Harold Sack knows most great American antiques personally. Perhaps that's why, at 75, even his face, which looks a bit like a carving on a table leg, seems to have acquired a patina instead of wrinkles.

Sack, president of Israel Sack Inc. of New York, is reputed to have invented the American antique market, an ornate and arcane world where thousands of dollars can turn on whether a paw foot on a table is hairy enough.

Sack denies it. "My father did," he says with some modesty. And to prove it, he wrote (with Max Wilk) "American Treasure Hunt: The Legacy of Israel Sack" (hardback from Little, Brown and Co.; soft-cover, just published by Ballantine Books), a delicious peep through the windows of American antique collectors, a history charting the phenomenal rise of the American antique, and a biography and a kaddish for his father, Israel Sack, who died in 1959.

At the big New York auctions of American antiques -- such as the Oct. 24 sale at Sotheby's that totaled $3.7 million -- Sack is as familiar as the auctioneer's hammer. Auctioneers and bidders alike watch him closely as he sits on a front seat and spends hundreds of thousands of dollars with a barely noticeable flick of his pencil.

At the Sotheby auction -- six days after the crash of '87, with stock prices falling like hail on Wall Street -- Sack, as he says, "put my money where my mouth is." He paid $143,000 -- considerably more than the high estimate of $30,000 -- for a "fine and rare" Federal mahogany extension accordion dining table, circa 1810, New York. And he paid $110,000 for the Chippendale carved mahogany block-front slant-front desk from Salem, Mass., circa 1775, that Sack Inc. had owned five years ago, intending, of course, to resell.

"Prices were high and strong all during the sale," Sack says. "At every major recession since World War II, antiques have brought record auction prices. The Flayderman sale in 1930, just a few months later than the crash of '29, broke all records.

"In 1932, business in general fell into the abyss. Still, even then the Depression didn't affect prices on masterpieces ... A large segment {of people} have funds to indulge their taste."

One of the legends about Israel Sack is that he bought up all the prize antiques from stockbrokers going bust after the 1929 crash -- and that his sons kept them in the basement until prices rose in the late '70s and '80s. But Harold Sack says that's not exactly right.

His account: In 1931 Israel Sack got a tip from a friend of President Herbert Hoover that the market was going to turn up. The elder Sack moved to New York and began to buy furniture collections in bulk on credit, just as brokers had bought stock on margin before the crash. But when the Depression settled in, and the cash flow was slow, the firm was almost ruined, and the Sacks had to liquidate their inventory. The Sacks still brood over the treasures sold in that auction. It was just as the elder Sack used to caution: "Sell and repent."

Today, Harold Sack predicts the market in antiques will hold up even better in the financial crisis than it did in the '30s because interest in antiques is broader, and so is knowledge. But he also thinks that people who buy solely for speculation in the antique market will "chicken out early," and that the middle classes will perhaps not be able to afford to buy antiques as easily.

"On the other hand, some people, fearing a crisis, may be willing to sell their treasures -- that's an opportunity to strike for those with courageous money."

Sack says there's no doubt that American antiques are good investments. And besides, unlike stock, you can sit in a chair while you wait for its price to go up.

The Business At the moment, the most expensive piece on the Israel Sack Inc. floor is a Chippendale Baltimore highboy at $450,000. The Sacks like to "decorate" their showroom with such important pieces. "It gives us status, background," Sack says.

He and his partner, younger brother Albert, are often seen on lecture platforms, explaining how to tell the fake from the true antique. Their father, a Lithuanian cabinetmaker who learned dovetailing in Birmingham, England, went to work faking antiques in Boston, the day after he got off the boat. But he fell in love with real American antiques and helped establish them, at least at home, as the equal of European.

Harold is business head of the family firm; he says Albert, who wrote "Fine Points in Furniture," is the scholar of the family. Another younger brother, Robert, and nine other assorted relatives mind the store.

When Harold Sack isn't at the store, it's likely he's in some grand mansion, or his own gallery at 15 East 57th St. in New York City, buying or selling some rare and important American Federal object.

Every top collector in the United States -- including television star Bill Cosby (who has outbid Sack a time or two), the Paul Mellons, Henry Ford, Ima Hogg's Bayou Bend Collection in Houston, various Kennedys and Rockefellers and former American ambassador to Austria Ronald S. Lauder -- buys from Sack's shop, commissions him to bid at auction on their behalf, takes his advice as to when not to buy. They fill the Sack Inc. pockets, and in turn, receive from the Sacks a bonus of knowledge about American objects.

The Sacks Soar The rise of Israel Sack Inc. both preceded and followed the great increase in prices of American fine furniture. While auction prices have yet to reach the Star Wars prices of paintings (such as the $53.9 million paid for Vincent van Gogh's "Irises" last Wednesday), the price difference is narrowing. A Cadwalader hairy paw foot wing armchair set a world auction record for furniture at $2.75 million when it was auctioned by Sotheby's last January at its Americana sale that totaled $14.5 million.

