A peek under the paint

If you thought you'd never have anything in common with a Renaissance painter, consider this: They, too, didn't always get it right on the first try.

For a long time, artists were the only ones who knew about a redrawn scene or an apprentice's help. But curators and restorers are becoming more aggressive about using technology to reveal what's underneath the surface of masterpieces.

The results might turn painters as crimson as the infrared light used to peek under their works. But what's being learned helps scholars and gallery-goers better understand the creative process.

Renaissance painters in particular liked to do preliminary drawings, and they often used materials that infrared equipment can detect (little did they know). Some of these artists – such as Raphael, Bruegel, and Altdorfer – are featured in an exhibit opening Oct. 30 at the National Gallery in London, "Art in the Making:

Underdrawings in Renaissance Paintings."

"We thought it was about time to show the results of this research to the public at large because the images are absolutely sensational," says David Bomford, the exhibit's curator. "We draw all sorts of wonderful conclusions about the creative process from studying these underdrawings."

Depending on how you look at it, these artists either had a lot or very little to hide. Some came to the sketch knowing exactly how a figure or a landscape would look, and they made only minor changes to the final painting.

Other underdrawings reveal that the painting was likely created assembly-line style in a workshop. An apprentice would either trace a master copy or several different artists would prepare the sketch, with the master wielding a brush to finish the job.

Anyone who has ever drafted an essay or made a bouilla baisse will understand that sometimes artists simply changed their minds about small things at the last minute – as did an unknown Dutch artist who thought the eyes and lips of the work known as "The Magdalen" needed a bit more tweaking.

Raphael offers a meticulous underdrawing for his small painting, "The Madonna and Child with Infant Baptist," but even he realized that his central figure was going to get lost in the busy background he sketched originally and drew a column behind her instead.

All of this leaves art experts somewhat breathless about how the technological advances are helping them make new discoveries.

"This technique is being more and more refined," Mr. Bomford says. "The fact that we're getting images as good as this, I mean, 10 years ago this simply wouldn't have been possible."

One look at the dramatic changes to Jacopo Pontormo's work "Joseph with Jacob in Egypt" and you can't really blame them for getting excited.

The artist moved an entire scene – that of the death of Jacob, Joseph's father – from one side of the painting to the other after he had started applying paint. He simply got out the sketch materials and started again.

A Dutch physicist developed the technique being used by the gallery, called infrared reflectography, in the late 1960s. It uses infrared light and a special video camera to detect what the human eye cannot.

Basically, the charcoal or ink underdrawings absorb the infrared light, and the background it's drawn on – often a panel covered in a white plaster – reflects the light back.

It's similar to the way people are able to see words and shapes printed on paper – the dark ink absorbs the light, the paper reflects it.

Initially, the technique was quite time-consuming and only small parts of a painting could be examined at a time.

But at London's National Gallery they've found a way to simplify the process, so that underdrawings that once took three weeks to uncover, now take as little as three hours.

"In the last 10 years since I've been at the gallery, we've developed a system of digitizing the infrared signal and then using a computer to join up the pieces," says Rachel Billinge, a research associate at London's National Gallery.

Only works from the gallery will be featured in the exhibit. The one that will greet people as they enter is by German painter Albrecht Altdorfer. To curator Bomford, "Christ Taking Leave of His Mother" represents the difference between the freewheeling spirit of drawing and the more controlled nature of painting.

"The sheer energy that you're seeing in the underdrawing gets quieted down," he says.

A dramatic mountain drawn next to the Christ figure is scaled down in the painting. A church with a steeple in a little nearby town disappears (along with the town). Only the figures remain relatively unchanged.

"He obviously had worked out what he was going to do with the figures beforehand and had done preliminary drawings," Ms. Billinge says.

Altdorfer did his underdrawing with brush and paint, resulting in strokes that resemble those of a watercolor. Other underdrawing materials include pen and ink, or charcoal, which was less desirable because it was powdery and would get mixed up with the paint.

Artists used whatever color was available in their studios for their sketches, however, including browns, and even red, which thwarts scientists who use the infrared technique.

Still, it doesn't always take fancy equipment to see an underdrawing.

Sometimes the paint on a work might become transparent over the years, or an artist might not have completed the top layer of painting. And even the experts say that the technology alone can't tell them everything they need to know about an artist.

"It isn't ever enough just to look at the underdrawing," Bomford says. "You have to have the painting alongside you, and look at it closely, so you can see how they actually put it together."