EXAMINATION METHODS AND SCIENTIFIC TERMS

CROSS-SECTIONAL MICROSCOPY

What are Cross-sections?
Methods of Preparing Cross-sections
Cross-sectional Microscopy
Other Types of Microscopy
Related Links and Resources
References

What are Cross-sections?

Cross-sections are microscopic paint samples that are carefully collected from an artwork. These samples (typically no larger than the head of a pin) can provide important information relating to paint stratigraphy, pigments, and the presence of unoriginal restoration layers. Ethical factors are always considered when choosing a sampling location and occasionally it may be decided that sampling is inappropriate for a given artwork. However, paint cross-sections have proved valuable information in many instances when questions remain regarding an artwork’s condition, authenticity, or provenance.

Methods of Preparing Cross-sections

Paint samples are generally collected along the edges of paintings or near extant losses or cracks in the paint/ground layers. A variety of tools and instruments can be used for sampling including micro-tweezers and scalpels used in surgical procedures. Often samples are collected with the aid of a microscope, a head loupe, or other types of magnifiers. The microscopic paint sample is then cast in a clear resin, typically polyester or resin-based, that hardens over a period of several hours. The resin is polished smooth using fine abrasives or with the aid of a microtome until the paint sample within the center of the resin block is reached. Ideally, all ground and paint layers will be exposed during this process, providing the scientist or conservator with a “cross-sectional” view of the artwork’s stratigraphy.

Cross-sectional Microscopy

Paint cross-sections are typically viewed through a microscope that is ideally equipped with an ultraviolet light source. Samples are examined using reflected visible light and ultraviolet light as both wavelength ranges can provide complementary information (see Ultraviolet Illumination section). For example, certain organic materials will auto-fluoresce when exposed to ultraviolet light (e.g. natural resins, organic red lakes). Additional information can also be obtained by subjecting the sample to other wavelength ranges or through the application of biological stains on the surface of the cross-section.

Other Types of Microscopy

Polarized Light Microscopy (PLM)

Occasionally questions relating to pigment characterization are addressed using Polarized Light Microscopy. Pigment samples are collected in a similar manner (using fine tools and magnification) as cross-sectional samples, although less material is needed to perform the analysis. These dispersed pigment samples are distributed across a microscope slide and cover-slipped, fixing the fine particles in place using a clear adhesive. The pigment sample can then be viewed under a microscope using transmitted light (as opposed to reflected light as is done with cross-sections), allowing light to pass through the pigment particles. Many factors are considered during PLM including the morphology, color, and shape of the particles and how certain pigments interact with light (e.g. visible light, polarized light). Such observations can assist with the characterization and identification of pigments present in a sample.

Scanning Electron Microscopy (SEM-EDX)

While microscopic examination is commonly performed on paint cross-sections and dispersed pigment samples, the use of a scanning electron microscope that is capable of energy dispersive X-ray analysis (referred to as SEM-EDX) can be of equal importance. A paint cross-section is placed inside a vacuum chamber and an electron beam is focused on the exposed surface of the sample. Two types of images can be generated using SEM-EDX. An SEM back-scattered electron image (BSE image) provides visual information relating to pigment morphology as well as the range of atomic weights present (e.g. light areas correspond to elements with higher atomic weights such as lead while darker areas indicate lower atomic weight elements). The second type of image, referred to as an EDX map, is generated using very similar technology described in the X-Ray Fluorescence section. X-rays excite electrons present on the surface of the paint sample, generating elemental information that can help scientists and conservators identify pigments (typically inorganic) present in discrete paint/ground layers.

References

Eastaugh, Nicholas, and Valetine Walsh. “Optical Microscopy.” In The Conservation of Easel Paintings, edited by Rebecca Rushfield and Joyce Hill Stoner, 306-318. Routledge: London and New York, 2012.

Meeks, Nigel, et al., eds. Historical Technology, Materials and Conservation: SEM and Microanalysis. London: Archetype Publications, 2012.

Taft, W. Stanley Jr., and James Mayer. The Science of Paintings. New York: Springer, 2001.

Townsend, Joyce, and Jaap Boon. “Research and Instrumental Analysis in the Materials of Easel Paintings.” In The Conservation of Easel Paintings, edited by Rebecca Rushfield and Joyce Hill Stoner, 344-45. Routledge: London and New York, 2012.

Wolbers, Richard C., Susan L. Buck, and Peggy Olley. “Cross-section Microscopy Analysis and Fluorescent Staining.” In The Conservation of Easel Paintings, edited by Rebecca Rushfield and Joyce Hill Stoner, 326-335. Routledge: London and New York, 2012.