Many chemical methods can be used to analyze, conserve, and restore artwork.
Some common means are spectroscopy, cross-sectional analysis and microscopy, UV light, and X-rays.
Spectroscopy can be used to identify materials, including pigments and varnishes, in artwork. It takes advantage of properties of various substances.
In different materials, atoms vibrate at different frequencies. Vibrational spectroscopy measures the frequency of atom vibrations in molecules to determine the molecule's identity.2
There are several ways to measure the vibrations of atoms. One method, called Fourier transform infrared spectroscopy, involves shining beams of infrared light on the material and measuring the absorption for beams with different frequencies.2 This is used to create an infrared spectrum unique to the substance.
Another technique using radiation is Raman spectroscopy. In Raman spectroscopy, visible light from a laser is shined at the painting. Most of this light is reflected from the material at the original frequency, but some is reflected at a different frequency.2 This phenomenon, called the Raman effect, can be used to identify the substance since it corresponds to the structure of the molecule.2
Spectroscopy does not need to use atomic vibrations to identify substances. Elemental spectroscopy, such as X-ray fluorescence spectroscopy, can also be used. Irradiating substances with X-rays causes some electrons to move to higher energy levels.2 When the electrons fall back to lower energy levels (after the substance is irradiated), they emit energy in the form of X-ray fluorescence.2 The energy of these rays can be used to identify the substance.
Combined, FTIR, Raman, and X-ray fluorescence spectroscopy can identify almost all pigments. See Pigments through the Ages for more information and examples.
Cross sections of paintings can be made by taking a small sample of the paint and embedding it in a plexiglass or resin cube.6
Optical microscopes can be used to examine cross sections. With different lighting, microscopes can reveal the layers in a cross section.1
Scanning electron microscopes can also be used to determine the pigments in a painting. After the microscope beams electrons on the paint sample, it collects information about reflected/scattered electrons.1 Information about the reflection and absorption of electrons can be used to identify substances, since metals and heavier elements reflect more electrons (and appear brighter in images) than lighter non-metals.1
Visit Pigments through the Ages for examples of micrsoscopy and cross-sectional analysis.
Ultraviolet (UV) radiation can be used to distinguish materials in artwork.
Organic materials absorb ultraviolet light and emit visible light.6 This process, called fluorescence, allows restorers to see the difference between original varnishes and pigments and newer ones. Older pigments and natural resins usually fluoresce while oil paints and newer varnishes do not.3
Different pigments fluoresce differently, showing where artists used multiple pigments or different strokes.3 This can help restorers imitate the artist's work to repair destroyed areas.
See Pigments through the Ages for demos on analyzing art with UV light.
X-ray radiographs can be used to show the materials in a painting. Made by exposing radiographic film to x-rays through the painting, radiographs show areas with different pigments.4 Generally, pigments with heavier elements and metals block x-rays while lighter nonmetals allow them to pass through. So, pigments such as white lead paint appear bright in x-ray radiographs.6
In the same way, x-rays can reveal different materials in statues and structures. Metals and heavier elements appear brighter than lighter nonmetals, allowing restorers to see where the artist used different materials.
View Pigments through the Ages for demos of X-rays revealing information about paintings.
Paintings are commonly preserved with varnishes, though some varnishes deteriorate over time. Sculptures are protected by stable patinas.
Varnishes are used by artists to improve paintings by making the colors deeper and the surface glossy. However, they can also either protect the painting beneath or deteriorate with it. See the deterioration page for more information about varnishes reacting with the environment.
Natural resin varnishes, used in many older paintings, tend to become cracked, unclear, and yellow.5 While they initially improve paintings, they block the painting underneath as they deteriorate. They also became harder to remove as they react with oxygen and other environmental factors.5
To restore paintings and prevent their further decay, art restorers often replace natural varnishes with synthetic ones. Natural resins help with color saturation and gloss because of two properties: low viscosity (spreading thinly) and low molecular weight.5 To conserve paintings, conservation scientists synthesized resins that had these properties and were stable and easy to remove.5
Today, synthetic varnishes with these properties include hydrogenated carbon resins and aldehyde resins.5 They improve upon natural varnishes since they react less with their surroundings and can be easily removed with organic solvents such as acetone, toluene, ether, and ethanol.5 These synthetic resins are ideal for restoration as the work can be easily reversed, it does not damage the painting over time, and it appears nearly identical to the original varnish.