"I always said the decorative arts prices will meet the fine arts," Sack says. He recalls that in 1911, his father bought a Salem Chippendale chest of drawers for $150. In the three-quarters of a century since, Sack Inc. has bought and sold it five times. Five years ago it went for a quarter of a million.

Sack has a severe, serious look, a bit like a cast iron bulldog doorstop ("a straight face is necessary equipment," he says), except when he thinks of some great coup, such as the time in 1982 when he won the bid on the great Gibbs kneehole desk at a then record auction price of $685,000.

The Kaufman Collection The American antique reached its apotheosis last fall through April when the National Gallery of Art showed the George and Linda Kaufman collection, displaying the pieces on pedestals, not as "decorative art" but as "fine art" -- sculptures in wood and brass.

Most of the Kaufman collection was purchased from Sack Inc. George Kaufman tells the story of how he was caught in the rain one day on 57th Street, and wandered into Sack's to stay dry -- and bought a fine American highboy that was the beginning of his collection.

Sack tells another one. He had heard a rumor about a fine John Townsend tea table belonging to a woman in Essex, Conn. She'd bought it for $180. Sack persuaded her to sell it to him for $350,000. The Kaufmans were happy to buy it from him for $675,000.

"I call it the Remarkable Discovery Syndrome," Sack says.

1600 Pennsylvania Avenue Ironically, the White House, long one of the chief catalysts for the appreciation of antiques, has slowed its collecting in the last few years.

"Nancy Reagan doesn't seem interested in adding to the White House collection," Sack says.

Not that Mrs. Reagan has asked his opinion, but Sack gives it anyway: "The White House is in presentable shape, but much needs to be done. The White House especially needs fine American paintings."

A spokesman says the White House indeed has been adding to its collection, most recently acquiring two early 20th-century paintings, 21 pieces of President James Monroe porcelain, an ice cooler once owned by Monroe, a James Madison-era tea service and two pairs of mahogany chairs.

Sack was influential in the furnishing of the White House as far back as the Eisenhower administration, when the National Society of Interior Designers wanted to redecorate the Oval Diplomatic Reception Room on the ground floor. The designers originally proposed to add some English furniture. Sack's insistence on American pieces, and his donation of a New York Federal sofa helped set the parameters of the more extensive American decoration under the Kennedys and the Nixons.

Sack advised Henry du Pont, chairman of the White House committee that passed on acquisitions during the major redecoration of the White House by Jacqueline Kennedy. Later, he advised Clement Conger, on the great redecoration during Pat Nixon's tenure and when Rosalynn Carter took up the collection. Mrs. Reagan fired Conger last year, to the dismay of the Sacks.

Washington Furnished Not to worry, Sack has many off-the-avenue reasons to come to Washington.

As ever, he stops by the State Department's Diplomatic Reception Rooms, where Conger still presides, and by Washington mansions in Kalorama, Georgetown and other circles and squares of affluence. He has advised Conger since the Diplomatic Reception Room collection was started more than a quarter of a century ago. "When a donor offers to give Clem a chair, he sends me a picture before he accepts it," Sack says. "I pass on everything for him."

He also continues his family's long curatorship of The Lindens. Israel Sack paid $10,000 for the historic house, when it was still in Danvers, Mass., before Miriam and George Maurice Morris bought it for $14,000 and moved it to Kalorama in 1934, and then paid considerably more to restore it.

Israel Sack had sold the house's paneled drawing room to the Kansas City Museum, but paid $2,500 for a craftsman to copy it for the Morrises. The Sacks also helped the Morrises assemble a collection to furnish the house. At Mrs. Morris' death, Harold Sack appraised the contents at $2.2 million, just about what Christie's auctioned it for.

After Norman and Diane Bernstein bought The Lindens about five years ago, they came to Sack Inc. and said, "We have English antiques, but now we must have American. Will you help us?"

Harold Sack replied, "We come with the house."

Wouldn't Give 'Em the Time O' Day Joseph Hirshhorn, who gave his enormous collection of 20th-century art and his name to a Smithsonian museum and sculpture garden, bought a wonderful 1800-1810 grandfather clock by Simon Willard with the original papers including the instructions for assembling and running it from Sack Inc.

"Joe loved his clock," says Sack. "Every time he got a divorce {Hirshhorn was married four times, divorced three}, he always called us before the fact to arrange to store the clock in our warehouse, so his wife wouldn't get it. When he remarried, he'd call for the clock to keep time for his new wife."

After Hirshhorn's death, the clock sold at Sotheby's in 1981 for $77,000 to Bill Cosby.

