Photography was thought to have made the mezzotint obsolete, but Sally Kerr has seen evidence to the contrary

Photography proved to be the undoing of the mezzotint, an engraving method with many processes, and a finite life. Messy,

time-consuming, and prone to disaster, the mezzotint looked to be confined to art dictionaries. Instead, it is enjoying a rebirth, as this remarkable show by the American artist Craig McPherson witnesses.

To appreciate the art of this man, look first at the creation of a mezzotint. The blank copper plate is marked with small burred dots, made by a chisel-like tool called a rocker - left like this the plate would print solid black. Half-tones and light are created by removing the burr with a scraper, or by polishing the plate smooth again with a burnisher.

There is a heavy price to pay for this lengthy work. In a piece such as Yankee Stadium At Night McPherson took 12 months to complete the initial ''rocking'', working six or seven days a week, unable to rock more than three hours a day as the pressure involved caused tennis elbow. The trick in printing is to leave layers of ink which create modulations of shadow and light, to be wiped carefully with fabric, the edge of the hand, the fingertip, or even, for the brightest areas, a cotton bud, which will absorb all ink. Then fabric must be used to blend the edges of light and darkness.

McPherson also controls the stiffness of the inks, giving a rough or silky finish. Trial proofs must be printed to examine work in progress, allowing for change in the move towards the finished print. Finally the print is made, and, with a steel facing protecting the copper plate, up to 70 prints can be taken.

A mezzotint's real strength is in its half tones, with the ability to build shadows almost on to themselves, something pure photographic technique cannot achieve. Because of the processes involved, a mezzotint creates light from darkness, with a quality of emotion, moment, and character quite unlike any other form.

Inevitably, the use of half tone and light is reminiscent of film noir. McPherson's Cityscape series wallows in the style, most notably in the third of the series, which depicts FDR Drive in New York. The looming tall buildings, the flash of lights and driving rain, with passing cars by the East river, is brooding and highly evocative. Early examples of his famous Yankee Stadium print have the barest suggestion of light, with everything focusing on the shouting brilliance of the stadium itself; later versions explore accents between street and bridge lights, the half light reflected in the sky and off buildings producing the very real feeling that the Stadium is the pulsating centre of city life.

McPherson explored the use of pastel in some discarded prints as part of his proposal for his mural cycle for American Express, the finished oils a rich evocation of the night scene, with shades of dusk and evening blue replacing the half tones of the mezzotint. He took this idea further in his series on Ocean Terminal in Hong Kong and the mills of Pittsburgh. McPherson used two plates (black and red) and excluded the terminal buildings to concentrate on the ripples and their emotional atmosphere, bathed in blood-red reflected neon. An alternative version uses pink and a thinner blue-black ink, giving delicate accents to the tips of the waves. A further one uses pastel to pick out the terminal buildings and ferry boats, a straight comment on the impact of man.

In his most recent and continuing work at Pittsburgh, McPherson takes this same theme but glories in the startling hell-like contrasts of the smoking stacks and iron walkways. Periodic bursts of steam as the coke is purified provide the perfect subject, garishly back-lighting the scenes. Here he explores the ultimate extremes of heat and cold, dark and light.

In his Chop Shop series he chose to change the scene for each print taken, allowing us to witness the gradual disintegration of stolen cars observed unseen from his studio window in Washington Heights. The scene is split between dark rooftops, one street light, and the disappearing cars. Each print is a subtle progression, until we are left with empty space and new graffiti on the streets.

It is astonishing to see the breadth of work gathered here, as if one had walked into a world of Chandler, Hopper, and Steinbeck. It is a reminder of the darkness of life, and the strength of human spirit, and a celebration of an art form once thought obsolete.

n Craig McPherson and the Art of the Mezzotint is at the Hunterian Art Gallery, Glasgow University, until June 20.