Architecture: By the contemporary book: Peter Dormer visits a Cambridge college where the craft of borrowing has brought a taste of machine-age beauty into the library

Lipstick on the gorilla. That was Sir Norman Foster's now famous phrase for describing the decorative detail applied so self-consciously to drab modern buildings and the Post-Modern confections that have littered Britain over the past decade. But surely it must be possible for a contemporary architect to design and decorate a building in a consistent and convincing style and manner? Possible, but extremely difficult, given how hard we find it to invent or develop a relevant and convincing form of modern decoration.

However, this is exactly what Edward Cullinan and his team have pulled out of the hat in the new library they have designed for St John's College, Cambridge. Given the severe setting of the building, perhaps their imagination was forced. The library forms an integral part of Chapel Court, which is bounded on its southern side by a range of lovely Tudor buildings. The trouble is that the court is overpowered to the east by the tower of Sir George Gilbert Scott's great beast of a college chapel. This 19th- century interpretation of 13th-century French Gothic pushes itself into the court like a cross-Channel ferry barging its way into a harbour of inshore fishing boats. And on the northern flank there is a dour Thirties college building made of grim, grey bricks and designed by Sir Edward Maufe, architect of the austere Guildford cathedral.

So, apart from anything else, the new building really did need to cheer the place up. This it does with an exterior of ember-red bricks with white stone details.

You enter the library through a two-storey porch, a powerful response to the braying forms of Scott's chapel. Crowning the library is a steel and glass lantern, a modern addition to a city skyline well peppered with such devices from previous centuries.

As well as making its presence felt in the St John's courtyard and on the Cambridge skyline, this cheery newcomer has nice manners: where it presses up against the existing Tudor range there is a new brick staircase tower which provides a comfortable visual cushion between new and old buildings.

The new building is also well crafted: Cullinan has a reputation for this. The texture as well as the shape and colour of the hand-made bricks he has chosen count as much as the depth to which the stone detailing has been cut. The stone, which is Ancaster Weatherbed, is cut crisp and deep to provide sharp, dark shadows that prevent the building ever looking mushy, as so many worthily intended new 'vernacular'-style brick buildings do.

Although well crafted and respectful of its historic setting, the library is also - as all modern libraries are - a sophisticated electronic machine. It is thoroughly computerised and, using smart-card technology, provides 24-hour access for readers as well as electronic access to most of the other libraries in Cambridge.

Inside, it is organised around the sane but often overlooked principle whereby a reader naturally takes a book to the light to study it; so reading bays are distributed around the windows. Each bay is provided with a sinuous desk with an oak-tipped aerofoil edge. The windows offer wonderful views, those overlooking the Master's Garden have an almost heartachingly beautiful outlook. These are among the major privileges offered by a Cambridge college. Nothing similar exists at most poorer universities.

Although books provide much of the decoration (as is proper in a library), the architects have designed patterns and decoration out of elements of modern technology and engineering. This is at its most powerful, apparently contradictory, yet convincing in the lantern and the rooms beneath it on the third floor. The lantern is part of the building's ventilation system. On the outside it bristles with steel shutters and looks, close up, like part of an electricity sub-station; inside it is layered in plywood, painted white and resembles a beehive. A scattering of spotlights around its base are pieces of bright, hi-tech jewellery. Immediately beneath the lantern there is an elaborate criss-crossing of decorative tension cables.

These evoke the engineering details of steel bridges, ships and even aircraft. They are a part of our natural, late 20th-century decorative vocabulary, as are other elements such as the desk-edge aerofoils. These are images we see in everyday life and so a logical basis for contemporary architectural decoration - particularly because, although faced in brick and stone, this building is a machine.

Many of the prettiest machine shapes of this century occur in aircraft. So the main door to the library is shaped like a large section of a wooden aircraft wing; it is centred on a central pivot and to open it you simply push on one edge. There is no reason for the door to be shaped like a piece of wing except that it functions nicely and is a recognisable part of our age.

Elsewhere in the library, in the design of staircases and lift shaft, radiators and lighting, the architects have interpreted rather than copied technology.

Of course, nothing in this building can compete for ornateness with the college's famous 16th-century gate tower. Above the gate there is a statue of St John, who charmed poison from a doped chalice, drank deep and lived. Contemporary architects trying to build in such sensitive settings as St John's College, Cambridge, are also faced with the offer of a potentially spiked drink. Design a radically new building and be accused of attacking context and history; design a pseudo-historic alternative and be labelled philistine. Cullinan and his team have learnt from St John: they have drawn off excessive decoration from the cup of architectural contextualism and drunk from the chalice of crafted, yet sophisticated modern design.

Peter Dormer is fellow in the critical appreciation of the applied arts, University of East Anglia.

(Photographs omitted)