Metals tend to react with their environment, producing compounds such as oxides, sulfides, carbonates, and sulfates.6 See the deterioration page for more information about metals reacting with the environment.
Reactions of metals with the enviroment affect metal sculptures. The material on their surface reacts to produce a coating called a patina. Unstable patinas can harm sculptures. They continue to react with the environment and are easily removed and eroded. This exposes new layers of the sculpture's metal, which again react with the environment.6 Over time, the entire statue can decay.
If stable, patinas can protect the interior of the statue so that it does not decay or erode over time. Restorers try to replace unstable patinas with stable, nonreactive ones.6 They also repair cracks in stable patinas to protect the interior of the statue.
1. Douma, Michael, Curator. "Microscopy." Pigments through the Ages. Institute for Dynamic Educational Development, 2008. Web. 14 Apr. 2014. <http://www.webexhibits.org/pigments/intro/microscopy.html>.
2. Douma, Michael, Curator. "Spectroscopy." Pigments through the Ages. Institute for Dynamic Educational Development, 2008. Web. 14 Apr. 2014. <http://www.webexhibits.org/pigments/intro/spectroscopy.html>.
3. Douma, Michael, Curator. "Ultraviolet (UV) Light." Pigments through the Ages. Institute for Dynamic Educational Development, 2008. Web. 14 Apr. 2014. <http://www.webexhibits.org/pigments/intro/uv.html>.
4. Douma, Michael, Curator. "X-ray Light." Pigments through the Ages. Institute for Dynamic Educational Development, 2008. Web. 14 Apr. 2014. <http://www.webexhibits.org/pigments/intro/xray.html>.
5. Ember, Lois R. "Chemistry & Art." Chemical & Engineering News 79.31 (2001): 51-59. Chemical and Engineering News. American Chemical Society, 30 July 2001. Web. 15 Apr. 2014. <http://pubs.acs.org/cen/coverstory/7931/7931art.html>.
6. Kabbani, Raifah M. "Conservation A Collaboration Between Art and Science." The Chemical Educator 2.1 (1997): 1-18. Print.
A. Museumphotographer. DSC_1514. Digital image. Flickr. Yahoo, 23 Aug. 2010. Web. 16 Apr. 2014. <https://www.flickr.com/photos/museumphotographer/4920461526/>.
B. Bertin, Gianluigi. P9121780. Digital image. Flickr. Yahoo, 12 Sept. 2010. Web. 16 Apr. 2014. <https://www.flickr.com/photos/drammatico/4982713457/in/photostream/>.
C. McCoy, Richard. Detailed Radiographic Image of an African Songye Power Figure in the collection of the Indianapolis Museum of Art. Digital image. Wikipedia. Wikipedia, 18 Sept. 2010. Web. 16 Apr. 2014. <http://en.wikipedia.org/wiki/File:Detailed_Radiographic_Image_of_an_African_Songye_Power_Figure_in_the_collection_of_the_Indianapolis_Museum_of_Art_(2005.21).jpg>.
D. Mbboston. Art Restoration, Painting and Frame Restoration, before and after, Oliver Brothers Art Restoration, Boston. Digital image. Wikipedia. Wikipedia, 20 March 2012. Web. 16 Apr. 2014. <http://en.wikipedia.org/wiki/File:Art_Restoration,_Painting_and_Frame_Restoration,_before_and_after,_Oliver_Brothers_Art_Restoration,_Boston.jpg>.
E. Left: Currier & Ives. Currier and Ives Liberty2. Digital image. Wikipedia. Wikipedia, 19 Feb. 2009. Web. 16 Apr. 2014. <http://en.wikipedia.org/wiki/File:Currier_and_Ives_Liberty2.jpg>.
E. Right: Clark, John L. The Statue of Liberty Pictures - New York. Digital image. Flickr. Yahoo, 15 Nov. 2008. Web. 16 Apr. 2014. <https://www.flickr.com/photos/jon-clark/3036537799/>.