IT IS COLD in the firehouse at ninth and R Streets. The brass pole has been taken out and the ceiling sealed off. Wounded double basses trussed in clamps lie on tables. Wood is piled everywhere, notably a knee-high stack of spruce boards, wedged tight in its packing wire with some old violin backs.

"We got these in Germany last summer," says Barbara Wolf. "Sitka spruce has a brighter sound, but we don't use it in the French instruments."

"We have $4,000 worth of cypress logs from Italy out back," says her husband, Tom. "A lifetime supply. Try to tell that to our accountant."

The Wolfs make harpsichords, clavichords, virginals, fortepianos and other ancestors of the piano. They are three years behind on their orders. But catching up.

"A big one takes 800 hours," he says.

"We make about 2 1/2 a year," she says.

"We try to make historically oriented instruments: You might as well have the right harpsichord for the purpose," he says.

"Replica is an awful word, it implies you're not doing original work, and we're original," she says.

Their shop list ranges from a $14,000 German fortepiano, a 1795 replica with "a variety of veneered finishes available," to a Flemish virginal, a 1620 replica, for $4,750. Some are superfine copies, some are just "based on" an antique original.

"She's the harpsichord player," he says. "She does the work on the strings and plectra pluckers , the action stuff, but we really do use a lot of the same tools."

"He does the cabinets. His father was a cabinetmaker," she says. "We share the chores."

"But sometimes when I go out to repair an instrument, the guy says, 'Oh, I was hoping the good-looking one would come,' " he says.

They are both from the Midwest, met at Interlaken Arts Academy, went to New England Conservatory together, got married, took summer jobs as apprentices to harpsichord makers.

"We were apprentices five years," he says. "We've always been together, had the same training. We'd worked on 60 instruments before we opened our own shop. That was 12 years ago."

He is the one with the double bass. He free lances with the Smithsonian Chamber Players and the Washington Opera. He's making a replica of a violone, an early bass.

The Wolfs used to repair and fine-tune kit-built harpsichords, but no longer. They find that kits make an interesting woodworking project but not exactly a first-rate cheap instrument. Sometimes they see one superbly constructed -- but with a key part upside down. Harpsichords being more or less taken for granted now, the new thing is replicas of specific 19th-century instruments: Mozart's piano, Beethoven's piano.

"You can see what a tremendous revelation the piano must have been, with its great expressive range, coming after the clavichord," she says. "They were going for bigger halls, a more public sound."

It is a world of fine tolerances. Barbara Wolf works part-time on early instruments at the Smithsonian, some of which the couple has copied, and she is apt to notice minute differences, for all the precision of their work, differences she can sense only by instinct: the width of a pen line.

"Plastic plectra are harder than quill, they make a different sound, but you can shave them down," she says. "We did one Italian restoration job this year that had both leather and quill stops."

But you can't use just any quills. They must be primary feathers from meat-eating birds, good fliers, crows, ravens, condors. "Those Eastern Shore crows aren't worth a damn, they're soft, they sit around and eat fish all day." There are 17 varieties of crow in the Encyclopedia Britannica, she adds.

"I spent four hours revoicing an instrument the other day," he says, "just moving the sound post a sixty-fourth of an inch this way and that way."

"We get depressed when we finish a job," she says. "We see all the flaws. We have to start another one right away. Maybe we'll get it perfect one of these days."

"When we're 75," he says.

He is 34, she 32.

"It can be very boring work," he says.

"Yes, we have to hit ourselves over the head not to work 16 hours a day," she says.