National Gallery of Art Conservation: Scientific Research
Glossary of Terms and Techniques
Glossary of Instrumental Methods
Instrumentation in the Scientific Research Department falls into the
three broad classes of chromatography, spectroscopy, and microscopy.
Chromatography involves the separation of a mixture of materials into
individual components. Separation is achieved by the partitioning of a
component via selective adsorption and desorption as the mixture passes
over a stationary phase. Spectroscopy involves the interaction of
"light" (electromagnetic radiation) with a sample. The instrumentation
is designed to examine specific wavelength regions: x-ray, visible,
infrared, or ultraviolet wavelengths. Optical microscopes use a system
of lenses to magnify a sample, while an electron microscope uses a
series of magnetic fields as lenses to focus a beam of electrons on a
sample.
1. Chromatography
A. Gas Chromatography (GC)
Gas chromatography (GC) is a separation technique that can be used for
both the qualitative and quantitative identification of materials. It
relies on
the selective adsorption and desorption of volatile components on a
stationary phase. The components are carried through the column by an
inert gas to a detector. Common detectors for gas chromatography
include flame ionization (FID), thermal conductivity (TCD), and mass
spectrometry (MS). Components are identified based on retention time,
and, where available, mass spectrum.
Gas chromatography is used at the National Gallery of Art for the identification of protein-containing binders, as well as for the identification of oil, waxes, and low-molecular-weight natural resins containing di- and triterpenes. Higher-molecular-weight polymeric materials can be identified by pyrolysis-gas chromatography (Py-GC).
B. Pyrolysis Gas Chromatography (Py-GC)
Pyrolysis is the thermal dissociation of materials in an inert
atmosphere or a vacuum. The sample is put into direct contact with a
platinum wire, or
placed in a quartz boat inside a platinum coil, and rapidly heated to
600o – 800oC. Large molecules cleave at their weakest points and
produce
smaller,
more volatile fragments. Various methylating reagents, which increase
the volatility of polar fragments, can be added to a
sample
before pyrolysis.
These
fragments can then be separated on a gas chromatograph (GC).
Pyrolysis gas chromatography is very useful for the identification of
synthetic polymeric media, such as acrylics or alkyds, and synthetic
varnishes.
C. High-performance Liquid Chromatography (HPLC)
High-performance liquid chromatography uses a liquid mobile phase to
transport and help separate the components of a mixture. The mixture
is injected into a column packed with a stationary phase under high
pressure. In the column,
the mixture is resolved, via absorption and desorption, into its
constituents.
The interaction of the solute with mobile and stationary phases is
dependent
upon the solvent gradient and stationary phase. High-performance liquid
chromatography
is useful in the separation of compounds that are too nonvolatile to
be
separated by gas chromatography. At the National Gallery of Art,
high-performance
liquid chromatography is used for the identification of organic dyes.
D. Size-exclusion Chromatography (SEC)
Size-exclusion chromatography is a liquid chromatographic technique in
which molecules are sorted according to their size. The sample is
introduced to the column(s) containing closely packed porous
particles and carried along by the solvent. The separation is achieved
by exchanges between the solvent and the pores of the packing. Analysis
of standards of known molecular-weight distribution allows for the
determination of molecular-weight distributions of polymeric materials.
At the National Gallery of Art, size-exclusion chromatography is used
to examine the molecular-weight distribution of conservation materials,
which has a direct relationship with their optical and handling
properties.
2. Spectroscopy
A. Fourier-transform Infrared (Micro)spectroscopy (FTIR)
The technique of infrared spectroscopy uses infrared radiation in the
range from 4000 to 600 cm-1, the mid-infrared region, to
observe the
vibrational changes in chemical bonds. The presence and intensity of
specific vibrational frequencies allows for determination of functional
groups in organic molecules. The class of material
(proteinaceous, cellulosic, and so forth) can then be identified from
these
functional groups. Infrared spectroscopy serves as a first step to more
complete identification using other techniques.
The system at the National Gallery of Art is equipped with a
microscope,
and samples are examined in transmitted or reflected light modes. A
micro-attenuated total reflectance (ATR) attachment for the microscope
is also available for studying the composition of thin coatings, such
as
the varnish on a painting or photograph.
B. Ultraviolet-visible Spectroscopy (UV-vis)
The ultraviolet-visible portion of the electromagnetic spectrum that is
most commonly studied covers the wavelength range from 200 to 700 nm.
