doodlexrays00HOW CONSERVATORS LOOK INSIDE
An X-ray sees what the eye cannot.
Museum conservators X-ray paintings to see how they were built: the heavy lead white an artist laid down, a figure that was later painted out, a repaired tear, or a whole earlier picture on a reused canvas.
Test a pigment01Principles
02THE SHORT ANSWER
An X-radiograph is a map of what absorbs X-rays.
In old paintings, that mostly means lead white.
HOW IT WORKS
Pigments made with metals of high atomic number absorb X-rays best. Lead white, the main white for centuries, is the strongest of all.
Test a pigmentWHAT IT FINDS
Artists sometimes reused canvases, so an X-ray can show an earlier picture underneath. A Whistler seaside landscape hides a vertical portrait.
See the numbersWHY IT MATTERS
X-rays spot repaired tears, holes in panels and cut-down edges, and can reveal pentimenti, the changes an artist made while painting.
How it is done03A QUICK PIGMENT CHECK
Pick a traditional pigment to see how strongly it absorbs X-rays. The bar shows how bright it tends to appear on the film: the more it blocks, the brighter it shows.
Based on the pigment absorption table in WebExhibits' Pigments through the Ages. Real images also depend on how thick the paint is and how much pigment it holds.
04TWO NUMBERS WORTH KNOWING
The number that makes lead white so visible,
and the year the rays were found.
Z = 82
Protons in a lead nucleus
Pigments made of lead, such as lead white, chrome yellow, Naples yellow and minium, block X-rays best. Carbon-based pigments, with atomic number 6, are the worst absorbers.
LEAD · ATOMIC NUMBER
1895
Year the rays were found
Wilhelm Röntgen noticed a faint glow on a nearby screen while studying cathode rays in 1895. He received the Nobel Prize in Physics in 1901 for the rays later named after him.
DISCOVERY · NOBEL PRIZE
Paintings are X-rayed at exposures far lower than medical X-rays, and WebExhibits notes the low dose does not harm or alter the painting materials.
05FOUR STEPS IN THE STUDIO
X-radiography is not photography. It records how much of the beam each part of the painting stops, layer upon layer, all at once.
The painting is held in a frame by its edges so nothing touches the paint surface.
A sheet of radiographic film goes against the paint layer, where it gives the sharpest image of the artist's working.
The X-ray tube sends its beam through the painting from beneath, onto the film.
Each sheet is only a bit larger than a printer page, so large paintings need several, joined into one image with software.
06X-RAY OR INFRARED?
X-rays mostly show dense, metal-rich paint, so they are good at finding lead-white figures, repairs and reused supports. They see little of a carbon-black sketch.
Infrared reflectography is used to reveal underdrawing, the artist's first outline on the primed support. The Phillips Collection says infrared showed that Picasso's The Blue Room was painted over an earlier portrait of a man.
07Questions
Lead has a high atomic number, 82, so it absorbs X-rays very well. Lead white was the principal white pigment for centuries, so it maps out much of an old painting's structure.
Yes, sometimes. Artists reused canvases and supports, and an X-ray can reveal the earlier picture. WebExhibits gives the example of a Whistler seaside landscape that conceals a vertical portrait.
The National Gallery explains it comes from the Italian for repenting or changing your mind: a change the artist made while painting, usually hidden under later paint. X-rays and infrared can detect them.
Paintings are X-rayed at exposures far lower than medical X-rays, and the low dose does not hurt or alter the materials. The operator needs protection, and facilities need licences.
They can show what an object is really made of. Forgers sometimes use a period panel or canvas, and an X-ray of a ceramic jug can show it was rebuilt from shards.
08Sources
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