The Star-Spangled Banner _ the Star-Spangled Banner _ which hangs in honored glory at the Museum of American History, has become so filthy with dust, grit and fibers from visitors' bluejeans that it is in danger of wasting away to rags.

The dust shows as nasty-looking fuzz, like an old furnace filter. The grit doesn't show but is worse, because it shreds threads.

The great Banner is to be vacuumed -- fore and aft, ever so lightly, from scaffolding -- and treated to a new installation. In the future it will be shown only for a few minutes each hour. The rest of the time it will rest out of sight behind a theatrical scrim -- the better to assure that it will endure for coming generations.

Hanging opposite the Mall doors, the vast flag has been the dramatic focus of the grand entranceway since the museum opened in 1965. All day long, every day except Christmas, tourists stand before it with chins uptilted. The Banner flew, all 30 by 42 feet of it, over Fort McHenry in Baltimore harbor in the early morning of September 14, 1814, toward the end of a fruitless British attack. The sight of it moved Francis Scott Key to write his "O, say can you see" poem, which was set to the tune of a popular drinking song and in 1931 became the National Anthem.

Tourists, overdosed on Disney World facsimiles, have to be told that this is the real Banner. Mary Young Pickersgill of Baltimore stitched it on contract in the summer of 1813, with help from her mother and her daughter Caroline. They used 300 yards of first-quality handwoven English wool bunting. Because her house was too small she laid out the pieces on the maltroom floor at nearby Claggett's Brewery.

Each of the 15 stripes is two feet wide; the 15 stars are 24 inches point-to-point, and never mind that there were 18 states at the time, the flag was officially correct. The jumbo size was specified by Major George Armistead, commander of McHenry, who wanted the British to be able to see it without their glasses.

Above battle-smoke and morning mists the British saw it, as did the anxious Key.

Key, 35, a personable Georgetown lawyer and amateur poet, was negotiating for the release of his elderly friend, Dr. William Beanes of Upper Marlboro. Beanes, under the impression that we had won the Battle of Bladensburg, had jailed three British stragglers. (In fact they whipped us so handily the "battle" is known as the Bladensburg Races.)

The British agreed to free Beanes because the wounded they left behind at Bladensburg had been humanely treated. But Key's party had witnessed preparations for the imminent attack on Baltimore, and so were not allowed ashore before the battle.

It began with furious shelling September 13. Seeing the flag by day and the rockets by night, Key knew the fort still held. But for some hours after midnight, when the British stopped firing for fear of hitting their own landing barges, the outcome was in doubt.

When the flag was still there at dawn it led him to write, on an unfinished letter, his exuberant patriotic verses. The song was instantly popular and was played at military retreats long before it became the National Anthem.

Skeptics are always asking Smithsonian docents if Key, eight miles away, really could have seen the flag. Answer: Yes, probably with the naked eye, certainly with the sort of spyglass always ready to hand on such a vessel. And: Is this the flag he saw? Answer: Yes, most probably, although probably not the one the rockets glared redly at (those are not shot holes that pepper it).

Smithsonian naval historian Harold Langley thinks a smaller storm flag flew through the night. "We know it rained. Any flag would have hung by the pole, waterlogged, and no one could have seen much of it. The flag that streamed out from the pole in the morning was dry. In size it was the equivalent of John Hancock's signature on the Declaration of Independence -- run up as the garrison's nose-thumbing gesture."

Langley cites a bill of sale, found in 1940, in which Mrs. Pickersgill acknowledges receipt of $405.90 for the large flag and $168.54 for a storm flag, 25 by 17 feet, made at the same time. He also cites Dawn's Early Light, a 1972 book by Walter Lord, which quotes the account of British midshipman Robert Barrett: "As the last vessel spread her canvas to the wind, the Americans hoisted a most superb and splendid ensign on the battery."

"The point is that this is the flag that inspired the poem, not that the holes in it were made by shells in battle," Langley said. "There is no indication it was hit by anything other than collectors." The museum has several swatches believed to have been snipped off as souvenirs, and knows of others. "There's every indication that these bits belong, but there's an element of faith. Once you separate fragments from the flag, the thread count is slightly different. The pieces become looser as they're handled, more gauzelike. They unravel."

The curious red V, stitched on as a patch to one of the Banner's white stripes, is believed to be a souvenir snippet that was returned. It appears in the flag's first photograph, taken in 1874 at the Boston Navy Yard, where the Banner acquired a heavy canvas backing. "No one was thinking then of V for Victory," Langley said.

Rita Adrosko, flaglady of the Smithsonian's textiles division, designed the conservation plan now under way. She reports that the red V was recently discovered to have a red B embroidered on it. It's her guess that the patch may cover Armistead's signature and the date, which are said to have been put on the flag. We'll never know, because museum people would never countenance removing the patch for a look, any more than they would reweave the holes or bind up the tatters. Or replace the six-foot section cut off to fulfill a dying soldier's request to be buried in The Flag's embrace.

Garrison commander Armistead, the Hero of Baltimore, got to keep the Banner. He also was brevetted lieutenant colonel and presented by the city with a silver bowl in the shape of a 13-inch bombshell. Family lore has it that the flag was carried in Armistead's funeral procession in 1818, appeared at a reception at the fort for Lafayette in 1824, and was sent to England late in the Civil War for safekeeping. In 1907 a grandson, Eben Appleton, lent it to the Smithsonian. Later, to the abiding sorrow of Baltimore's city fathers, he made the loan a gift.

At that time the flag's conservation was the charge of Theodore Belote, an assistant curator of history remembered at the Smithsonian for such attention to duty that he saved packets labeled "dust from G. Washington relics," "dust from Jefferson relics," and so on. The flag was frail when he got it, damaged by its too-heavy canvas backing. A conservation contract went to Amelia Fowler of Boston, who, leaving the holes and rents as they were, stitched the flag to a pliable linen backing. Her work shows as a network of tiny rectangles; the buff backing shows in the white-stripe areas, where the original bunting is grey.

The project was begun in May, 1914, in the Chapel (now the Commons Room), of the Smithsonian "Castle," and was completed as promised on June 30. Cost: $1,243 ($500 to the needlewomen, $500 to Mrs. Fowler, the rest for materials). Curator Belote said the flag "should now last for centuries. It could "be flown from any flagpole if necessary, without a particle of injury," the donor was informed.

The flag was displayed in the Arts and Industries Building -- except during World War II, when it was bundled into tarpaper for safekeeping in Luray Caverns -- until the new building opened. For the new vertical installation, which finally got the Banner into the field-left position specified by the Flag Code adopted in 1923, tapes were sewn onto the backing to equalize stress. A "curtain" of air was supposed to block out dust.

But dust insinuates itself through the air curtain even as it does through a "dustproof" case, and now has built up like snow on windowpanes on Mrs. Fowler's stitches and in the creases acquired at Luray.

Samples of the dust have been found to contain, along with grease and grass, minerals from the soil and paths of the Mall and an abundance of fibers, many of them abraded from museumgoers' jeans. As the Banner moves gently in drafts, the dust chews away at flag's fibers.

The dust has helped to preserve the flag's colors, though, by muting the damaging light that falls on the indigo-dyed blue of the field and the tin-mordanted cochineal and madder-dyed red of the stripes.

Conservation procedures will take all summer and, like last winter's rewaxing of the elephant at Natural History, may provide some spectator entertainment. Workers will stand on a huge window-washer's platform and use special low-suction, battery-operated vacuum cleaners.

The job, which started on Flag Day, June 14, is scheduled for completion by September 14, the 168th anniversary of Fort McHenry's bombardment. There will be ceremonies.