with Piet Mondrian, among others.

The extraordinary house that Rietveld built for his client, lover, and eventual partner, Truus Schröder, is often described as the first truly modern building. Rietveld had trained as a furniture maker and by 1908 was already experimenting with modern forms—straight lines, geometric shapes. In 1917, he designed the Red and Blue chair (a dramatic composition of planes and angles that were painted in discrete colors), which became an icon of the De Stijl movement. From there, he began designing whole interiors, advocating very simple living and the use of open-plan layouts. The interiors of the Schröder house are open plan, with movable walls and otherwise transformable features; the windows can be opened out completely, removing the indoor/outdoor boundary. None of this had been done before.

But by 1974, when Truus Schröder asked Mulder to restore her home, Rietveld had been dead for ten years and the 50-year-old structure had some serious problems. Moreover, “restoration” and “modernism” were antithetical terms then—decrepit modernist buildings were bulldozed with indifference, and restoration was still anathema to modernist thinking. Rietveld himself had suggested demolishing the house when a new public road was laid down, transforming its context. “He didn’t think any building should last longer than a generation anyway,” Mulder recalls. “I once asked him which he considered his most important building, and he replied, ‘The next one.’”

Mulder had no official restoration credentials, but he had worked for Rietveld, befriended him, and even moved into his home when the older architect vacated it. The original furnishings and details were all there, and Mulder subsequently inherited a number of pieces, including Rietveld’s first modern chair design of 1908. “I inhabited Rietveld’s universe,” he says, and this was ultimately his main qualification for the task.

After a half-century, Rietveld’s diminutive masterpiece featured a honeycomb of widening cracks and an interior that had been modified over the years and was far removed from the original vision. “The job scared me,” says Mulder. “I had absolutely no idea how to go about it. But actually that proved to be a huge advantage, because I didn’t have to follow any of the usual conventions.”

Many restorations later Mulder came to realize that his starting point was based on principles he learned in the 1950s from the Bauhaus-trained Max Bill, at the Hochschule für Gestaltung in Ulm. The approach, says Mulder, is basically “define the assignment first and the solution arises from that. I studied the Rietveld Schröder House,” he continues, “until I understood how it was put together. I made a model of it. Rietveld saw architecture as the art of space—he wasn’t making buildings as such, he was carving space. So this house is a spatial constellation of independent elements, a composition like his Red and Blue chair. I looked at old photos and spent hours talking to Truus Schröder and others about it. In the end, the house told me what to do.”

First, there were huge structural problems to solve. “Rietveld had wanted to use concrete, but it had been too expensive at the time,” says Mulder. “So he combined steel posts and brickwork, in a totally experimental way. The combination didn’t age well, and it was impossible to repair, so I used new, synthetic materials and new techniques.” However, such adaptations raised the issue of authenticity and angered the Dutch Historic Monuments Commission. “But did Rietveld mix the plaster himself in 1924?” asks Mulder. Similarly, in choosing the exterior colors, he looked at five different layers of old paint to assess the ideal shades, “which were not necessarily the first colors Rietveld put on,” Mulder says. “Or why would he have painted over them?”

For Mulder, the object of the restoration was “to return the house to what it was intended to be at the time it was built: a manifesto, a blueprint for a new architecture and a new way of living.” Some of Rietveld’s own later interventions were overturned. His increasing minimalism, for example, meant that he had stopped using the primary colors that had made the house resemble (in the words of the critic Reyner Banham) “a cardboard Mondrian.” He had also removed some of its more sculptural elements, like a yellow stacking cabinet. Mulder decided to replace it, because it accorded with the architect’s original De Stijl–inspired vision of the harmonious integration of all the arts.

“Rietveld and Schröder made the house together—their cooperation was what allowed Rietveld to go so far,” he says. “But the house continued to evolve over time and was much changed by Rietveld himself. After he died, I restored it to the original idea in cooperation with Schröder. So the house was finished when the restoration was finished. The circle was complete.”

A notable feature of the restoration is the amount of imperfection it retains—small, handcrafted, human touches that fall short of the elegant precision of Rietveld’s drawings. On this point, Mulder shows a faithfulness of another sort. “Restoration has to start with the object itself,” he says. “Not with the architect’s intentions, or what he wanted to build, but with what he actually managed to build.”

At the same time, when Mulder built a “new” Aula hall in Hoofddorp (the old one was sidelined by an airport expansion), he used 1990s technology to execute Rietveld’s 1960s design, thereby improving its functionality. “I’m often asked if the Aula is a Mulder or a Rietveld,” he says. “The answer is, it’s a better Rietveld. The texture of the materials is the same; the quality of the building is better.”

The Building, the exhibition space in Utrecht, was for Mulder a chance to achieve the purity that had eluded Rietveld. “It’s just planes and lines,” he says. “Technology allowed me to hide everything else. You know, this is what Rietveld wanted to build, but couldn’t, because the technology wasn’t up to it.” Partnering with the famously temperamental Brouwn was not difficult, he says, although he did take the precaution of noting down exactly who was responsible for what, since “collaboration can get messy.” But not, you feel, for Mulder.

“For me, architecture always involves a group of people working together,” he says. “The design was a sketch of Stanley Brouwn’s. I expanded the scale of it, since it had to be large enough to be an exhibition space, and I made it modular, because it will eventually be moved to a new site. For the rest, I found the solutions to reach the vision that he had—a vision he could have only because he isn’t an architect and doesn’t know what’s not possible. And so together we could make this piece of pure architecture. This is modernism with our technology. It’s modernism for the 21st century.”