Sleights of screen put the magic in Oz

Citizen Kane opens with a sequence made up of a series of paintings of Xanadu palace, supplemented by miniatures, models, live action and props in the foreground, all punctuated by optical dissolves. The grand palace on the hill, like its celebrated owner, consists of jarring elements that make sense from a distance but that on closer inspection seem suspiciously like empty rhetorical flourishes. The overall structure of the palace was based partly on the fairytale castle in Snow White, partly on William Randolph Hearst's San Simeon and partly on Mont-Saint-Michel. The individual details included the campanile of San Marco in Venice, the colonnade of the Palazzo Pitti in Florence, a Victorian Gothic arched entrance, the rose window in Chartres Cathedral, the battlements of a medieval fortress and the roof of one of Ludwig of Bavaria's mad castles. With today's Photoshop software the assembly of these elements would have been a straightforward mechanical process, but in 1941 it involved the laborious preparation - with oils and large brushes - of a series of matte paintings, "matte" meaning a mask used to block out part of the image and to allow another to be substituted. In the case of Xanadu, iron railings and Kane's private zoo were added to the painted images.

The paintings were by Chesley Bonestell and Mario Larrinaga, members of a team assembled by Orson Welles at RKO studio. But they were never intended to be examined in the kind of detail I have just used. Mark Cotta Vaz and Craig Barron estimate that typical matte shots would appear for a maximum of six seconds on screen "before an audience might suspect an illusion". These visuals are now being scrutinised by scholars as if they were an "art form", hence the title of this book. An invisible art form, because the Hollywood hierarchy was so sensitive about revealing its craft secrets. Today, a glimpse behind the scenes is thought to enhance the magic rather than diminish it.

Design for film is a surprisingly undeveloped area of film scholarship.

Serious books about film design - the painstaking work of art directors, production designers, heads of art departments and the small army of craftspeople who supported them - are hard to find. There have been studies of the Hollywood studio moguls and their visual flair (most of them originated in the rag trade, and understood the importance of product differentiation through design), of the different "looks" created by major studios, of individual cult films such as Citizen Kane, Gone with the Wind and The Wizard of Oz, and of architecture in film in relation to architecture in the real world, but the field is cluttered with picture books, collections of interviews and souvenir brochures.

Apart from anything else, no one seems confident about exactly where to place the work of film designers. Is it art? A couple of years ago, an exhibition of the work of Ken Adam - designer of Dr Strangelove and the best of the James Bond films - at the Serpentine Gallery in London presented his work-in-progress materials in individual frames as if they were old master drawings. And yet they were intended to be only very preliminary steps towards the finished image on the screen. And can film design be separated from the other crafts? The relationship between the lighting cameraman and the designer is key - as was, in the studio era, the relationship with the head of department or design manager and, in the post-studio era, with the director.

Another problem is the survival of artefacts. Before studio theme parks and the re-valuation of back catalogues for issue through proliferating new media, the Hollywood studios were not particularly interested in their own heritage. The most famous example of such short-sightedness was the closure in May 1970 of the design departments of MGM and the sale of thousands of props, costumes and artefacts: 3,000 pastel matte paintings were dispersed, stolen or smashed. This question of survival might explain why books on film design keep returning to the same examples: the Douglas Fairbanks Robin Hood (1922) and The Thief of Bagdad (1924), King Kong (1933), The Wizard of Oz (1939), Gone with the Wind (1939), Citizen Kane (1941), The Ten Commandments (1956), the late works of Alfred Hitchcock and Star Wars and its offspring. Sure, these films contain some very memorable images: the gigantic Nottingham Castle - created for Doug to slide down the curtains; the hapless ape climbing the Empire State Building; the yellow brick road; the Cloud City launch pad in The Empire Strikes Back (1980).

But the fact that the reference materials for these matted images happen to have survived may also have helped to give them an artificial prominence in film scholarship.

The Invisible Art contains some terrific pictures - especially of original matte paintings and the final composites that incorporated them. It provides an outline history of matte work from glass paintings created on location to glass paintings hanging in studio settings to "original negative" matte paintings to the work of today's digital painters. It revisits all the films I've mentioned, plus a few less well-known ones such as Hitchcock's The Paradine Case (1948). It contains spectacular visual subsections on ancient Rome in the movies, Bonestell's space-age illustrations and the secret government warehouse full of wooden boxes at the end of Raiders of the Lost Ark (1981) - a painted shot that was on screen for a record 45 seconds. But the text itself consists largely of anecdotes, interviews conducted nearly 20 years ago and gushing phrases about the "promised land for moviemakers". European cinema does not seem to exist except for the trick films of Georges Meli s and Michael Powell's The Thief of Bagdad (1940), which was completed in America. The cinema of cultures beyond Europe is not even mentioned. The "invisible art" is traced all the way back to Meli s' flapping backcloths, which in fact created a totally different effect: matte paintings were hung very near the camera as forecloths to create an illusion of three-dimensional depth and scale; the backcloths were static backgrounds to the action taking place on the stage. So shelve The Invisible Art with all those other picture books. I'm afraid the great Wizard of Oz has not given away many of his secrets after all.

Sir Christopher Frayling is rector, Royal College of Art.