Articheck

There are several spectroscopy techniques that a conservator can employ in material analysis. The Fourier Transform Infrared Spectroscopy (FT-IR) is commonly used first in the process of sleuthing out the composition of an artwork. By examining the vibrational, stretching and bending energies that are activated in compounds when they absorb light within the infrared region, FT-IR can identify unknown materials, determine the quality and consistency of a sample and determine the amount of components in a mixture.

  The FT-IR technique was developed to overcome the limitations of other older infrared processes – namely dispersive and filtered methods that only measured infrared frequencies individually.The problem was solved by using an interferometer; a simple optical device that produces a cipher-like signal called an interferogram that allows for simultaneous scanning of infrared frequencies, reducing scan time from several minutes to a matter of seconds.

  Additionally, there are a number of other benefits for using FT-IR.The technique is non-destructive and requires only a small sample for analysis (milligrams or micrograms). Its internal calibration is established by a HeNe laser as an internal wavelength calibration standard, a helpful feature freeing users from manual calibration thus reducing the chances of error. Finally, FT-IR’s sensitivity is a vast improvement from earlier infrared models. Its high optical throughput results in much lower noise levels and its fast scans enable the coaddition of several scans in order reduce random measurement noise to a desired level.

  However, the technique is not omnipotent and it is often used in conjunction with Ramen spectrography. FT-IR is generally more sensitive to organic material, but only Ramen can analyse sulfide pigments (such as vermilion and cadmium yellow), along with carbon black pigments derived from vegetables. It also is noted to have difficulty with individual components of complex mixtures and identifying sources of materials, e.g.: proteins from animal glue vs protein from an egg yolk.

  In the conservation of art, FT-IR is a powerful tool in detecting both organic compounds and some inorganic materials with either crystalline or amorphous structures. It has been used in the analysis of various mediums and/or structures, such as sculptures, frescoes and oils on canvas. Notably, FT-IR was instrumental in a large international study that included several of Van Gogh’s paintings, ‘Portrait of Gauguin’ and ‘Vase with Sunflowers’, whose chrome yellows were fading to a dull brown. Its analysis (along with other testing techniques) provided the answer. While confirming that Van Gogh used both stable and unstable chrome yellows in his work, it was determined that the LED lighting in the museum was a reactant to the unstable pigment. After a few hundred hours of exposure the unstable chrome yellow began to fade to the dull brown. The discovery adds new information to the discussion within the community about the effects of various lighting schemes to specific pieces of art and what actions could be take to avert continuing damages. Luckily, conservators have a resource where they can contribute and compare their results in the comprehensive IRUG Database that includes hundreds of spectra of pigments, fillers and other materials.

Credit: University of Antwerp, Department of Chemistry – Van Gogh Museum.

      Sources:

-Nicolet, Thermo. “Introduction to Fourier Transform Infrared Spectroscopy.”Introduction to Fourier Transform Infrared Spectrometry(2003): 1-7. Web.

-“Art Conservation at the University of Delaware : Other Technical Terms.”Art Conservation at the University of Delaware : Other Technical Terms. N.p., n.d. Web. 20 Jan. 2015.

-“Spectroscopy.” Pigments through the Ages. N.p., n.d. Web. 20 Jan. 2015

A newbie on the conservation technology scene, thermal quasi-reflectography (TQR), turns the process of thermography on its head and offers a new diagnostic tool that reveals subtle details in paintings that other imaging tools have missed and that the naked eye cannot perceive.

An innovative technique, TQR was developed in 2012 by a collaboration of scientists from the Universities of L’Aquila and Verona and the National Institute of Optics in Florence. The process is a noninvasive analysis of pictorial surface layers in artworks, but has shown to be highly sensitive; allowing for a more refined insight into past restoration processes and serves as a helpful tool to explore current and future conservation needs as well.

 TQR was first tested on a small piece of the Zavattari frescoes (1440-1446) in the Chapel of Theodelinda in the Monza Cathedral. It revealed several findings previously missed in earlier imaging studies, one showed older restorations that had simply repainted over the original gold decorations. Another piece in the initial TQR study, Piero della Francesca’s painting, The Resurrection, yielded interesting findings as well, such as: previous restorations retouched several areas in the work with highly reflective pigments and several differing fresco techniques were used in an area around a soldier’s sword. The research was published in Optics Express, along with several photos showcasing the power of TQR. The aforementioned frescoes were compared in traditional photography, near-IR imaging and mid-IR imaging, offering evidence that TQR has the ability to peel back yet another layer of understanding.

(Part of the fresco by the Zavattari brothers in the Chapel of Theodelinda. Color photography (a) and imaging in the near-IR (b), compared with the thermal quasi-reflectography image (c); thermal quasi-reflectography (TQR) captures details not detected by other techniques. Images courtesy of Optics Express.)

How does it work, exactly? Opposed to traditional thermography where infrared radiation emitted from a surface provides a measure of temperature, TQR does something slightly different, it doesn’t detect heat - it tries to minimize it.  This is done by using mid-infrared light, (which also reduces any residual emissions to a negligible 1%), on to a surface where its reflected light is recorded back into a camera.

In a stable temperature environment, paintings typically emit more energy in the longer infrared wavelengths. This effect is reduced in mid-IR. By using underpowered halogen lamps at their mid-IR source, researchers reversed the tools of thermodynamics, allowing for a new, clearer picture of details that have been previously obscured with traditional IR imaging by utilizing mid-IR’s shorter wavelengths. TQR holds an advantage over both IR and low-IR, in differentiating material used, provides better contrast and sharper images.

TQR is still in its infancy, while its initial studies are “wow-worthy”, it’s full range is yet to be fully uncovered. Since its introduction, several studies, including collaborations with Opifi cio delle Pietre Dure, are in the process of carrying out studies to further advance our understanding of the TQR system’s capacity for imaging various types of artwork, imagery integration and its relationship to spectrometry. In the cutting-edge renovation of San Sebastian's church, resting place of the great Paolo Veronese, one of superheroes of 16th-century Venetian painting, TQR was employed when the side chapel frescoes were scheduled for repair. Thanks to this new technique, it was discovered that the use of “smalt” blue, (a less expensive pigment that can deteriorate radically) had been used in several areas of the frescoes. This new imaging has allowed restorers get a better sense of Veronese's true colors, which in some cases have changed substantially over time. When the project is finished,  Melissa Conn, director of the Venice office of Save Venice Inc., believes that scholars may have to "re-evaluate what they say about Veronese's palette."

Sources:

"Reflected Light Helps Preserve Frescoes." http://www.photonics.com/. N.p., 2012. Web.

Marcus, J.S. "Looking up at Veronese's Grandeur." WSJ. Wall Street Journal, 14 Mar. 2014. Web. 16 Dec. 2014.