Museums Sacked The Sack sons endowed three rooms in the Metropolitan Museum of Art's American Wing in their father's name. Harold, deprecating his effort to honor his father and to benefit the museum, says the $350,000 donation "is the best advertising money the firm has spent."

Sack Inc. also gave a gallery to Hood Museum at Dartmouth, where Harold graduated. The Boston Museum of Fine Arts, Colonial Williamsburg and the Metropolitan (of which Israel Sack helped assemble the nucleus, the Eugene Bolles collection) are among the places that also have works of art either sold or appraised by the Sacks. Israel Sack is credited with suggesting the ploy (later perfected and practiced by Conger) by which a museum buys a piece of furniture or art -- and then finds a donor to pay for it.

The Internal Revenue Service is as interested in museum donations as the recipient. Because of such a donation, an Israel Sack sale to a Washington collector involved Harold Sack in a battle with the IRS many years later.

In 1952, Israel bought a superb highboy chest from the Newport workshops of the Goddard-Townsend families for $20,000. He sold it for $2,000 more to Admiral and Mrs. E.P. Moore of Washington, with a guarantee that "it will go up a thousand dollars a year in value."

Harold Sack found out that his father had been wrong about that appreciation upon the admiral's death, when Barbara Moore gave the highboy to the Metropolitan Museum in 1980. Asked to appraise the piece for the tax deduction, Sack valued it at $235,000. The IRS office in New York said $40,000.

After Sack put together a 100-page presentation, an appellate court sent the case to Washington, and eventually Sack's valuation held. Sack said he recently looked at the highboy again, where it stands in the Metropolitan's American Wing: "If I ever saw one that good again, I'd pay a lot more for it than that."

Founding Father Harold and Albert Sack learned the care and feeding of collectors from their father. "My father slept in more houses than George Washington," Sack says. "He was always willing to spend the night with some collector in Detroit, swapping stories."

And collectors came to him. Israel Sack was fond of bringing home a prospective buyer for dinner, selling the dining room sideboard over cigars and port, and sending a truck to pick up and deliver it the next day -- all without giving his wife warning. Sack says that to this day the brothers don't collect antiques themselves because they were traumatized by their mother's disappearing sideboards.

It was in the 1920s that the millionaires of the day began to be seriously interested in American colonial and federal furniture and architecture, and so discovered Israel Sack. The story goes that Henry du Pont, whose Winterthur Museum is still the biggest and best American collection, first bought antiques from a New York firm, called Collings & Collings. The Collings couple played bridge Monday through Saturday. On Sundays they would buy furniture from Israel Sack and then sell it to Henry du Pont. Finally du Pont discovered their source.

Henry Ford, the Model T man himself, bought Wayside Inn, Sudbury Mass., the site of Henry Wadsworth Longfellow's "Tales of ..." and one day went into Israel Sack's shop and asked how to furnish the inn. Sack said Ford could afford the best and should have it. Ford "didn't ask me what, where, or how much," Israel Sack told his son.

But the antique dealer and the car manufacturer almost broke up when Sack found the tycoon was having his antique furniture pieces refinished as though they were cars. Ford's automobile assembly line painters laid 16 coats of shellac on an innocent early American desk, destroying the patina.

They made up, and Israel Sack gave Ford a cane that once belonged to John Hancock, the signer of the Declaration of Independence. Ford gave Israel Sack an enormous Lincoln touring sedan, the most expensive made. The Sacks used it as a delivery wagon.

When Harold Sack visited his father in a nursing home near the end of his life, he found Israel Sack talking to a psychologist, trying to sell him a fine Martha Washington chair from the magnificent collection in his mind.

No Sale Despite the reputation of American antiques in the United States, they don't sell well abroad.

Sack says: "To our knowledge there is not one collector of American furniture outside the United States. There's no market for American antiques -- they aren't exhibited or sold. American arts have always been Europe's stepchild. Neither the sheiks nor the Argentines want anything except French and English furniture. About the only exhibit of American furniture is at the American Museum in Bath."

Sack predicts, however, that all this may change. "With our treasures bringing these million-dollar prices at auction and the story told by the Kaufman collection, they'll have to pay attention," he says. Especially, now that the Japanese have our dollars and we have their VCR sets.

Sack thinks that a great American decorative arts museum in the nation's capital would help the international status of American antiques.

"The White House is full of appropriate things, all classical American furniture, but it's a house. The State Department Diplomatic Collection is marvelous. But these are furniture to be used," he says. "If we had a national museum, it would be an inducement for the great collectors to give their greatest pieces. The Metropolitan Museum of Art in New York has a great wing, as do other museums, but American antiques need a building of their own."

Sack is not one to think that there are no more great antiques yet to surface. "Three great masterpieces came on the market last year. I know where there's more. People will give to the museum because the values are so great.

"My father used to say, 'Money is honey.'