Sometimes I think I am getting to know too much about a subject, but then I check out a detail and find that even the detail has details — and I enter a whole new level of minutiae and am fascinated all over again.

The basic story here is simple enough: they are taking down the Star-Spangled Banner, the giant American flag that hangs in the National Museum of American History, for a three-year conservation project. When it is back onstage in 2002, it will be protected from all the things that attack 189-year-old flags and will be visible from two floors of the museum in a special showcase.

The conservation will cost $5.5 million, which is coming from the Pew Charitable Trusts and other foundations. The new Flag Hall will add another $5.5 million to the $18 million project, which itself is part of President and Mrs. Clinton's call for a refurbishing of our national icons to celebrate the millennium. In response to the Clintons' campaign, Polo Ralph Lauren Corporation recently pledged $10 million to help save the flag.

Now for a few details.

The flag was ordered in 1813 by Maj. George Armistead, commander of Fort McHenry in Baltimore, who wanted something big enough for the British to see when and if they invaded his harbor. He hired Mary Young Pickersgill, age 29, a professional flagmaker (creator of ships' colors, signals, and so forth), to sew a flag with 15 stars and 15 stripes, according to provisions in the 1794 Flag Act, and measuring 42 by 30 feet. The contract was for $405.90. Such a large flag was not unusual for the time; garrison flags frequently were flown from poles at least 90 feet tall. Mrs. Pickersgill may have bought the wool bunting and the cotton for the stars from her brother-in-law, Jesse Fearson, who owned a dry goods store in Baltimore. Then she and her 13-year-old daughter, Caroline, put the huge thing together on the floor of a local brewery.

The flag, made in six weeks of ten-hour days, required 300 yards of wool (cotton, then a luxury material, was used only for the stars, which are 24 inches in diameter) and weighed 50 pounds. It took 11 men to raise it.

Today the flag weighs approximately 135 pounds, mostly because of a linen backing added in 1914 to strengthen it, replacing a loosely basted canvas backing installed 41 years earlier. The new backing was sewn on by a team of seamstresses who put in, so I am told, more than 1.7 million stitches.

Now there's a statistic. We know there were 12 stitches per square inch, which comes to 1,728 stitches per square foot. When multiplied by the total number of square feet, that comes to a whopping 1,762,560 stitches.

Over the years other things have happened to the banner. Armistead's widow cut out one of the stars to give to a political personage, but no one knows who the recipient was. There is a mysterious upside-down V, or a chevron, in one of the white stripes, which could have been an unfinished A for "Armistead." Lonn Taylor, a Smithsonian historian, cites a letter from Georgeanna Armistead Appleton, the owner's daughter, which says, "My mother sewed the red A on the flag," which would seem to rule out the other possibilities.

And the flag is now only 34 feet long, because the Armisteads gave away souvenir swatches. Taylor says this was common back then, there being no law against it.

During the turbulent night of September 13, 1814, in the heat of the British bombardment, a smaller flag that was used during bad weather, called a storm flag, very likely was run up over Fort McHenry. This flag, 25 by 17 feet, was also made by Pickersgill and cost $168.54.

In the morning, Armistead's men ran up the big flag. An English midshipman wrote that, at dawn on September 14, "as the last vessel spread her canvas to the wind, the Americans hoisted a most superb and splendid ensign on their battery." This was, of course, the moment when Francis Scott Key, detained aboard a British ship, saw the sight that inspired him to write the famous lines that became our national anthem.

Armistead acquired the flag sometime after the battle, and his wife inherited it upon his death in 1818. She willed it to their daughter Georgeanna Appleton. Armistead's grandson Eben Appleton loaned it to the Smithsonian in 1907 and made a gift of it five years later.

During World War II the flag was hidden away for safety in a Luray, Virginia, warehouse, appearing again in 1944. In 1963 it went to what is now the National Museum of American History. After the war, it was spot cleaned with gasoline and later was vacuumed. Luckily, as the flag aged, conservation techniques grew steadily more sophisticated. In the 1970s plans to enclose the flag were proposed and dropped; by the 1980s conservators had worked out a cleaning plan. In 1984, to protect it from light and dirt, it was covered with an opaque screen that was lowered for just five minutes every hour for an unobstructed view while the national anthem was played. Then, in 1994, the cables holding the screen broke, leaving the flag newly exposed.

Conservators soon discovered that the screen had not kept out plant material, debris from construction projects, or lint from paper and clothes, including some blue cotton fibers that may have come from blue jeans worn by many of the five million people who visit the museum annually.

Also, the wool fibers have been decaying over the years, says Suzanne Thomassen-Krauss, the conservator in charge of the entire flag project. Sunlight, heat, chemical interactions, even the oxygen in the air, cause deterioration.

But help is on the way. "We're going to lay it flat to work on it," says Thomassen-Krauss, "and there will be a big moving bridge that we call a gantry, 32 feet across, so we can work on any part of the flag."

It will lie on a platform in the Flag Hall, inside a 50-by-50-foot room. There it will be examined and stabilized so that in early 1999, it can be moved to a special laboratory in the museum. In this $1 million lab, the three-year conservation process will begin. Experts will clean the flag, evaluate whether the stitches can be removed, add new support material and attach a new mounting system.

One point of interest will be that upside-down V. Conservators will be looking for traces of the crossbar that would show it had been intended for an A.

The flag comes down this month. If the linen backing is removed, it means that each of the 1,762,560 stitches will have to be snipped, at two snips per stitch. That is 3,525,560 snips. I love stats.

When finished, the flag will probably be displayed on a tilt from the vertical to ease the stress on the hanging points. It will be housed in a four-story climate-controlled display case that could cost up to $2 million. It will be the largest case ever built for a museum artifact.

While the Star-Spangled Banner is being worked on, programs are planned to give visitors an idea of what is happening to the flag. Also, the Smithsonian and the History Channel are collaborating on a documentary, scheduled to air this fall, featuring reenactments of the War of 1812 and the bombing of Fort McHenry. Complementary educational materials are in the works, too.

And with luck, we will all be able to wake up on the morning of the millennium and know with delight that, despite all that time could do to it, the flag is still there.

By Michael Kernan