Tools For Art Material Analysis

In order to identify the materials used by an artist on a piece of artwork, a scientist will utilize a variety of techniques to gain a full understanding of the materials that make up the work.  The scientist is interested in knowing the color, shape and composition of pigments, paint media, metals, woods, or minerals used.  This scientific understanding provides an objective means of evaluating a piece of artwork relative to a date of creation, for comparison with known works, and general knowledge for piece of artwork which can aid in conservation efforts.

 

Microscopy/Magnification
magnifying glass
The first step is to begin to examine the piece at magnification greater than what can be observed with the naked eye.  This can be as simple as a magnifying glass or loupe to see features more clearly such as a signature, craquelure in the paint, the type of application of ink on a print, lithograph or etching.  Examination of the canvas of a painting with low magnification can aid the examiner in seeing if the canvas was handmade or machine produced.

If samples are removed from a painting or drawing, they can be examined with a microscope at magnifications from 30X to 1000X, to reveal mixtures of pigments, the presence of additives and extenders. The shape and size of pigment particles can reveal information about how they were manufactured.  Examination of fibers from papers canvases allow identification of fiber types, treatments and additives.

Hematite_in_Scanning_Electron_Microscope,_magnification_100x

For examination at even greater magnification use of the scanning electron microscope allows the scientist to look at individual clusters of pigment particles magnifications as high as 100,000X.  This type of magnification allows differentiation of pigments that are very hard to tell apart such as black pigment from carbon black, charcoal or bone black.  These very high magnifications it is often possible to see the crystal structure for modern pigments which may be below the resolving power of a regular optical microscope.

Molecular Spectroscopy1024px-Sucrose_molecule_3d_model

Once particle types have been identified is important to understand the type of chemical making up the particles or fibers. Molecular spectroscopy involves the interaction of light ranging from the visible to the infrared with the materials, as well as techniques they can reveal the weight of molecules and molecule fragments.

Ultraviolet\visible spectroscopy is used to identify absorptions of visible light by material.  This type of technique can be used to show how different colors can be combined to give new colors (for example blue and yellow make green).  This type of spectroscopy is often general in the type of data it provides but can give some insight, particularly with mixtures.

Infrared spectroscopy can be more informative than ultraviolet\visible spectroscopy because the light that is absorbed by material such as dried linseed oil, rose madder pigment, or chalk is related to specific vibrations of the atoms in the molecules that make up these compounds. The spectrum which is produced for a material is often referred to as a fingerprint because it is specific for the material or mixture being analyzed. This take me can also be used for microscopic analysis measuring regions as small as 1/10 the thickness of a human hair (10 um).

microscope-385364_640A complementary technique to infrared spectroscopy is Raman spectroscopy. In this technique a low-power laser is directed through microscope onto a sample mounted on a slide. The laser light causes the material to emit additional light that has chemical information. The advantages of Raman microscopy are an even smaller sample size area of 1/100 the thickness of a human hair. And it allows enhanced analysis capabilities for inorganic pigments such as titanium dioxide, carbon based pigments such as carbon black or bone black, iron oxide pigments including ochres, chromate pigments, and synthetic pigments including azo compounds.

Mass spectroscopy also provides molecular information for material but the analysis is performed in a very different way.  Material such as a dye or a synthetic polymer, or natural resin must the vaporized and ionized for analysis. The first step of vaporizing the sample can be done by dissolving some of the material in a solvent and passing it through and instrument such as a gas chromatograph which heats the sample above the boiling point and flows the gas sample towards the mass spectrometer. Another option is for some of the material to be transferred to a probe, which is inserted into the mass spectrometer in the sample is rapidly heated electrically or by a strong laser exposure.  Once the material has been vaporized and makes its way to the mass spectrometer, it must be ionized to give it a charge.  This is often accomplished by passing material through a stream of electrons which results in the material having a charge and being attracted to and repelled from magnetic poles. By controlling the electric fields and magnetic charges in the system is possible to screen through different molecular weights and read these out as well as causing further breakdown of the molecule in looking at the weights of the component parts. With knowledge of the molecular structure of the materials of interest, the scientist can identify materials based on their molecular weight as well as their characteristic fragments.

Elemental Analysisimage330

Understanding what elements are present in a material is often extremely helpful in identifying the composition of the material.  When combined with information from the other techniques, understanding the elements that are present allows the scientist to narrow down the number of options for identification of the material.

Scanning electron microscopy with energy dispersive x-ray spectrometry takes advantage of the behavior of materials when they are examined with the scanning electron microscope (SEM).  In the SEM, a beam of electrons is focused onto the sample to allow high magnification images to be collected.  This exposure to the beam of electrons also produces x-rays, which scatter back up from the sample and can be detected.  The energy of the x-rays is related to the elements which produce them so it is possible to generate an elemental spectrum during the analysis with the SEM. This technique allows very detailed and specific examination of individual pigment particles, mineral grains, small metal pieces, and glasses.

X-ray fluorescence is another technique which can be used to identify the elements present in a material. In a similar manner to examination in the SEM, the exposure of the sample to high energy produces an emission of energy related to the materials present. In x-ray fluorescence, instead of exposing the sample to electrons materials exposed to x-rays, which cause the sample to produce x-rays with slightly lower energy which can be identified and related to the elements present. An advantage of x-ray fluorescence is that there are handheld instruments available which allow the scientist to nondestructively characterize pigments in paintings, minerals in sculptures, metals, and components of glass.

By combining the data collected using these different tools, the scientist can identify materials used by artists in a wide variety of objects. Seeing the size, shape and color of materials as well as understanding their composition can lead to identification of not only the type of material, but also the manner in which it was manufactured. By knowing the full story of the materials present, the scientist can compare the identifications with known materials as well as data the scientific literature, which leads to an understanding of when an object was created, the relationship to materials known to been used by an artist, and a comparison of more modern materials which may be used to produce copies or replicas of an artist’s work.