Frames without pictures. Frames with blanks where portraits or landscapes ought to be. Not a cheat, but a rare treat of an exhibit that sets modest limits, then explores expansively.

"The Frame in America 1700-1900" at The Octagon offers two rooms of these frames, framing nothingness, plus one small room of gilder's tools and quirky trappings. Spare and elegant, it invites us to see what we've skimmed 10,000 times, to focus on frames -- in their own right and as tokens of changing tastes.

Specifically American are the 1700s' Basic Black, made of window and door molding, white pine, hand-gouged and chiseled, painted matte black; and the mid-1800s' Thomas Sully, named for the painter, made of angular molding and gilded to reflect ambient light onto the canvas and to catch colors from the painting onto its own bright surface. Both are plain, unadorned, no- nonsense Yankee inventions, suiting the American character.

They are two in a range of styles that includes Faux Bois (stained and painted to look like marble and high-class veneers), Dutch Ripple (not an ice-cream flavor), Gothic Revival Tramp Art (chip-carved from cigar boxes), Cut Leather (worked by arts-crafts Victorians) and Renaissance Revival, including the classic continuous acanthus-leaf design favored by the American Impressionists. Here, too, is one of Whistler's fluted reed frames; he designed his own, gilded to his taste with a cool 18- karat green gold.

Labels point out corner joinery (whether miter, spline, lap joint or mortise and tenon), as well as types of iconography and ornament (lamb's tongue, rope and bead, padroon, corn). One small room introduces period nails, hangers, braided wire and the fittings of a gilder's kit. Here in an appealing clutter is the hammer for beating gold to its 1/250,000th-of-an-inch thickness and the Arctic hare's foot (with its extra tuft on the pad) for "talcing" the sheet of ox intestine on which the beating takes place. And the gilder's cushion (with its little windscreen against currents of air) where the squares are further cut.

The show and its splendid catalogue were created by William Adair, a gilder who learned the art from the National Portrait Gallery's Oliver Anderson and now runs the Gold Leaf Studios Inc., in a building that houses what he believes to be the city's only gilded elevator.

At his studio Adair undoes the damage to gold frames of ill-advised repairs such as sprayings-on of bronze radiator paints or the fillings-in of missing ornament with chewing gum. He employs a gilder who apprenticed in Belgium and a Cambodian refugee carver whose tools are fashioned from bedsprings set into bamboo-root handles.

Adair favors rabbit-skin glue, bought in sheets, though a predecessor he greatly admires, the 13th century's Cennino Cennini, preferred goat. For an ideal glue that slices like bread, Cennini wrote, use feet, sinews, skin and muzzles of goats killed in winter frost. He commended garlic, not too young, as a mordant, and burnt bread for buffing. And though Adair uses agate burnishers only and stows these tools in fur-lined bags, Cennini said the teeth of dog, cat, horse or wolf would do -- "any animal which feeds decently upon flesh" -- though the tools must be kept in the bosom, he said, against damp and scuffing.

Adair laughs about the refinements and little tricks of his craft, such as knowing just how to blow -- straight down -- onto the middle of leaf to flatten it, or when a tricky cleaning job calls for urine as solvent.

During gilding times at his studio no one roller skates or whistles or talks -- or breathes -- says an assistant, lest the leaf flutter away or be crushed.

The frames at the Octagon deserve that kind of attention. THE FRAME IN AMERICA 1700-1900 -- Through September 11 at the Octagon, 1799 New York Avenue NW; 10 to 4 Tuesday through Friday, 1 to 4 Saturday and Sunday. 638-3105. IT'S A FRAME-UP You can follow up the frames exhibit with a new-perspective tour of the National Portrait Gallery and the Freer. On Gilbert Stuart's full-length "Landsdowne" George Washington portrait at the portrait gallery, there's an English-style ornate carved-wood 6f X 10f frame, gilded, with just about every kind of ornament in the books -- lamb's tongue, pearl beading, acanthus leaves, fluted reeds and straps and a plum-colored bole showing through the gold. This one took 400 hours of cleaning and repair. Around the corner is a good example of the indigenous American Thomas Sully- type frame, on a portrait of Robert Morris by Robert Edge Pine. John Brown's portrait has the kind of stenciling and flocking on its frame that lent itself to cleaning only by uric acid. And the same treatment removed grime from the lavish plaster ornamentation, gilded, on Bierstadt's huge mountainscape of the Sierra Nevadas. Don't miss the huge gold-and-black frame on Ole Peter Hansen Balling's portrait of U.S. Grant at Vicksburg that hung in a Saratoga Springs hotel lobby. It's loaded with eagles, flags, acorns, oakleaves, the names of battles. Sale price of $50,000 was said to be $10,000 for the painting and $40,000 for the frame. At the Freer, see Whistler's fluted-reed frames, also the frame with oriental motifs that he designed for his "Princess from the Land of Porcelain" before he then painted the whole Peacock Room as a kind of extension of her frame. See, too, in the room of Whistlers, one frame with Whistler's butterfly cipher painted onto it, matching the butterfly in the painting -- "Variations in Pink and Grey."