Chroma: Ancient Sculpture in Color

Glossary

Brief Glossary of Analytical Techniques and Scientific Terms

Cross-section
A cross-section is obtained by mounting a microscopic sample removed from an artwork in a transparent resin, which, once hardened, is cut and polished until all the sample’s constituent materials, sometimes organized in layers, are exposed in one plane. This type of sample can be studied with different analytical techniques, including optical and scanning electron microscopies.

Digital microscopy
Digital microscopy is a type of optical microscopy that captures high-resolution digital images directly on a screen, by combining motorized lenses and digital elaboration of the source images. 

Electromagnetic radiation
Electromagnetic radiation is energy that propagates through space and materials both as waves and particles. Visible light is a form of electromagnetic radiation, as well as radio waves, microwaves, infrared light, ultraviolet light, X and gamma-rays.  The main difference between these radiations is their associated energy. The range of all types of electromagnetic radiation is known as electromagnetic spectrum.

False color image processing
False color imaging techniques process, modify and combine digital images to enhance or isolate specific information contained therein. These elaborations can be applied to various types of images, including those obtained by capturing visible, ultraviolet and infrared radiation to help recognizing patterns and pigments on works of art.

Fiber optic reflectance spectroscopy (FORS)
Fiber optic reflectance spectroscopy is a type of ultraviolet-visible spectroscopy that is carried out by illuminating the object and collecting the radiation reflected by it using a fiber optic probe. It can be used to identify pigments and colorants by measuring how light is absorbed by materials. Pigments and colorants absorb and reflect light in different ways depending on their atomic and molecular structure. The use of fiber optic probes (flexible light guides) makes the instrumentation much more convenient to be used with the complex shapes (and often large sizes) of works of art.

Infrared-reflected imaging (IRR) 
This imaging technique captures the contrast between materials that reflect and those that absorb infrared light when an object is illuminated with infrared radiation. Infrared light can penetrate deeper into paint layers, revealing hidden details.

Multiband imaging (MBI), also known as multispectral imaging (MSI)
While traditional photography captures images using the whole spectrum of visible light, this group of techniques captures images only at discrete intervals (bands) of the electromagnetic spectrum, from the ultraviolet (UV) to the near-infrared (NIR) region, by using modified cameras and filters. With MBI, certain pigments can be identified depending on how much radiation in a specific band they reflect or absorb. MBI includes imaging techniques such as infrared-reflected imaging, ultraviolet-reflected imaging, visible-induced infrared luminescence, and many others.

Non-destructive analysis
Analytical techniques are called non-destructive when the sample removed from the works of art is not consumed during the analysis and can be investigated with other techniques or stored for future analysis. 

Non-invasive analysis
Analytical techniques are called non-invasive when they can be used directly on works of art without the need for a sample to be removed from the objects. 

Optical microscopy
Optical microscopy includes a variety of techniques that use visible light and a combination of optical lenses to create a magnified image of a specimen or of an object. Under high magnification, it is possible to identify characteristic features of microscopic pigment particles, such as shape, size, and color.

Photomicrograph
A magnified image of an object captured through a microscope.

Polarized light microscopy
Polarized light microscopy is a type of optical microscopy that uses polarized light to study small amounts of crystalline particles, such as mineral pigments, by means of a compound microscope (a type of high magnification optical microscope). Under these conditions, light travels through the specimen, interacts with and is modified by the minerals, permitting their identification based on characteristic optical properties.

Polychromy 
A modern term from the Greek words meaning “many colors” referring to multi-colored nature of ancient sculpture. Polychromy includes not only applied pigments but also combinations of different colored media such as inlays and gilding.

Raking light imaging
Raking light imaging uses a source of bright light placed at an oblique angle to illuminate the object.  This simple examination technique can help see subtle differences in surface topography.

Raman spectroscopy
This spectroscopic technique is used to identify molecules based on their vibrational characteristics, allowing to characterize minerals, pigments and, occasionally, some organic colorants Raman spectroscopy takes its name from the Indian physicist Chandrasekhara Raman, who first described the anomalous light scattering effect at the basis of the analytical technique and was awarded the Nobel Prize for this discovery.

Scanning electron microscopy (SEM)
A scanning electron microscope is a scientific tool that takes advantage of the interaction between a beam of electrons and a specimen to obtain high resolution images at magnifications that are not achievable by means of a visible light microscope. In addition, the different signals produced at and near the surface of the specimen by the electrons can be detected and measured to obtain a wide variety of physical and chemical information. For example, by adding to the SEM an energy dispersive spectrometer (EDS), it is possible to perform localized chemical analysis by detecting the X-rays emitted by the sample.

Ultraviolet-induced visible luminescence imaging (UVL)  
This imaging technique captures the visible light emitted from an object that is illuminated with ultraviolet radiation.  

Ultraviolet-reflected imaging (UVR) 
This imaging technique captures the radiation in the ultraviolet region of the electromagnetic spectrum reflected by an object illuminated with ultraviolet radiation.

Ultraviolet-visible absorption spectroscopy (UV-Vis spectroscopy)
Ultraviolet-visible absorption spectroscopy identifies pigments and colorants by measuring how the incident electromagnetic radiation is absorbed by the material when illuminated by radiation in the ultraviolet (UV) and visible (Vis) regions of the electromagnetic spectrum. Pigments and colorants absorb and reflect UV-Vis radiation in different ways depending on their atomic and molecular structure.

