BELLA ROSS WAS on the third floor of W & J Sloane's downtown store recently. She was scheduled to tell the public where brass beds come from and how to buy one. To do this, she had traveled from South Brunswick, N.J., where she and her husband run a company called J/B Ross Inc. Bella and John Ross make antique reproduction beds from brass.

Norma Lee Mahdavi, Ross' press agent, was trying to get her to tell how the business started in a chicken coop. "Tell him about the chicken coop."

"Aw," said Ross, wincing sort of childlike, "I don't think I like to call it a "chicken coop.""

The story goes like this.

Bella Ross, American, and John Ross, Englishman, were living near London until five years ago. He was an attorney (solicitor) in his father's law firm. She spent her time trying to sate her consuming passion for antiques.

On a trip back to the United States, they decided to stay and build an antique import business.

"John was in a very prestigious law firm," says Bella Ross. "His father is a very prestigious man. But he didn't want people at cocktail parties saying behind his back, "There goes John Ross. He has a very prestigious law firm. But really, it was his father who built the business." John didn't want that."

John and Bella Ross made frequent trips to Europe and imported thousands of antique brass beds. They had several collectors commissioned to save beds for them. "Sometimes one of our collectors would have 100 beds," says Bella Ross. "I'd pick out the 50 I liked best, then release the others."

One day a dealer asked if John Ross could make copies of the antique brass beds he was selling. Ross said sure, if the dealer would order 10. "We never expected he would take us up on it. But he took out a wad of money and started peeling off $100 bills." The dealer asked for a dozen.

Bella Ross' father is a wholesale egg distributor. The new makers of brass beds took over 4,000 square feet of chicken coop and turned it into a factory. Since 1975, the company has expanded many times over and become a million-dollar business with distributors across the country and in several forieng countries.

Bella Ross tells the public that a brass bed is not always a brass bed. There's a difference. Some brass beds are made of solid brass. Others look like brass beds, but are only brass-plated metal. Solid brass beds carry the insignia of the Copper Development Assoc., the copper industry trade group.

"I remember seeing this big full-page ad in (she gives the name of a leading department store in New York City). They were advertising brass beds for $90.They couldn't possibly have been real brass beds. But here was this big ad and you had to look way down at the bottom where it said, in little tiny print, "Brass Finish.""

The Ross reproductions are made from brass tubes and brass castings from the originals. As nearly as possible, says Bella Ross, they try to construct their beds in the exact manner of the antiques. This includes no "brass to brass" fittings that buyers might find on other newly constructed brass beds.

Some manufacturers, for instance, make joints where posts are fitted into holes half-way through the cross-bars. On antique brass beds, the posts have metal fittings in them.A long screw that extends through the cross-bar is secured on the other side with a nut that is a brass ball.

"One way to spot a good brass bed," says Ross, "is to look for all the little brass balls." This method of construction also makes for easier disassembly.

The same is true of joints around the frame. Some of the newer ones are joined by screwing one brass part to another with wing nuts. On others, joints are fitted into slots. With daily pressure on the bed, and because brass in not an exceptionally strong metal, these joints will weaken and cause the bed to loosen and rattle. Antique beds have metal castings inside the brass tubing with the joiners attached on the outside. The Ross beds are built in the latter mode. The frame parts sanp together and lock tight.

Another manufacturer, Bern Hynes, vice-president and national sales manager of Swan Brass Beds, made in Los Angeles, says: "The trouble with a lot of these beds is that they need to be polished."

The Ross beds are lacquered to eliminate the need for regular polishing. They have a high golden sheen, which also eliminates the need to guess whether they are antiques or not.

Lacquer does not last forever, though. And when it starts to go, the bed must be stripped clean of it. Hunes says the Swan beds are made with epoxy rosins bonded "permanently" to the metal. "Most companies," he said, "will eventually go with the epoxy." Ross will sell beds without the coating on request.

Reproductions, including the Ross line, are selling for $600 to $2,500. "One of the ironies in this, as well as other antique items," said Lee Arney of Miller and Arney Antiques in Georgetown, "is that the reproductions will often sell at the same price, sometimes more, sometimes less, as the antiques." Prices on antique originals range from $250 to $2,500.

"The biggest problem," said Len Berber, who has 14 brass beds on hand, ranging in price from $250 to $950 at his Country Bard Antiques in College Park, "is that everybody now has a queen size or a king size bed. In the old days, they didn't make them that big."

Reproductions come in all sizes. CAPTION: Picture, Bella Ross: All that glitters isn't brass.