Sculpture is probably the oldest of human arts.

It seems a human instinct to make shapes out of whatever material is handy. Yet computers are changing even this ancient art that produced Michelangelo's David, Trafalgar Square's Lord Nelson and the Statue of Liberty.

Those sculptures all have something in common: they are big. David is five times life size, Nelson is three times life size, and the Statue of Liberty, from base to torch, is just over 46m (152ft) tall. How do sculptors do it? This is the question Manhattan sculptor Meredith Bergmann faced in creating a 2½m (8ft) bronze portrait of the first African-American opera singer, Marian Anderson.

The first step is to make a maquette, a clay model a few feet high - large enough for detail work, but small enough to see around. That took Bergmann two years of research. Once the maquette is finished, the technical fun starts. To get from clay to bronze is a reasonably simple, if destructive, 4,000-year-old process of alternating positive and negative images, ending with a ceramic shell into which you pour molten bronze. Of these many steps, the one being changed by computers is the intermediate one of scaling up the maquette to the sculpture's final size.

Traditionally, this was done by "pointing up", which recreates the sculpture point by point. The computerised method is done by several companies. Bergmann sent the Anderson maquette to Kreysler and Associates, a small company north of San Francisco that uses a laser scanner made by Cyberware, the company that laser-scanned actor Robert Patrick's movements for the liquid-metal cyborg in the 1992 movie Terminator 2: Judgment Day.

The scanner traces a line from top to bottom, turns the sculpture a fraction of a millimetre, and does it again until it has scanned the entire thing. If the maquette is too big, or if some of the block detail is overhung from a particular angle, the sculpture is turned on its side and scanned again, or sawn into pieces to be scanned separately. Eventually, the computer holds a complete three-dimensional representation.

The software then draws a milling path, imaged as a network of fine lines. The cutting tool itself is a long, smooth, metal lance-like object, much like a drill bit without the scrolling. A chunk of very dense urethane foam is placed on a turntable or flat bed, and a computer-driven machine uses the spinning tool to cut those lines back into the foam block, or blocks. It was the latter with the Anderson sculpture, which came back to Bergmann as a jumble of puzzle pieces to put together. The artist and her budget determine how close the cutting distance will be: the closer the lines to each other, the more detail - and the more time, which costs more.

Three kinds of things need correction when you scale up:

Mechanical distortion: the Statue of Liberty had several interim enlargements, each correcting mechanical errors endemic to the available tools;

Adjustments of proportion, though expert sculptors tend to foresee the impact of size correctly: the head of Michelangelo's David is enlarged to correct for its distance from viewers;

Errors of execution or details that weren't noticeable when the sculpture was small become glaring on a larger scale.

"The further you go [ in enlargement], the more undescribed areas and surfaces you find," says Bergmann. "When it's big, you want more details to the drapery, and you have to fill in the expression." Bergmann spent four months correcting and adding detail to the Anderson piece, 10 hours of it on the fingertips alone. "This method [ computer enlargement] is much more faithful. The laser gives you many more points."

It also takes less time - weeks instead of months. On a typical commission schedule, the artist gets more time to work.

Foam blocks also have the advantage of portability over a giant lump of clay. "The fact that it was lightweight was fantastic," says Bergmann. "I could get on a ladder and pull the head off and set it on a peg on a table and work on it at eye level because it weighed six pounds instead of 45 or 50, and it wasn't fragile."

Back at Kreysler there are some wacky action figures that offer clues to the computer-altered future of sculpture. They are enlargements of the little moulded plastic cowboys you used to find in cereal boxes. They point the way to economic opportunity.

Using computer milling, sculptors will in future be able to sell scale models of their works. - (Guardian service)