The U.S. State Department building isn't one of the more notable works of architecture in the nation's capital. From the outside, the 1960s modernist box deserves few second glances.

Inside, it's a different story.

Tucked away on an upper floor are some of the nation's most historically and elaborately furnished rooms, created over time, years after the building first opened its doors.

These 42 rooms cover about 28,000 square feet and serve a dual purpose. Their primary role is to receive foreign ambassadors and other dignitaries, providing them with an elegant space in which to interact with our nation's vice president, cabinet secretaries and other diplomats and leaders.

But their secondary role is to delight visitors interested in seeing some of the best of this nation's early decorative arts.

It's sort of like the the U.S. Capitol or Charleston City Hall for that matter: a functioning government space first that also is open to the public as the business of government permits.

It's Marcee Craighill's job to oversee these diplomatic reception rooms. She recently visited to address the Charleston Heritage Symposium and tell the story of their creation.

"I want to say it’s a little-known collection," she says. "It’s always a surprise to people to know that these remarkable rooms and this historic collection exists in this building."

The rooms open for tours three times a day, provided no events are taking place. "It's a little like the White House,” she says.

When the State Department moved into its new offices, it had public reception rooms. But they weren't very nice. Craighill's predecessor, Clement Conger, began changing that in 1961 and eventually created one of the most remarkable collections of its kind in the nation, almost solely through donations from patriotic-minded citizens, companies and nonprofit groups.

"They began with loans and remarkable gifts," Craighill says. "People understood the importance of what is trying to be done here. Now we own 99 percent of the collection, but we continue to receive and improve and elevate the collection each year. We just acquired through partial gifts a beautiful painting of the West by Maynard Dixon. It’s called 'The Prairie.' "

In total, the rooms display more than 5,000 objects whose cumulative value is estimated at about $125 million. The office is able to raise money to maintain and conserve the items, which unlike most museum furnishings, still get used.

And the rooms are closed much of August, when Congress is gone and business slows, to tackle maintenance and conservation work. 

The collection focuses on the nation's founding era (roughly 1750-1825). One of its most prized pieces is where the United States was born: the desk on which the Treaty of Paris was signed, ending the Revolutionary War (donated in 1963 by a family that had it in their living room in Chicago).

“What better object could you have in the diplomatic rooms of the U.S. State Department?" Craighill asks. "What more important treaty could there be? When they see it on a tour, students stop tweeting. Suddenly, it becomes real.”

Unlike other museum spaces, the State Department doesn't have much in storage. "We only accept or go after things we display," she says. "We actually do not borrow, but we lend all the time." Last year, it loaned a painting of Niagra Falls to the Senate for a presidential inaugural luncheon.

The architectural retrofitting of the rooms was handled largely by the late neoclassical architect Edwin Vason Jones of Albany, Ga., who was inspired by detail in historic houses in Virginia and Philadelphia. Most of the collection also comes from there, as well as New York and Boston.

"We do have a few pieces from South Carolina, wonderful pieces," she says. "We have a beautiful library bookcase and a really wonderful silver collection.”

Craighill says what has meant a lot to her is when a particular object in the collection can be displayed in a room for a meeting that speaks to the shared history of the United States and the other nation.

But the rooms also are powerful because they reflect how individuals not only support the State Department's work promoting negotiation and peace but also the ideals of the nation itself. Craighill notes a Hungarian man donated portraits of George and Martha Washington as a salute to freedom.

"The essence of these rooms — and the spirit of American philanthropy reflected in the collection — is as inspiring as the collection itself,” she says. "It's something I think is a real powerful message. ... What America did was an idea succeeding."

Reach Robert Behre at 843-937-5771. Follow him on Twitter @RobertFBehre.