22.3 I am here too. Investigating the invisible

24/03/10

“What is essential is invisible to the eye”; this beautiful quote from The Little Prince by Antoine De Saint-Exupery, suggests some reflections that can cover the field of artworks: how about if a matter the utmost importance is concealed inside what we cannot see?

Fortunately analytical techniques helps us with the investigation and the disclosing of “hidden truths”.However, most of the analysis are the prerogative of high skilled technicians and require expensive equipment that give results in terms of spectra, often very difficult to be interpreted. Despite of that, some techniques allow even less experienced users to collect important pieces of information, useful for their restoration projects; for example, light microscopy, fluorescence UV and infrared reflectography. In this article we will discuss just about IR reflectography, the technique that exploits the ability of infrared radiation to pass through the outer layers of a polychromatic work, from the paint across the painting substrate, down till the preparatory drawing. Traces of retouches, fills, abrasions and detachments of the paint or preparation signs are easily disclosed and the more important thing is that you can obtain information on the techniques of running through the lines of the preparatory drafts under the surface.

Before going deeper into the analysis of some aspect of reflectography, we should open a parenthesis that enables us to better understand this technique. Over the centuries the entire work of men who have focused on visual arts, from prehistoric cave 'painters' to the contemporary avant-garde artists, including those men striving for the preservation of those artworks, is based on light. 
Light is no more than a small portion of the electromagnetic spectrum that the human visual system is able to process. The electromagnetic spectrum is classified according to the wavelength of radiation that compose it, ranging from several meters ("long-wave", such as radio waves) to nano and picometers (for example X-rays). The visible spectrum is placed in the "shortwave" section, ranging from 400 nm to 700 nm (millionths of a millimeter).


The overall effect of electromagnetic radiation when encountering any object depends on three factors: 
1) the type of incident radiation (light bulb, sunlight, neon, etc.);
2) the composition and structure of the object encountered by the radiation;
3) the device which detects the radiation coming back from the object (human eye, infrared camera, etc.). 


That is useful in many fields of conservative restoration, such as for the operations of retouching in which you should always use a light source as similar as possible to the sunlight.

In the case of infrared reflectography technique, the basic prerequisite for the success of the diagnosis is that the light source contains a good amount of infrared radiation in its emission spectrum. That can interact with the paint layers and it is easy be detected by a specifically calibrated “eye” such as a camera that is sensitive to wavelengths between 700 and 1100 nm, the so called near infrared. An incandescent lamp will be appropriate in this case and make sure to place it not too close to the art work, in order to obtain a diffused light and to avoid overheating.
When looking at the graph we can easily see the difference between the emission spectrum from the sunlight and that of a normal incandescent bulb: the intensity of the natural light emission is well distributed over the entire range of the visible spectrum, while the light of the bulb is rather shifted to the higher wavelengths (we mean the range of yellow / orange / red spectra).
The lamps reproducing sunlight effect emit only in the visible, both the UV components (because photochemically harmful) and the infrared ones(because of the heat) are cut off.
This suggests that a good lamp for retouching purposes will be absolutely useless when making diagnosis by means IR reflectography. On the other hand, the spectrum emitted by a tungsten wire filament has a very strong infrared component, which can not be seen by our eyes, but that is perceived at a thermal level (infrared rays "heat") and can be used in reflectography.

Of course, the optimal light source is that having 100% infrared components and a convenient solution is given by IR lamps with E27 plug at 250 W that can be installed on the common lamps in place of the incandescent bulbs. However, a simple infrared source is not enough: the radiation reflected by the surface of the work, when interacting with the paint layers, needs to be adequately "filtered" and recorded by a camera, by means of the lenses and of the detectors. Therefore, the secret for good outcomes in the range of 'near infrared is the adoption of a proper mix of light sources, filters and detectors, combined in such a way to emphasize the intensity and detection efficiency of those wavelengths (700- 1100nm) that can reach the deepest layers of a painting and reveal preparatory drawings, alterations and changes of the author: in other words “bring to light what is not visible in the sunlight”. The interpretation of what has been recorded and displayed on the screen is not very easy. Firstly, the preparatory drawing is not always present. And when it is present, that becomes visible just in the cases the artist used charcoal, graphite or similar devices. Black, in fact, absorbs radiation of the near IR while other materials, such as hematite (that gives the red hue to the sanguine) reflects radiation and therefore it is "invisible" to IR camera eye. It is this selective absorption of incident radiation coming from different materials that made possible the development of spectroscopic techniques, such as IR spectrophotometry. Even with a simple IR reflecto-meter is possible to identify some of the involved materials: note that the organic-based pigments and lead-based pigments are generally transparent to the IR, while copper or iron cyanide based pigments (as Prussian blue) are opaque and do not enable to see what hides underneath.

Last, but not least, that is concerned with the choice of the proper equipment. IR reflectography dates back to the '30s and began as IR photography, and in the '60s there was a great turning point because the use of Vidicon tube cameras. At present we use cameras equipped with CCD (Charged Couple Device) which work with a higher ratio signal/noise, and a faster response, and less geometric distortions and greater stability at high intensities of input light(performing much better than Vidicon tubes). In order to "clean" the signal detected by the CCD, a filter with band width of 720-1100nm is adopted; this device eliminates the wavelengths of visible, that may interfere with IR: in this way you can have clear and sharp images with good contrast levels, and without "scattering" caused by the reflection of wavelengths below 700 nm (remember that the level of penetration is directly proportional to wavelength).

The cost of an equipment for IR reflectography can be very high, depending on the technical characteristics, such as accuracy and resolution of the images. If you are looking for a relatively low cost device, we recommend the portable system MIR 10 (available in CTS catalog), combined with other simple and affordable techniques that enable the restorer to get a complete picture of the work under investigation.

References
  • Walmsley, Elizabeth, Fletcher, Colin, Delaney, John, Evaluation of system performance of near-infrared imaging devices. Studies in Conservation, Volume 37, pagg 120-131, 1992
  • Palazzi Sergio, Colorimetria, la scienza del colore nell’arte e nella tecnica, Firenze, Nardini, 2000
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