The right picture frame brings a painting to life, the wrong one kills it. John Windsor meets the master matchmaker of a lost art
The kitchen-sink artist John Bratby used to paste a printed slip on the back of his canvases with instructions to frame them with no more than thin strips of battening nailed along the narrow width of the stretcher, and the stricture: "Ideally, there should be no frame."

Well, who needs picture frames? You can hang a canvas from its stretcher well enough. Apart from telling us where the wall ends and the picture starts, what use are they? Look at the two framed reproductions (opposite) of Matisse's portrait White Plumes of 1919. Given the chance of carrying off one of them, which would you choose? Not much doubt, is there?

Whenever picture and frame are happily married, something extra gets created. The unexpected twist is that this happy marriage is an anachronism - the spectacular bold frame with stylised fleur de lys motif was made in 17th-century Florence. After four centuries, its virility is undiminished. The fleurs de lys echo the shapes of the pendants of the sitter's headdress, the bright gold offsets the red background and the frame's thrust-forward (bolection) profile projects the image from the wall.

As it happens, the other frame is 17th-century, too - French, Louis XIII. It was chosen by a previous owner, who thought a classic French setting would complement a French modern master. But the couple are clearly not hitting it off. The dull old frame is trying to flatten the brassy young sitter.

The successful matchmaker is 51-year-old Paul Mitchell, the New Bond Street picture framer and restorer. He is today's leading custodian of the almost lost knowledge of picture framing. The original Matisse, in the Florentine frame he chose for it, hangs in the Minneapolis Institute of Arts. He is currently choosing frames for four European and 10 American museums, including the National Gallery of Art in Washington and the Getty Museum. No other framer travels to more museums or has a bigger photographic archive of frames, both with and without pictures - 200,000 of them.

He framed the only Leonardo in North America, the portrait of Ginevra de' Benci at the National Gallery of Art, in a soft, subdued late 16th- century Italian frame, replacing the strident, distracting architectural one. And Raphael's Colonna Madonna, at Germany's national Gemadegalerie, Berlin, having languished in a cut-down, overscale 17th-century Baroque frame, now has a delicate contemporary 16th-century Tuscan frame with a frieze of vines.

The most expensive Old Master picture sold at auction - Pontormo's vibrant propaganda portrait (c.1537) of Cosimo I de Medici, bought for $32.2m (pounds 22.3) by the Getty Museum at Christie's, New York, in 1989 - had the 18-year-old newly-acclaimed head of the Tuscan state enclosed in a fussy gilt 17th-century frame made in the rival republic of Venice. The young nobleman is now swaggering in a mid-16th-century Tuscan Mannerist frame with bold gadrooning.

Mitchell hopes that publication of Frameworks, his 480-page masterwork co-authored with the frame historian Lynn Roberts - the fulfilment of 20 years' study - will change the way we look at picture frames, or rather fail to look at them. Few of us appreciate just how unaware of frames we have become. Hardly any auction catalogues of picture or country house sales illustrate or describe the frames that pictures are in. As a result, frame dealers frequently snap up bargains. Scholarly, expensive art books show pictures cropped from their frames - even if the frames were chosen by the artists. "Most art historians are confused by frames," says Mitchell. "To them, they are a side-issue, a nuisance. They complicate their study."

Few pictures in museums are still in their original frames. They have been framed and re-framed, matched and mis-matched over the centuries. Two historical horror stories illustrate the point. Napoleon framed the paintings he looted from European art collections in a uniform livery - gilt Empire style with moulded ornaments - so that they could hang in orderly ranks on the walls of the Louvre. Never mind the artworks, look how the frames match the architecture. The Impressionists chose plain white or coloured frames for the paintings they showed at their ground- breaking exhibitions. Only a handful survive. Dealers such as Durand-Ruel re-framed them in ornate gilt (sometimes buying up Napoleon's chuck-outs) in an attempt to soften their revolutionary impact. Never mind the artworks, think of the clients' neo-Louis interiors.

Degas, an imaginative framer, is said to have stomped out of a friend's dinner party, never to return, carrying under his arm a painting of his, originally in a green frame, that his host had hung re-framed in gilt. His solitary gesture did not prevent French 17th- and 18th-century-style gilt frames with their ornate foliate friezes becoming the "correct" style for Impressionists, a solecism that persists to this day.

So salute the judicious anachronisms of Mitchell. History has seen quite enough framing to match architecture, nostalgia for antiquity or the pomp of collectors. He matches frames to paintings. And he maintains that the repertory of frame styles from past centuries is more than sufficient for framing modern masters. After all, the effects of "form, function and ornament", the sub-title of Frameworks, are universal. He cites de Chirico's self-portrait of 1911, in a black early-17th-century Italian frame with cartouches of gilt foliage at the corners and centres of the frieze, which hangs in the Metropolitan Museum of Art, New York. "When I first saw it, I stopped in my tracks. It works as well today as it must have done 400 years ago. I have no idea when the frame was applied to the picture.

"You don't need to invent frames. There's little that hasn't been said in frame-making in the last 500-600 years," says Mitchell. Sure, the moderns themselves have done funny things with their frames. Consider the inlaid and emblematic frame designed by Jan Toorop for his The framemaker Joosstens of 1898, or the Vienna Secessionists, especially Gustav Klimt's broad side-panels full of emblematic designs. And then, of course, Howard Hodgkin. Such artist-designed picture/frame combinations are peripheral to Mitchell's expertise - although he cites approvingly frames by Whistler, Rossetti, Brown, Degas, and especially Holman Hunt. Mitchell's quest is to discover frames that will reinforce an artist's composition while maintaining their integrity as frames. "The excitement comes when you put picture and frame together and the picture makes a sudden leap that has to be seen to be believed." The rules, please? "A great marriage is instantly recognisable. Only afterwards do you analyse why you like it."

Look, for example, at the stunning Ophelia by Gerald Brockhurst (d.1978), withdrawing into the shadows but being drawn forward by the foliate corners and centres of her brightly gilded mid-17th-century Spanish bolection frame. The ensemble has the tension of a Velazquez.

Paul Mitchell's company sponsored the National Portrait Gallery's exhibition "The Art of the Picture Frame" earlier this year. His own gallery, on four floors, has wall panels displaying much of the exhibition that he mounted to complement the National Portrait Gallery show, for which "Frameworks" was written. He has co-authored with Lynn Roberts the entry on picture frames in Macmillan's 34-volume "Dictionary of Art", which is published separately as "A History of European Picture Frames". Order forms for the "History" (pounds 25) and "Frameworks" (pounds 75), both published in association with Merrell Holberton, are available from Paul Mitchell Ltd, 99 New Bond Street, London W1Y 9LF (0171-493 8732).