Infrared reflectography and X-radiography can expose evidence that ordinary light leaves hidden: underdrawings, altered compositions, structural features and, in some cases, an earlier painted image. Neither method is a universal way to see through a canvas. What becomes visible depends on the pigments, drawing materials, layer thickness, support and the contrast produced by the imaging process. Conservators therefore treat each image as evidence to interpret alongside the visible painting, ultraviolet examination, microscopy and material analysis.
What these images actually record
An imaging system does not make an old painting transparent in the everyday sense. It records how particular forms of radiation interact with the materials in the object. Contrast in the resulting image may correspond to paint, preparation layers, drawing, a change in composition or the support itself. A feature that is absent from an image may still exist if the relevant materials do not produce enough contrast at that wavelength or energy.
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That distinction matters when people ask whether an X-ray can show an earlier painting. Sometimes it can reveal concealed forms, but the result is generally a tonal map of material differences, not a normal-colour view of a complete earlier version. Infrared images likewise show responses to infrared wavelengths, not a literal photograph of everything beneath the surface.
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How infrared reflectography works
Infrared reflectography (IRR) records reflected infrared radiation from the painting. Some upper paint layers transmit portions of the infrared range, while materials such as carbon black absorb it. When an underdrawing or preparatory mark absorbs infrared beneath more transmitting paint, it can create visible contrast in the reflectogram.
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This makes IRR particularly useful for detecting charcoal or graphite underdrawing, construction lines and some changes made during painting. A shifted contour, replaced object or altered pose may appear as a pentimento when the original and final materials respond differently to infrared. The image can show where an artist changed direction even though the final visible surface conceals the earlier decision.
What an infrared image may reveal
- Charcoal or graphite underdrawing beneath paint that transmits the relevant infrared wavelengths.
- Preparatory marks used to establish proportions, contours or placement.
- Some compositional changes, including earlier outlines or altered forms.
- Occasionally, broad regions where paint layers differ enough in infrared response to indicate a concealed design.
These are possibilities, not guarantees. The drawing material must absorb infrared in a useful way, and the paint above it must transmit enough of the radiation for the detector to register a difference.
Why a missing mark proves little
Pigments vary widely in their infrared behaviour. Some block the wavelengths needed to reach an underdrawing; others transmit them. Opaque or heavily built-up layers can hide marks that would be visible beneath a thinner passage. Consequently, an IRR image with no apparent underdrawing does not establish that the artist worked without one.
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Interpretation also requires caution with dark features. Getty’s historical painting analysis notes that a broad dark area in a reflectogram may be paint material rather than an underdrawing. Conservators compare the shape, location and behaviour of a feature with the visible image and with other examinations before assigning it to a drawing or an earlier painted form.
X-radiography: mapping internal material differences
How X-radiography works
X-radiography makes a photographic or digital record of how X-rays are absorbed or transmitted as they pass through the object. Areas with different material composition, thickness or density attenuate the beam differently, producing tonal variation in the radiograph. The result can include information from the paint layers, ground, canvas or panel support, and structural repairs.
What an X-ray may reveal
- Internal construction and changes in the support, such as joins, cracks or repairs that create radiographic contrast.
- Variation in paint materials or thickness where those differences affect X-ray transmission.
- Concealed painted forms when their materials produce a distinguishable pattern.
- Evidence of an earlier composition, although usually without the colour information needed to reconstruct it as a normal-colour image.
X-radiography does not separate every layer. If two materials absorb X-rays similarly, they may merge into the same tone. Conversely, a dense pigment or thick passage can dominate the image and obscure less attenuating features beneath it.
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X-radiography is not XRF imaging
X-radiography and X-ray fluorescence (XRF) imaging spectroscopy both use X-rays, but they answer different questions. Radiography records transmission through the object. XRF imaging detects characteristic secondary X-rays emitted by elements in the materials and can map where those elements occur. A conservation study may use both, but an X-ray radiograph should not be described as if it identified every pigment chemically.
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| Question | Infrared reflectography | X-radiography |
|---|---|---|
| Interaction that creates contrast | Absorption or transmission of reflected infrared wavelengths by materials | Absorption or transmission of X-rays through the object |
| Findings it may expose | Some charcoal or graphite underdrawing, preparatory marks and compositional changes | Internal structure, pigment-related contrast and some concealed painted forms |
| Main factors controlling visibility | Pigment response, drawing material, paint opacity and layer structure | Material composition, layer structure and contrast in X-ray transmission |
| Safest wording for a result | “The image suggests a mark or change that conservators interpret in context.” | “The image records internal contrast that may reveal structure or concealed paint.” |
The methods are complementary rather than interchangeable. IRR is often strongest for certain drawing materials and changes in contour. Radiography can show features governed by density and elemental composition, including information from the support that infrared cannot capture.