The
energy of this wavelength range of light causes electronic transitions
in molecules and depends on whether transitions involve nonbonding or
bonding (π or π*) electrons. The position of absorption of the
radiation
depends on the degree of conjugation and the nature of substituents on
the
molecules. In conservation science, this technique is most useful for
the identification of dyestuffs through solution spectroscopy.
C. X-ray Fluorescence Spectrometry (XRF)
X-rays, when striking an object, can cause ejection of inner-shell
electrons in atoms. The vacancy is filled by an outer-shell electron,
which causes secondary
x-rays to be emitted. These x-rays are of characteristic energies for
each
element present as they represent the binding energy of the two
electronic
states. Therefore, a spectrum can be obtained of energy versus
intensity.
The open architecture x-ray fluorescence spectrometer at the
National
Gallery of Art allows a whole object to be analyzed without removing a
sample.
X-ray fluorescence spectrometry can be used to determine the alloy composition of a metal object and the elemental composition of paintings (from which pigments may be inferred), drawings, and photographs. This technique is sometimes used in conjunction with other techniques such as x-ray diffraction or optical microscopy for pigment analysis.
D. Energy-dispersive Spectrometry (EDS)
Energy-dispersive spectrometry, similar to x-ray fluorescence
spectrometry, provides a means by which characteristic x-rays emitted
by elements present in a sample can be detected and processed. The
analyst can use the x-ray data
for either qualitative or quantitative assessment of the sample. EDS is
frequently
combined with scanning electron microscopy (SEM). The combination
of these two analytical methods makes possible the analysis of much
smaller areas of the
sample, and it provides for the detection of lighter elements
when
the analysis is carried out in a vacuum. SEM/EDS is routinely used at
the
National Gallery of Art to provide elemental data along with visual
information on inorganic materials encountered in samples taken from
art objects.
E. X-ray Diffraction (XRD)
X-rays are diffracted when directed at a crystalline material according
to its lattice structure. This is accomplished through the use of a
micro x-ray diffractometer that is capable of focusing a collimated
x-ray beam (20 to 800 micron diameter range) onto areas of interest
within the sample. X-rays diffracted by the sample strike a detector
and are converted to an electronic signal that is then further
processed using proprietary software. The generation of an x-ray
diffraction pattern that is characteristic for the crystalline phases
contained within the sample is the result of the data collection
process. Search-match software is then used to match the unknown
diffraction pattern to a database of diffraction patterns collected
from reference compounds. At the National Gallery
of Art, x-ray diffraction analysis is used for the identification of
pigments, corrosion products on metal objects, or efflorescing
materials. Analysis of materials using the micro x-ray diffractometer
may be carried out on small samples removed from an object, or the
analysis can be conducted in situ on objects small enough to fit within
the analysis chamber. Such analyses can provide precise pigment
identification, rather than
pigment inferences obtained by x-ray fluorescence spectrometry (XRF) or
energy-dispersive spectrometry (EDS).
F. Color Spectroscopy
A spectral reflectance curve represents the fraction of incident light
reflected at each wavelength from a material. From the reflectance
curve, tristimulus values can be calculated by combining the
reflectance curve with the three standard color matching curves as
defined by the International Commission on Illumination (CIE) and the
power spectrum of the incident light source. The three tristimulus
values roughly correspond to the amounts of red, green, and blue light
required to match the observed color. Changes in color can then be
determined by calculation of the CIE deltaE*94 values using a standard
equation. In conservation science, measurement of color is used for
evaluation of products, monitoring of fading, and pigment
identification. At the National Gallery of Art, reflectance spectra are
measured
using a dual beam xenon flash spectrometer (wavelength range 360 to 750
nm)
with a six-inch diameter integrating sphere.
G. Mass Spectrometry (MS)
In mass spectrometry, an instrument is used to produce ions from
molecules. Two common ways of doing this are bombarding the sample
either with
a beam of electrons (electron ionization) or with small molecules such
as methane (chemical ionization). The initial ion, called the molecular
ion, often undergoes fragmentation into
a pattern of smaller molecular weight ions. The ions are separated
according to their mass-to-charge ratio and their relative
intensities,
producing a mass
spectrum. In a quadrupole instrument, the ions produced in the source
enter
a high-vacuum area between rod electrodes maintained at opposite
polarity. The application of varying radio frequencies and DC and AC
voltages allows only ions with specific mass-to-charge ratios to be
ejected to the detector. In ion trap mass spectrometers, the ionization
and storage of ions occur in
the same location. The ion trap electrodes create a
three-dimensional electric field that holds the ions. Application of an
appropriate radio frequency voltage is then used to eject ions with
specific mass-to-charge ratios. Mass spectrometers frequently serve as
detectors for gas or liquid chromatographs.
