The design of the National Museum of African American History and Culture was affected by cost-cutting decisions. (Michael R. Barnes/National Museum of African American History and Culture)

The basic shape of the Smithsonian’s new National Museum of African American History and Culture has been clear to anyone strolling the Mall for months now. Set into an irregular plot at the intersection of Constitution Avenue and 14th Street NW, the museum sits on a glass base, with a three-tiered facade of canted metal panels running around all four sides. It is aligned with the regular march of museums along the north side of the Mall, but it serves as a transitional spot to the open grassy reaches that surround the Washington Monument.

Like so many things in Washington, it has been debated, refined and amended. In the process, it has become both a better and a worse building than the 2009 winning design concept presented by the team of Freelon Adjaye Bond. Its most distinctive element, the metal “corona” that gives the museum its instantly recognizable and appealing geometry, was a constant throughout the process. But it was originally meant to be a cap on a large, aboveground rectangular volume that stretched almost to the sidewalk along Constitution Avenue, space that was eventually moved underground. By removing that aboveground space — and placing the corona directly over a glass atrium with views in all four directions — the idea got simpler, stronger and more appealing.

That was a huge improvement, emphasizing the bold, upward-thrusting energy of the metal skin and diminishing the bulk of the building and its impact on the views and openness of the Washington Monument grounds. But the corona has also become darker, and under some lighting, it appears almost leaden. In early renderings it had a brassy luminosity, and looked to be made of continuous, porous, meshlike metal sheets. Now it is constructed of many smaller panels of aluminum, coated to give them a dark bronze color. That, unfortunately, was not an improvement.

Cost and construction difficulties are to blame.

“I really wanted a real bronze building, but the weight issues and maintenance became much too difficult,” says David Adjaye, the lead designer for the team that also includes the SmithGroup. Bronze is heavy and expensive, says Philip Freelon, head of the Freelon Group, the architect of record. And although the corona is the most distinctive element of the building, the “how to” for making it was a work in progress even as excavation for the project was underway.

The exterior of the building features a three-tiered facade of canted metal panels. (Jahi Chikwendiu/The Washington Post)

“In the beginning, we were not 100 percent sure how we were going to do it, to be completely honest,” Adjaye says. Unfortunately, the technical solution came with significant aesthetic compromise, a drama played out like a slow-motion train wreck in design oversight hearings before the Commission of Fine Arts.

“The Commission members emphasized that the treatment of the corona is the single most important element of the design,” wrote Thomas Luebke, secretary of the CFA, in a September 2012 letter to Smithsonian officials. “They recommended that great attention be given to determine the final detail, finish, and color of the corona panels to achieve the intended lacy and glimmering effect.”

A year later, the commission was even more worried: “Noting again the importance of the corona as the most iconic element of this new institution, the Commission members reiterated their support for the use of actual bronze in the finish of the character-defining corona panels.”

But the commissioners eventually compromised on the use of aluminum panels with a five-step “bronze” finish that doesn’t look much like bronze.

You can see what was lost in the process by studying the interior space of the glass atrium at ground level, where bright copper cladding is used to lighten the color palette. On the outside of the building, hinges affixed to doors that allow for smoke clearance in case of an emergency also have a lovely, brassy sheen. These are remnants of a bold and attractive color scheme that Adjaye says was inspired in part by the bright look of copper cookware, a fixture of Colonial-era kitchens staffed by African American slaves.

Unfortunately, although breaking the corona into smaller aluminum panels solved the weight and cost problem — and allowed the designers to recall the metalwork distinctive to African American artisans in cities such as Charleston, S.C. — it made the whole thing seem heavier. From the inside, as you look up into the interstitial space between the internal glass box of the museum and its metal facade, the support system is complex and ponderous, leaving an impression not of light and a pleasing interplay of angles and lines, but rather an encumbrance of structural clutter.

Visitors may not notice, and once the building is open, the many ways in which it may have been better will seem far less important than all the ways in which it is reasonably good. And perhaps the rather cheap, airport-looking materials, including the ceiling panels in the atrium, will seem less obtrusive.

The building probably will be hailed as a dramatic departure from other Smithsonian museums — in part because its interior space evolved during the same design process that vitiated its exterior. When the designers moved the bulk of exhibition space below ground, they may have taken inspiration from the National September 11 Memorial Museum in New York, where the exhibition space fills a vast subterranean cavern stretching down to the bedrock below the original World Trade Center towers. That museum was designed by Davis Brody Bond, which also serves on the team that designed the African American museum.

The exhibitions covering the darkest chapters of African American history are located in a similar subterranean space, a deep underground chamber built out with ramps and platforms, and home to some of the largest and most dramatic objects in the collection, including a Pullman rail car, a plane used for training by the Tuskegee Airmen and a slave cabin.

It’s too early to say whether that space works and how it will relate to the more conventionally Smithsonian-style exhibition galleries in what Adjaye calls “the attic,” aboveground. A critical transitional space underneath a round skylight, or “oculus,” wasn’t open during the tour last week. Adjaye says that room is essential to the experience: It is a “sacred” space, offering a place for reflection and quiet as you move from studying a painful history to aboveground spaces that celebrate the accomplishments and contributions of African Americans.

It may seem presumptuous to complain about the impact of cost cutting when the building’s price tag is well north of $500 million, but it’s important that anyone who cares about Washington, its museums and its urban design understands how the process unfolded. By choosing a symbolically dramatic site — on the Mall, near the White House and Washington Monument — museum leaders built into the project huge and costly challenges, including a high water table, and the need to keep the visible part of the structure congruent with the buildings around it. At every turn, there became fewer ways to solve critical problems, which necessitated more compromises.

Many people are deeply invested in the success of this museum. The events of the current political season have only deepened the urgent need for incorporating African American narratives into the larger narrative of American history. Raising the $270 million in private contributions to build the museum has been an Olympian challenge from the beginning.

But if there were one building in Washington where the imperative to cut costs should have been resisted, this was it. There will be plenty to celebrate when it opens in September, but for now it’s worth acknowledging some disappointment. Yes, it has taken years to make this essential museum a reality, but let’s reiterate this fundamental truth of making civic architecture in the nation’s capital: Do it slowly, deliberately and meticulously, because it can never be undone.