SOME IMPROVEMENTS IN THE STUDY OF CROSS SECTIONS
JIA-SUN TSANG, & ROLAND H. CUNNINGHAM
3 METHODS THAT CAN BE APPLIED TO THE STUDY OF THICK AND MICROTOMED CROSS SECTIONS
SOME SIMPLE, rather accessible microscopic examination methods (3.1, 3.2, 3.3, 3.4) can be very valuable in identifying and understanding the material; suitable microscopes are available to most conservation laboratories. In many of these tests, however, the cross section is destroyed in the process of analysis. With other methods some portion of the prepared section is used up in the process of testing (GLC, HPLC, XRD and SEM). With FTIR the section is not used up.
3.1 EXAMINATION WITH A STEREOMICROSCOPE
This method permits visual study and photographic recording of the cross section and its layered structure together with observation of its color, texture, thickness, and the pigment particle size. It can provide preliminary identification of the pigments. The solubility studies can be conducted by applying various solvents to the cross section. Characterization (not identification) of the media and overpaint can facilitate decision making for treatment purposes. Staining tests can be conducted for the study of binding media (Johnson and Packard 1971).
3.2 HOT-STAGE MICROSCOPY
Micro-melting-point studies can be used to divide the medium into subgroups such as resin, oils, and protein. Thin sections allow faster heat transfer and make the studies more effective. Traditionally, melting points are carried out using much thicker castings.
3.3 FLUORESCENCE MICROSCOPY
This method can be used to identify pigments and conduct staining tests of binding media (Wolbers 1988).
3.4 POLARIZED LIGHT MICROSCOPY
With this method, pigments can be retrieved from the cross section for examination under microscope.
3.5 GAS-LIQUID CHROMATOGRAPHY (GLC) AND GAS CHROMATOGRAPHY–MASS SPECTROGRAPHY (GC-MS)
Fragments can be saved from the original sample or samples can be retrieved under a stereomicroscope with needles from the thin section for more accurate identification of binding media.
3.6 HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Fragments can be saved from the original sample or samples from the thin section can be retrieved for more accurate identification of the amino acids in a protein-binding medium.
3.7 X-RAY DIFFRACTION (XRD)
This method provides accurate identification of inorganic pigments. Samples can be retrieved from the cross section. The sample can be reused for another type of analysis if separated from the mounting media.
3.8 SCANNING ELECTRON MICROSCOPY (SEM), WITH ENERGY-DISPERSIVE X-RAY SPECTROMETRY (EDS)
The morphology of each layer from the cross section can be studied at high magnification using the SEM, and elements composing each pigment can be identified in each layer of paint using EDS. The SEM sample can sometimes be reused for another type of analysis if it is large enough for repolishing, which is necessary to remove the carbon or gold coating.
3.9 FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR)
Analysis of an individual layer in the microtomed cross section can be conducted using transmitted light. This analysis provides a determination of the general chemical class of the layer without destroying the cross section and without involving complicated derivatization procedures. FTIR can be used to determine which further analytical techniques would be most appropriate.