Visible-induced infrared luminescence imaging (VIL)
Visible-induced infrared luminescence is a highly sensitive imaging technique that captures the infrared radiation emitted by an object when illuminated by visible light.  Only a few pigments, most notably Egyptian blue, luminesce strongly in the near-infrared region of the electromagnetic spectrum so that even microscopic remains can be detected, in contrast to other non-luminescing materials which appear dark under the same conditions. 

Visible-reflected imaging (VIS)
This technique corresponds to standard everyday photography and records the light in the visible region of the electromagnetic spectrum reflected from an object illuminated with visible light.

X-rays
X-rays are a form of high-energy electromagnetic radiation.

X-ray diffraction (XRD)
X-ray diffraction comprises a range of analytical techniques that use a beam of X-rays to investigate the structure of inorganic or organic crystalline materials. By measuring the intensities of X-rays that are scattered at specific angles (represented as a diffraction pattern), it is possible to reconstruct the materials’ atomic arrangement, as well as to determine other fundamental characteristics of such crystalline structures. The technique is also commonly used to identify unknown mixtures of crystalline compounds.  

X-ray radiography
This technique captures an image created by a beam of X-rays aimed at an object placed in front of a film or photo-stimulated luminescence screen. X-rays travelling through the object are absorbed (blocked) or pass through, depending on differences in radio opacity (density) of the materials. The resulting image often permits the visualization of the internal structure of objects, very much like medical X-ray radiography.  

X-ray fluorescence spectroscopy (XRF)
X-ray fluorescence spectroscopy is an analytical technique that provides the elemental composition of a material probed by a beam of primary X-rays, by collecting and analyzing the secondary (fluorescence) X-rays emitted from the material. These secondary X-rays are produced by the interaction of the primary X-rays with the atoms present in the analyzed material, a phenomenon called ionization. As each element of the periodic table produces a characteristic set of fluorescent X-rays, this technique allows for the identification of the elements present in the analyzed material. 

Brief Glossary of Pigments

Azurite (blue)
A copper carbonate hydroxide mineral naturally occurring in copper ore deposits (often alongside the more common malachite). Azurite was ground to create a bright blue pigment. Relatively rare and expensive, in antiquity azurite was mined in Egypt, Cyprus, and Spain.

Lazurite (blue)
A blue mineral which occurs naturally inside metamorphose limestones and marble.  It is the main constituent of the well-known lapis lazuli.

Lapis lazuli (blue)
A naturally occurring rock formed by a mixture of minerals, among which calcite, pyrite, and lazurite (the primary constituent). Lapis lazuli was incredibly rare and coveted, and was mined primarily in the Kokcha River valley in Afghanistan. To obtain the precious rich-blue pigment from Lapis lazuli, a complex and time-consuming process of refinement was required.

Cinnabar (red)
A bright red mercury sulfide mineral that forms naturally in veins associated to volcanic and hot spring activity. Because of its relative rarity, cinnabar was highly prized and very expensive, so its usage in ancient sculpture was a mark of great financial investment. Cinnabar is a rather unstable mineral and darkens upon prolonged exposure to light.

Egyptian blue (blue)
A royal blue-like pigment and the first synthetic (man-made) pigment in human history. The primary constituent of Egyptian blue was a calcium copper silicate mineral, called cuprorivaite, made by firing together different compounds, including powdered limestone, a copper-containing compound, such as copper filings or malachite, quartz, and plant ashes. While Egyptian blue was widely used in the Mediterranean, the exact method of its creation in antiquity is still subject of research studies. 

Malachite (green)
A green copper carbonate mineral naturally occurring in copper ore deposits (often alongside the rare azurite). Deposits of malachite are mined in Attica, the region surrounding Athens, Greece. 

Ocher (red, yellow, brown, purple)
A general term for colored rocks and soils containing oxides and hydroxides of iron. Ocher comes in a range of red, orange, yellow, and purple hues depending on the types and relative abundance of iron-containing minerals in it. It was one of the most frequently used components of pigments in antiquity due to its versatile earthy tone and ready availability. 

Hematite (red)
The most commonly occurring iron ore and the primary colorant of red, brown, and purple ocher. The name is derived from the ancient Greek word for blood in reference to its deep-red color.

Madder lake (red)
A dye made from the roots of the Rubiaceae family of plants which produces a pinkish red tone. The plants are native to the Mediterranean where they commonly grow.  

Lead white
A general term referring to any lead-containing white pigment, mostly lead carbonate hydroxide. The chemical form of lead carbonate hydroxide is the same as the naturally occurring mineral hydrocerussite, which was extremely rare. Lead carbonate hydroxide was synthesized in the ancient times to create the basis for lead white pigment. 

Tyrian purple
A pigment made from the crushed shells of marine mollusks in the Muricidae and Thaisidae families. The dye was famously difficult to produce and extremely expensive, hence its association with the trappings of the imperial family.

Mimetite
A bright yellow lead arsenate mineral found in lead ore deposits. This mineral is relatively rare, occurring only in a few localities, among which the silver mines at Laurion, near Athens, in association with other minerals, such as pyromorphite.