Cross-section Microscopy

Overview

Note: This section deals with cross-sectioning of layered surfaces. It does not deal with cross-sectioning of fibers.

Technique: Cross-section Microscopy (CSM)
Formal name: Cross-section Microscopy (CSM)
Summary description of this technique: Cross-section Microscopy (CSM) is a technique which allows for the examination of a sample from a layered structure, such as a painted surface, at the microscopic level. The sample is typically mounted in a clear resin, which can be cut, ground, and polished to reveal the cross-sectioned surface, which is examined with a high-powered microscope using reflected visible and ultraviolet light at magnifications ranging from 40x – 400x. The number and nature of layers such as paints, papers, varnishes, and other coatings can be observed in their order of application. Other important information can be gleaned from a cross-section, such as the original finish applied to an object, the working practice of an artist (ie: working ‘wet into wet’, or 'glazing', as with easel painting), method of manufacture, and/or the presence of overpaint or other more recent interventions. Comparison of paint layers from various locations on an object or within a building can assist with understanding an object’s/building’s physical history.

Details

What this technique measures: Cross-section microscopy is a visual investigation method. In some cases, it can be relatively straightforward. For instance, if a chair that is currently painted red contains twenty layers of multi-colored paints, all with significant layers of dirt in-between them, then the chair clearly has a long paint history and the current red color is not original. Some coatings/materials can be assessed visually, based on the analyst/conservator’s experience. For instance, certain materials such as natural resin varnishes, wallpaper starch-pastes, and animal glues exhibit a bright bluish-white autofluorescence in UV light. Some pigments also have distinct characteristics in UV light, for instance, verdigris-based paints will appear black, while zinc-based pigments, such as zinc white (ZnO), have a bright, ‘twinkling’ autofluorescence. If possible, these observations should be confirmed with an instrumental technique, as visual analysis can be subjective.
Limitations of this technique: Due to the variation in a layered material, a sample may not always be representative of the whole. If a sample is too small, it may not contain all of the information needed to answer the research question. Surfaces that have experienced wear, use, and repair can be missing early evidence.
Can this technique be made quantitative?: Cross-section microscopy is not a quantitative technique.

Samples

Phases it can be used to examine: Solids (papers, paints, varnishes, adhesives, wood, stone, limewash, etc…).
Is this technique non-destructive?: CSM is a destructive technique which requires a sample large enough to contain all layers in question.
How invasive is this technique?: CSM is invasive as a sample is required. If possible, care should be taken to sample from inconspicuous or already cracked, lifting or damaged areas to minimize loss to the object. Ideally, one would want to collect all layers, intact and attached to the substrate (wood, plaster, paper, metal). This is very difficult in practice, as many coatings are thin and brittle and can crumble or delaminate during collection. Often multiple fragments of a single sample must be cast and the stratigraphy reconstructed by the analyst, based on visual clues and observations made during sampling.
Minimum size of sample necessary to use this technique?: Sample size can vary depending on the object, but the goal is always to collect as little material as possible but enough to answer the research questions. Samples from small or precious objects can be < 1mm in diameter and collected with the aid of a stereomicroscope. Samples collected from large sculptures, murals, or architecture can be (but do not need to be) larger, approx. 2-3mm. Uncast sample portions can be used to run additional analysis such as PLM for pigment composition, FTIR for media analysis, and colorimetry if original color matches are requested.
Time to run one experiment?: Strongly dependent on the materials used. If the resin must cure overnight, at least 24 hours. Cutting, grinding and polishing should take less than 15 minutes. The examination (microscopy) and interpretation can take much longer.
Sample preparation methods?: Preparation methods and materials vary widely across institutions and practitioners. Samples are transferred to a casting tray (a common type is a silicone tray for making 1/2" ice-cubes), and mounted in a cube of synthetic resin, such as that used for mounting biological specimens. Once cured, the resin cures to a crystal-clear, rigid cube with the sample fixed inside. Next, the sample in its cube is ground (on a belt-grinder, 200 grit sandpaper) or cut down to expose its cross-section surface, and then polished with polishing cloths of increasing fineness (1500 – 12,000) until its surface is mirror-smooth and ready for microscopy.

Applications

How is this technique is used in the field?: CSM is used to better understand the layered structure of an object to determine: its finish history and original appearance or appearance at a particular point in time, its treatment/repair history, working techniques of the artist or creator, method of manufacture, presence or absence of coatings such as overpaints, varnishes, adhesives.
Risks associated with using this technique?: The mounting resins should be used with adequate ventilation/extraction and PPE should be worn when grinding/polishing resin cubes. Any dust created during and after polishing should be removed with a HEPA vac.

Budgetary Considerations

Approximate cost to purchase equipment for this technique?: CSM requires a high-powered research microscope (40-400x magnifications) with reflected visible and ultraviolet light capability (for instance, as with an epi-fluorescence microscope). A new system with all accessories can cost around $50,000, but used microscopes can be obtained at a fraction of the cost. A dedicated digital microscope camera is very helpful for documentation (and strongly recommended), although a digital SLR can be attached to most microscopes with a camera tube. A new digital microscope camera can cost around $10,000 and may require additional software. A range of digital microscope cameras can be obtained for much less (as well as iphone to eyepiece adapters), but you get what you pay for.

(It should be noted that stereomicroscopes do not have the adequate magnification or illumination for high-performance CSM).

Cost of maintenance?: Annual microscope maintenance by a professional can cost around $150, but is not always necessary if your microscope is working well. Beyond the initial investment in equipment, consumables such as bulbs, resin, microscope slides, coverslips, are around $200/year, with heavy use.
Sample analysis costs?: Microscopists in private practice may charge as much as $175 per sample (including report writing) for cross-section microscopy.
Time it may take to get results from a contract laboratory?: Depends on the laboratory/analyst and their backlog. Anywhere from within a few days to months. Also strongly dependent on number of samples submitted and the research questions.

Additional Information

Complementary Techniques: SEM-EDS of a mounted cross-section can be very helpful, as particular layers can be targeted for elemental analysis, which can help identify pigments present and “date” certain paint layers or better characterize their composition. SEM-EDS elemental mapping is a very helpful visual technique in which the elemental distribution within a sample is presented in a color-coded elemental map. In some instances, a mounted cross-section can be analyzed with FTIR-ATR, as well as by Raman spectroscopy.

References

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