Catching Art Forgers with Chemistry

A painting appears at auction, said to be a lost work by a seventeenth-century master. The brushwork looks right, the style is convincing and the provenance papers seem in order. Millions of dollars depend on one question: is it real? Art historians judge style, but style can be imitated. Chemistry is much harder to fake. A forger can copy what a painting looks like, but not easily what it’s made of.

The central idea: anachronism

Every material has a history. A pigment invented in 1920 cannot appear in a painting finished in 1650, unless someone added it later. So the most powerful question in art authentication is: does this painting contain anything that didn’t exist when it was supposedly made?

This is called an anachronism, and pigments are the classic example.

A timeline of telltale pigments

Pigment	Chemical	Available from roughly
Lead white	basic lead carbonate, 2PbCO₃·Pb(OH)₂	ancient times
Ultramarine (natural)	from the mineral lazurite	medieval Europe (imported, very expensive)
Prussian blue	iron(III) hexacyanoferrate(II)	early 1700s
Zinc white	zinc oxide, ZnO	as an artists’ paint, mid-1800s
Synthetic ultramarine	sodium aluminosilicate with sulfur	late 1820s
Cadmium yellow	cadmium sulfide, CdS	mid-1800s
Titanium white	titanium dioxide, TiO₂	1920s
Phthalocyanine blue	copper phthalocyanine (organic)	1930s

So, for example:

• Titanium white in a painting “by” a seventeenth-century artist is a clear warning sign, because titanium dioxide pigment wasn’t commercially produced until about three centuries later.
• Prussian blue can’t be in a painting from 1600.
• Zinc white makes an early date unlikely.

The logic only works one way. Finding an old pigment doesn’t prove a painting is old, because a skilled forger can use traditional materials. But finding a modern pigment in original, unrestored paint is strong evidence of forgery (or of later restoration, which experts must rule out).

Non-destructive tools

Museums prefer methods that don’t harm the object.

X-ray fluorescence (XRF)

An X-ray beam knocks inner electrons out of atoms in the paint; as outer electrons fall into the gaps, each element emits X-rays of characteristic energies. XRF identifies which elements are present: lead for lead white, titanium for titanium white, mercury for vermilion (mercury sulfide), cadmium for cadmium pigments. Portable instruments can be taken into galleries, and scanning XRF can map the distribution of each element across a whole painting, sometimes revealing hidden earlier compositions underneath. See X-ray fluorescence.

Raman spectroscopy

A laser is focused on a tiny spot of paint, and the scattered light reveals the vibrations of chemical bonds. Raman spectroscopy identifies specific compounds, not just elements. It can tell apart two white pigments that both contain calcium, or natural from synthetic ultramarine. See Raman spectroscopy.

Infrared spectroscopy and infrared reflectography

Infrared spectroscopy identifies binders (the oils, egg or gums that hold the pigment) and varnishes, including modern synthetic resins. Infrared imaging passes through the upper paint layers and reveals the underdrawing, the artist’s preparatory sketch. Genuine works often show changes of mind in the underdrawing; copies usually don’t, because the copyist already knows the final image.

X-radiography

Ordinary X-ray images show dense materials, especially lead white, as bright areas. They can reveal a painting hidden beneath another. A forger who reused an old canvas may leave a completely unrelated image underneath.

Ultraviolet fluorescence

Under UV light, old natural varnish fluoresces with a greenish glow, while recent retouching often appears as dark patches. This quickly shows which areas are original.

Methods that need a tiny sample

Sometimes a microscopic sample, often smaller than a pinhead and taken from the edge or a damaged area, is needed.

Cross-sections

A tiny sample is set in resin and polished to show the layer structure: ground, underpaint, paint layers and varnish. The sequence and materials should match what’s known of the artist’s technique and period.

Lead isotope ratios

Lead white was used in almost every European painting for centuries. Lead ores from different mines have slightly different ratios of lead isotopes (²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb and ²⁰⁴Pb). Measuring these ratios by mass spectrometry can indicate where and roughly when the lead was produced. See mass spectrometry and isotopes.

Radiocarbon dating and the bomb peak

The canvas (linen or cotton) and the oil binder are made from plants, so they can be carbon dated. See carbon-14 dating.

There’s a twist that’s extremely useful for catching modern forgeries. Nuclear weapons tests in the 1950s and early 1960s roughly doubled the amount of carbon-14 in the atmosphere, peaking in the mid-1960s. Plants growing after the mid-1950s absorbed this extra carbon-14. This is the bomb peak.

Any linseed oil or canvas made from plants grown after about 1955 carries this distinctive high carbon-14 signal. So a painting whose binder shows bomb-peak levels of carbon-14 cannot have been painted before the 1950s, however old it looks.

Famous cases (in outline)

• Han van Meegeren sold fake “Vermeers” in the 1930s and 1940s. He used old canvases and traditional pigments, and hardened his paint with a synthetic resin (Bakelite-type phenol-formaldehyde) to mimic centuries of drying. Later analysis found that synthetic resin, a material Vermeer could not have used. In the 1960s, scientists also measured radioactive lead-210 in his lead white: freshly refined lead white contains far more lead-210 relative to its parent radium-226 than white lead that is centuries old, and the results showed his pigment was modern.
• Wolfgang Beltracchi created fake works in the style of early twentieth-century artists. One was exposed partly because analysis found titanium white in a painting supposedly dated before that pigment was widely available.

Both forgers were skilled painters; both were caught by chemistry.

Beyond paintings

The same toolkit authenticates other objects. Ceramics can be dated by thermoluminescence, which measures radiation dose absorbed since the clay was last fired. Metal sculptures are checked for alloy composition, since modern bronzes and brasses often differ from ancient ones in trace elements. Documents are examined for paper fibres, optical brighteners (fluorescent whitening agents added to paper from the mid-twentieth century) and ink composition.

Why authentication is still hard

• Restoration introduces modern materials legitimately, so analysts must check whether a modern pigment lies in original paint or a later repair.
• Forgers learn: some use genuinely old canvases, frames and pigments scraped from worthless old paintings.
• Absence of evidence isn’t proof: a painting with only period-appropriate materials might still be a clever fake, so science is combined with art history and provenance research.

Key takeaways

• Authentication looks for anachronisms: materials that didn’t exist at the claimed date.
• Pigment timelines are key; titanium white, for example, dates from the 1920s.
• XRF shows elements, Raman and IR identify compounds, and imaging reveals hidden layers and underdrawings.
• Lead isotope ratios and radiocarbon dating, including the bomb-peak signal after 1955, add powerful evidence.
• Science can prove a painting is not what it claims to be more easily than it can prove it genuine.