Washington's new convention center is looking a little drab, so here's an idea: Let's ask all our biggest industries to pay our greatest artists to put work on its facade. The steel industry could get Richard Serra to do a giant arc of metal. The advertising lobby could commission one of Jeff Wall's backlit color photographs. The Motion Picture Association of America could sponsor a projected video by Matthew Barney.

Okay, so it's not my idea. I stole it from the citizens of Renaissance Florence. After they rebuilt the combined church and grain depot called Orsanmichele, they assigned each of the niches on its facade to a different guild, with the idea that they'd eventually be filled with great works of art. As things panned out, those niches came to house some of the most important and influential objects in the history of Western sculpture.

Three of the best of them, newly or recently restored, are at the National Gallery of Art on loan from those in charge of art in Florence.

You could write a volume on each of these works and still not come to the bottom of them. Instead, here are their images to whet appetites for the real thing, and a few words on each.

Nanni di Banco

Medieval legend says that, in the heyday of the Roman Empire, four Christian sculptors refused to carve images of a pagan deity, and were put to death for it. Around 1409, four leaders of the sculptors' guild of Florence thought it would be a good idea to fill their niche at Orsanmichele with a statue of their four sainted predecessors. They commissioned Nanni, son of one of their top members, to do the job.

He turned out a radical work that had no real precedents, and that few of his artistic successors have ever had the nerve to imitate. His strikingly realistic sculpture shows four middle-aged men standing in a loose semicircle in their niche, turned to face one another as though in the middle of debating some administrative point. ("What do you think, should we carve that pagan god or not?" Or maybe: "Who should we get to fill our spot on Orsanmichele, anyway?")

Each man's head is level with the next, and there's no sign of social hierarchy among them. Nanni hasn't even given them a compositional hierarchy, the kind of variety in scale and pose that almost every artist uses to guarantee drama and energy in a work with several figures. Equality and a very literal level-headedness are the crucial qualities that Nanni seems to want to get across, even at the expense of purely artistic force.

One claim about Nanni's sculpture that seems to make some sense is that it captures the proudly egalitarian, republican values of the self-governing guilds that ruled the city state of Florence -- values that were the more dearly held by some at just this date because they were facing giant threats. (Wealthy merchant families were busy jockeying for autocratic power; the Medici more or less achieved it after 1434.)

But what's most especially impressive about Nanni's absolutely even-headed crowd of sculpted men is that it foreshadows one of the most important aspects of European perspectival painting, which was invented in Florence a few years after Nanni's work. That new one-point perspective made it possible to create a feeling of coherent painted space that artists could then populate with realistic figures. One striking feature of such a peopled perspectival space, however, is that the top of every head in it will fall along a single line across the surface of the painting. That is, so long as all the figures in a painted crowd are standing straight, and they aren't mixed with kids or dwarves or giants, the effect is likely to be close to Nanni's Orsanmichele group.

Nanni's sculpture can't be said to have influenced the look of perspectival painting -- that depends on the scientific facts of optics and perception and geometry. But maybe there was something in the air in Florence in the early 15th century -- maybe it was a whiff of the approaching Medici -- that gave a special force and instant appeal to any art, in any medium, that could body forth equality.

Lorenzo Ghiberti

In 1419, Ghiberti, one of the greatest (and richest) sculptors of all time, was commissioned to make a statue for the bankers' guild. The contract said Ghiberti's image of Saint Matthew had to be at least as big and fine as the bronze Saint John the sculptor had completed for a rival guild -- which itself had been the first monumental figure cast in bronze since Roman times. Ghiberti came through: Saint Matthew ended up overtopping his holy colleague by a head, using up 3,000 pounds of expensive copper and 200 pounds of tin.

Ghiberti's sculpture is all about displaying the precious stuff it's made from. Matthew's robe's improbably thick folds descend in smooth cascades of bronze that almost conjure up the liquid metal flowing from its crucible. A recent restoration, which is a kind of pretext for the National Gallery show, has cleaned centuries of corrosion off the sculpture. We can now get at least a partial sense of the strikingly metallic gleam it would have had.

But Ghiberti had to work hard to contrive, almost to fake, that look of flowing, malleable metal. Unlike easily worked gold or the sheet steel of armor -- two Renaissance models for the "wet look" of metal -- cast bronze is rough and pitted and seamed when it's first taken from its mold. It took great labor to file, fill and polish the statue's surfaces so they gave a convincing sense of the valuable material they were made from -- or, rather, so that they would fulfill viewers' ideas of how that material should look.

Andrea del Verrocchio

Verrocchio seems to have received the commission for his Orsanmichele niche in 1467, when, at 32, he was on his way to becoming the top sculptor of his day. When his bronze sculpture of Christ showing his wounds to Doubting Thomas was unveiled, 15 years later, his supremacy must have been evident. He doesn't just take his predecessors' work to a new level. He rethinks the things religious statuary does.

A medieval work of sacred sculpture, when boiled down to its essence, was a block of inert stuff that viewers could then use to make holy things happen. It wasn't so much about the way it looked as the divine powers it acquired. The church on the ground floor of Orsanmichele attracted crowds and huge donations because of a miracle-working painting. But it was the holy status of the object, rather than its force as art, that drew worshipers. When the painting was damaged in a fire, a new and very different one could be substituted without losing any of the magical power.

Verrocchio's Christ at first looks like that kind of work. It is your classic statue of a freestanding holy man, ready to receive the multitudes who pass, to bless them with his upraised hand and maybe, if their luck is good, to perform miracles on their behalf. But thanks to the figure of Saint Thomas, we get to watch a moment of holiness take place before our eyes.

Verrocchio's sculpture isn't just a vehicle for miracles, as most older sacred art had been. It spells out just what it would be like to witness a holy scene.

Saint Thomas seems to be one of us -- walking past Christ's plinth and showing off his fancy shoes just like any other Florentine on business at the grain exchange. Through the sculptor's art, however, he then gets a custom-tailored dose of the divine. His Savior's image doesn't just suggest God-become-man. Verrocchio's sculpted Jesus parts his robes, displays his wound and offers to allay the doubts of passersby.

Verrocchio's miracle, that is, is as much about the power of art as about the power of divinity. It takes the old idea of what religious sculpture can do, then fleshes it out for us to see. It takes a sacred aura that depended on faith and grace and turns it into the hard facts of visible phenomena cast in an artist's bronze.

Two of Nanni di Banco's martyred saints, part of the National Gallery's show of Renaissance sculptures.Nanni di Banco's four martyred saints: A work that foreshadowed pivotal advances in perspectival painting.Lorenzo Ghiberti's monumental Saint Matthew, left: All about displaying the precious stuff it's made from. Below, Andrea del Verrocchio's Christ and Saint Thomas: A rethinking of religious statuary.