3. Microscopy
A. Optical Microscopy
Optical microscopy (also called light microscopy) is a technique for
the
visual characterization of materials. It is possible to evaluate both
physical and chemical properties of materials based on light
interactions with a sample while observing it at magnification. Various
reagents, optical filters, and preparation methods are available that
may either enhance or provide additional visual cues, facilitating
characterization. The basic configuration consists of two lens systems,
in which the objective lens forms a magnified image of the
object, which is further magnified by the eyepieces. Transparent
samples are
typically viewed with transmitted light, while opaque samples are
observed with reflected light. Both lighting configurations may be
necessary to characterize a sample.
Broad ranges of artists' materials lend themselves to light microscopic analysis. Materials such as pigments, textile fibers, paper fibers, woods, and corrosion products are examined using high-magnification techniques that require a prepared sample. Information about gross surface features can be gleaned using low-magnification microscopes and do not require sampling the object. In the case of multilayered objects (for example an easel painting or polychrome sculpture), samples prepared as cross sections allow the analyst to address stratigraphy issues and questions about artist's technique. A cross section is an excised sample that has been mounted in a resin block, which is cut, ground, and polished perpendicular to the layering. The cross section is then viewed in reflected light. Details of the number of layers and their ordering, thickness, condition, and composition can be assessed. The analysis of cross sections is one of the more widely used optical microscopy techniques in conservation.
B. Scanning Electron Microscopy (SEM)
Coexistent with optical microscopy in our laboratory is scanning
electron microscopy (SEM). SEM is also used to examine a diverse range
of artists' materials and their associated degradation products;
however, the sample is
visualized by electrons rather than light. A beam of electrons is
scanned
across the sample, and the interaction between the electron beam and
the
sample
allows the analyst to image and analyze the material in question. The
sample
can be imaged by detecting electrons from the beam that the sample
scatters,
or by detecting electrons ejected by the sample itself. X-rays emitted
in
this interaction can be analyzed to characterize the elements present
in
the sample. The technique of x-ray characterization used in conjunction
with
our SEM is known as energy-dispersive spectrometry (EDS). Although
capable of very high magnifications (hundreds of thousands of times),
much
of the SEM work in conservation science is carried out at the lower
magnification
ranges (that is, from a few hundred times to a few thousand times).
Scanning electron microscopy is typically used at the National Gallery of Art to examine samples that have been taken from easel paintings. These may be either pigment scrapings (dispersed particles) or cross sections. The latter reveal the layered structure of a painting from the lowermost to the uppermost layers. Other types of materials examined using SEM include fibers, metals, corrosion products, glass, and ceramics.
4. Miscellaneous Instruments
A. Weather-Ometer®
The Weather-Ometer®, a chamber used to carry out accelerated
aging of materials, employs rigorous conditions to simulate the
acceleration
of natural degradation processes. The system at the National Gallery of
Art is equipped with a xenon arc lamp that, with appropriate filters,
can simulate daylight with or without an ultraviolet light component.
Testing of materials to be used in conservation treatments, including
clear coatings and paints, can be carried out in the
Weather-Ometer®. In
addition, the degradation processes occurring in artists' and
conservation materials can be studied.
B. Rheometer
A rheometer measures both the viscosity and the elasticity of resin
solutions.
A resin solution of interest is injected between the fixed and the
moveable plates of the rheometer, completely filling the gap. When
stress is applied to the moveable plate, the deformation of the resin
solution and its rate of shear are measured. The handling properties of
synthetic resin varnishes developed at the National Gallery will be
greatly improved through characterization of the rheological properties
of natural resin varnishes.
C. Differential Scanning Calorimetry (DSC)
The thermal behavior of materials can be characterized by differential
scanning calorimetry. In this technique, separate chambers for the
sample and reference are heated equally. Transformations taking place
in the sample are detected by the instrument, which compensates by
changing the heat input so that there is a zero temperature difference
between the reference and sample. The amount of electrical energy
supplied to the heating elements is then proportional to the heat
released by the sample. Differential scanning calorimetry is used
to study the thermal behavior of polymers.