March 18, 1983

By dawn it was apparent for miles around that the long-awaited day had finally arrived. Clouds of bluish-white smoke billowed from the carbon-caped stacks. And gleeful but skeptical workers, some returning from months of unemployment, asked, ''How did it go?'' as they ambled through the plant gates. For the first time in 21 months, one of the aging blast furnaces at the United States Steel Corporation's Edgar Thomson Works is smelting iron ore, coke and limestone into pig iron. Shortly after 4 o'clock this morning, dozens of workers on the graveyard shift gathered around the furnace to watch the lighting, hoping they were witnessing the start of a lasting recovery in the feeble steel industry.

IT did not take quite so long to assemble ''The Great East River Bridge'' exhibition that opens tomorrow at the Brooklyn Museum as it did to build the bridge that is its subject, but the show is a singular accomplishment nonetheless. It is clearly the definitive event to mark the centennial of the Brooklyn Bridge - short of the celebrations that will occur on and around the bridge itself May 24, the anniversary of its opening. But those festivities will go on for only a day or so, while ''The Great East River Bridge'' will be with us all the way through June 19. That is a good thing, because there is no reason why anyone with the slightest passing interest in Brooklyn, New York City, bridges, technology, urban development, engineering, 20th-century painting, 19th-century art or photography should miss this exhibition. And three months should be time enough to make sure that everyone with an interest in any of these areas finds the time to get to the museum on Eastern Parkway.

France lost about $3 billion of foreign-exchange reserves in the week ended last Thursday as it struggled to support the declining franc and avert another devaluation, according to figures issued today by the nation's central bank. The loss is one of the largest ever experienced in a single week by the Bank of France. Meanwhile, the franc remains under pressure, with the Socialist Government still debating whether to devalue it a third time since taking power in May 1981 and introduce austerity measures meant to curb inflation and reduce the country's huge trade deficit. This continuing disagreement over economic policy has already forced President Francois Mitterrand to postpone a Cabinet reorganization originally scheduled to be announced last Tuesday following the Socialists' weak performance in municipal elections earlier this month. Paris is swept by rumors about which ministers might stay in office and which might depart.

Trading in shares of the Baldwin-United Corporation remained halted on the New York Stock Exchange yesterday for the third consecutive session and two of its insurance subsidiaries were fined $100,000 in Arkansas for violation of state insurance law. Meanwhile, in a reflection of growing concern over the unfavorable publicity the company has been getting in recent weeks, Kidder, Peabody & Company told its salesmen yesterday to stop selling Baldwin's insurance products, including its annuities. The move leaves Merrill Lynch, Pierce, Fenner & Smith Inc., Baldwin's largest seller of such annuities, as one its few remaining important outlets. Baldwin's stock last traded at $23.50. The cash-short financial services company continued to negotiate a further extension of $440 million in loans originally due Baldwin's lenders last Monday. A 12-day extension until March 28 was granted Wednesday.

The International Business Machines Corporation, continuing its aggressive moves against both domestic and foreign competitors, filed suit today against an American rival and five of its own former employees, charging theft of trade secrets. I.B.M. accused the Cybernex Corporation of San Jose, Calif., and the five former I.B.M. executives who founded it of misappropriation of trade secrets and unfair competition. The complaint, which seeks an injunction and unspecified damages, involves the principal component of a computer disk storage device known as the ''head.'' The component is used to ''read'' and ''write'' data on the disk's surface for storage.

There is seldom anything poignant about an oilproducing facility. But the words scrawled on a giant ball valve at the central compressor plant situated above the vast Prudhoe Bay oilfield tell of a dream deferred: ''To future gas plant 1986-plus.'' The gas plant, intended as the final processing point for Alaskan natural gas before moving it south by pipeline, has never been built. Neither has the pipeline. The original goal - to have it in service this year - has slipped to 1989 at the earliest, and probably far beyond. The reasons include the persistent abundance of oil worldwide, a similar abundance of far cheaper gas in the United States, the steep rise in estimated costs and severe financing problems.

THE party broke up about 200 years ago, but on Sunday you can catch its tail end: piles of dirty napkins, crumbs on the floor, half-finished glasses of wine, a broken clay pipe and - thoughtlessly left behind - a tricorn on the wall. This is how the Long Room at Fraunces Tavern Museum looks today. The recreated chamber, where Washington said farewell to his officers, has been furnished to resemble a tavern moments after the departure of a drinking party, with all the attendant jumble. Those who do not work or live around Wall Street rarely get to see this museum, which is closed weekends. But it will be open this Sunday while the museum offers tavern-style entertainment. A visit to Fraunces Tavern could be combined with a tour of what remains of Manhattan's oldest waterfront, an area undergoing dramatic change. There is perhaps no better time to see this part of the city, where the 18th and 20th centuries meet, than on the quiet of a downtown weekend. Such a walk might begin at the Battery, pause at the tavern, end at the South Street Seaport Museum and never stray more than a block from Manhattan's original shoreline at Pearl Street. (Advice on restaurants along the route appears on page C27.)

The Great Western Financial Corporation, parent company of the nation's second-largest savings association, is to acquire Financial Federation Inc., owner of another large thrift unit, for $184.8 million in stock, the two companies announced late today. The agreement in principle, which was approved by directors of both companies, would increase the assets of Great Western Savings to $15.8 billion, from $12.7 billion, and establish the state's largest network of savings and loan branches, with 261 offices. Home Savings of America, with headquarters in Los Angeles, is the nation's largest thrift institution, with $16.9 billion in assets, but has fewer branch offices. Financial Federation's principal holding is United Savings Association. The agreement is subject to approval by state and Federal regulators.

Seven Cambodian families who fled war and starvation in their homeland and crowded refugee camps in Thailand to take up new lives in New York have fled again, this time from robbers, muggers and young toughs on the streets of Brooklyn. Taking only luggage, 100-pound bags of rice and their clothes, the 44 members of the families, including many children and some grandparents, boarded a chartered bus Wednesday. They left Flatbush, where they had failed to find safety, and migrated again, to Harrisburg, Pa., where furnished homes, food, interpreters and other volunteer help awaited them. ''They're disappointed,'' Truong Ngoc Phuong, director of the Indochinese Service Center in Harrisburg, said in an interview yesterday as the refugees settled into row houses recently renovated by Presbyterian Church volunteers.

The Senate broke a weeklong stalemate today and approved a recession relief bill containing $5.2 billion to provide jobs and humanitarian aid for the unemployed. The stalemate ended when Senator Robert W. Kasten Jr., sponsor of an amendment to repeal tax withholding on interest and dividend income, agreed to withdraw his rider in exchange for a Senate vote on the measure next month. Supporters of tax withholding had been stalling a vote on the repeal proposal, which apparently commanded majority support. President Reagan had vowed to veto the job bill if it contained the amendment.

The nation's 10 largest banking companies had $43.7 billion at risk to financially troubled developing countries at the end of 1982, more than half again the value of their equity, according to figures contained in their recently released annual reports. That means if none of those loans and investments were repaid, the investments of the banks' shareholders would be wiped out and the banks would not be able to repay all their creditors. The likelihood of that happening is considered zero by most bankers and bank analysts. Even when a company goes bankrupt, a bank usually gets back at least a third of what it lent.

A review Wednesday of the Equity Library Theater's production of ''Where's Charley?'' incorrectly attributed the choreography of Charles Abbott's dancing. It was done by Dennis Grimaldi.