How conservators interpret a hidden feature
Start with the visible painting
Investigators first compare an unusual infrared or radiographic feature with the visible composition. A line that follows an earlier contour, or a radiographic shape that corresponds to a plausible figure, is more meaningful than an isolated patch with no relationship to the painting. The question is not simply whether a dark or light shape exists, but what material arrangement could have produced it.
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Compare multiple imaging modes
Visible-light photographs establish the final appearance. Infrared may indicate drawing or changes; radiography may reveal density-related structure; ultraviolet examination can help with some surface coatings and retouching; microscopy can show the physical sequence of layers. The National Gallery of Art describes imaging as one aid within broader conservation and art-historical investigation, not as a standalone verdict.
Test the material explanation
When a feature appears significant, conservators examine samples or microscopic details where appropriate and may use elemental or molecular analysis. In a Vermeer study, XRF imaging spectroscopy and microscopy helped relate imaging patterns to materials and painting process. This additional evidence can distinguish an underdrawing from a dark paint passage or clarify whether a concealed form belongs to an earlier design.
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Girl with the Red Hat
A portrait of a man beneath Vermeer’s Girl with the Red Hat was first noticed in an X-ray taken in the early 1970s. Later work used clearer imaging, reflectance methods, XRF imaging spectroscopy and microscopy to investigate the underlying portrait and its materials. The episode shows both the power and the limits of radiography: the original X-ray revealed a concealed form, while subsequent methods supplied clearer and more specific evidence about what that form represented.
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Woman Holding a Balance
In Woman Holding a Balance, false-colour infrared reflectance imagery showed freely brushed highlights on a wall that were hidden by the final paint. Other analyses, including XRF imaging spectroscopy and microscopic examination, contributed information about materials and working methods. The finding is not that infrared alone identified every step of the painting; rather, each technique contributed a different part of the explanation.
These examples should not be generalized into a rule that every old painting contains an earlier picture or that an ordinary X-ray can identify all pigments. They illustrate why conservators combine methods and interpret images in the context of the object’s construction.
What the images cannot establish by themselves
- Not a universal view through every layer: Paint opacity, pigment chemistry, thickness and support structure can block or confuse the signal.
- Not automatically an underdrawing: A dark infrared feature may be paint, ground or another material rather than a preparatory sketch.
- Not a colour reconstruction: A radiograph’s tones reflect X-ray attenuation, not the original hues of a concealed scene.
- Not proof of authorship or authenticity: Imaging can support conservation and art-historical arguments, but attribution requires broader evidence.
- Not definitive proof of artistic intent: A change may reflect planning, correction, reuse of a support or later intervention; context is necessary.
A concealed form can therefore be important without answering every historical question. The responsible conclusion identifies what the image demonstrates, what interpretation is likely and what remains uncertain.
How an examination is normally planned
- Document the surface. Conservators photograph the painting in visible light and record its dimensions, support and condition before interpreting any hidden feature.
- Choose the method that matches the question. IRR is appropriate when underdrawing or infrared-responsive changes are suspected; radiography is useful for internal construction and density-related contrast.
- Capture and calibrate the image. Professional equipment records the work systematically so adjoining frames can be assembled and compared with the visible composition.
- Inspect suspicious features in context. Investigators check whether lines or masses align with plausible forms, layer boundaries, support features or known restorations.
- Corroborate with other evidence. Microscopy, ultraviolet examination, XRF imaging spectroscopy or other analyses may test the material explanation.
- Report the conclusion with limits. The final account distinguishes direct imaging observations from interpretations and states when a feature cannot be identified securely.
This is a museum and conservation workflow, not a do-it-yourself imaging recipe. The reviewed conservation sources describe professional examination and do not establish a validated consumer-camera substitute or a safe home procedure for exposing artwork to X-rays.
Questions to ask when reading an imaging claim
- Which method produced the image: infrared reflectography, X-radiography or XRF imaging spectroscopy?
- Is the reported feature tied to a specific material response, or is it only a visual resemblance?
- Could paint, ground, varnish, a repair or the support create the same contrast?
- Was the interpretation checked with microscopy or another analytical method?
- Does the claim concern a complete earlier composition, a small adjustment or merely an internal structural feature?
- Is the conclusion about appearance, materials, conservation history, attribution or intent—and does the evidence support that level of claim?
Why the methods matter
Infrared and X-ray imaging expand the record of a painting beyond its final visible surface. They can reveal how an artist planned, revised and built an image, while also documenting the physical support and later changes. Their value comes from the combination of sensitivity and restraint: they make otherwise inaccessible evidence available, but they do not eliminate the need for material knowledge or historical judgment.
The most reliable interpretation is therefore layered as well. An infrared mark may suggest an underdrawing; a radiograph may show a concealed form; microscopy and elemental mapping may explain the materials; and art-historical study may place the discovery in context. Taken together, these observations can transform understanding of an old painting without pretending that any single image is the whole story.
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