Infrared Reflectography and Hiding Thickness

When it comes to the study of under-sketching rather than underpainting, scientific emphasis switches to infrared reflectography. X-rays are scarcely able to make any distinction between a plain chalk ground (Fig. C.6) and one only slightly altered by addition of a thin sketch line in charcoal and bone black. But infrared radiation of wavelengths around 2.0 microns, having penetrated the upper paint layers, in particular the pigment, will be reflected at the level of the sketch and by virtue of their contrast will sharply distinguish between the whiteness of the ground and the darkness of the sketching medium. Then, although the human eye does not see the infrared image reflected from the un-dersketching, it is able to see that image once it has been converted to a visible one using a vidicon television system (Figure C.7). Infrared reflectography may facilitate the localization of retouchings, damaged areas, etc., and has therefore been used as an aid in the restoration of paintings. Infrared reflectography, however, has been most successful in partially re-

Paint Thickness Oil

Paint layer thickness, ¡j.m

Fig. C.6. X-ray transmission characteristics for various pigments in an oil medium for an oil binder-to-pigment ratio of 0.2 for chalk, azurite, lead white and lead-tin yellow.

Paint layer thickness, ¡j.m

Fig. C.6. X-ray transmission characteristics for various pigments in an oil medium for an oil binder-to-pigment ratio of 0.2 for chalk, azurite, lead white and lead-tin yellow.

Infrared Reflectography
Fig. C.7. Schematic diagram of the application of infrared (IR) reflectography to reveal the presence of a charcoal sketch underneath an IR-transparent paint layer. An IR vidicom camera is used to display the image on the video monitor.

vealing underdrawings such as are frequently encountered in medieval panel paintings. In Flemish fifteenth-century painting a mixture of chalk and animal glue was applied on a carefully prepared oak panel to form a smooth ground. The artist then made a drawing with a brush, using paint with bone black as pigment on this ground. This underdrawing was then the guide for the overlying paint layer.

Different pigments have different reflection and absorption properties at different wavelengths. When viewed in the visible spectrum each pigment has a certain thickness that is required (hiding thickness) to cover up an underlying pigment. In the present case we are interested in finding photon wavelengths at which the underlying pigments can be detected. Our focus is to observe the carbon-containing pigments used in underdrawings.

In the detection of underdrawings the hiding ability of the paint has to be minimized. Long-wavelength infrared (IR) radiation (1.2 to 2.2 mi cron, ^m) is used to decrease hiding ability or to increase the paint layer thickness (hiding thickness) required to cover up, or "hide," an under-layer of charcoal or other pigment.

The paint-layer thickness cannot be modified, and therefore the only way to penetrate it is to decrease absorption scattering. From fundamental light scattering theory it is known that for nonabsorbing particles, small in themselves compared to the wavelength of incident radiation, scattering decreases with increasing wavelength. Because paints consist of pigment particles embedded in a medium, by increasing the wavelength, the hiding ability of the paint layers will decrease. The absorption of paints used in fifteenth-century Flemish paintings is not strong in the near infrared except for the carbon-containing pigments used in underdrawings.

In Figure C.8 the hiding thickness XD has been plotted against the wavelength A for a number of pigments encountered in medieval paintings. For all wavelengths it has been assumed that the reflectance of the underdrawing is constant. It is seen from Figure C.8 that lead white and vermilion show only a moderate increase in XD with A. The increase in hiding thickness is most pronounced for verdigris, a pigment that covers well in the 0.7 fxm region but shows a rapid increase in hiding thickness at larger wavelengths. Ochre also covers well in the visible region of the spectrum, but its hiding ability, defined as 1/XD, decreases rapidly with A. The green pigment malachite, which is very frequently used in Flemish primitive painting in landscapes and trees, also covers considerably less in the region beyond 1.0 ^m. The covering thickness of vermilion is almost constant over the wavelengths displayed in Figure C.8. Although the hiding thickness as a function of the wavelength can only be approximated, Figure C.8 shows that XD increases by a factor of 2 to 3 from the visible spectrum to A = 2.0 ^m. This increase is enough to predict a considerable improvement in detectability of underdrawings using imaging devices capable of detecting radiation up to 2.0 ^m. The ability of infrared photography to partially reveal underdrawings can be explained by the increase in XD from the visible to the photographic infrared spectral region, which for thin paint layers is often sufficient. A comparison between calculated XD values and measured layer thicknesses in actual Flemish primitive paintings shows that in the spectral region around 2.0 ¡¿m, the paint thicknesses are less than the hiding thicknesses and an underdrawing may thus be detected.

1 1 1 1 1 Verdigris

Ochre

/

/ ___

/'Malachite

/ / S

. ST

\ / / /■ Y J. '

1 1

Vermilion i i i

Fig. C.8. The hiding thickness, XD, plotted against wavelength in microns of light in the infrared region of the spectrum. The hiding thickness is the thickness that is required to hide an un-derlayer. All the pigments were ground with linseed oil with pigment volume content (PVC) between 11% and 31%: vermilion (PVC = 11%), malachite (PVC = 21%), raw ochre (PVC = 31%), and verdigis. (From, J.R.J. Van Asperen de Boer, "Infrared Reflectography," thesis, University of Amsterdam, 1970.)

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Freehand Sketching An Introduction

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Responses

  • layla
    How to hide from infrared?
    7 years ago
  • Veijo
    What is ultraviolet reflectography painting analysis?
    8 months ago
  • tullio cocci
    How does infrared reflectography see beneath paints?
    6 months ago

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