Polarized Light Microscopy

Overview

Technique: Polarized Light Microscopy (PLM)
Formal name: Polarized Light Microscopy (PLM)
Summary description of this technique: PLM is a technique that uses transmitted plane-polarized light (PPL) and cross-polarized light (XPL) conditions to examine very small samples, usually of pigments, particles, and fibers, mounted on a glass microscope slide and embedded in media of choice (such as Cargille Meltmount with known refractive index, or water). Physical properties (such as size, shape, color) and optical properties (such as relative refractive index, pleochroism, anisotropy, extinction) can be observed which helps to identify the materials present in the sample as compared to reference standards, published literature, or databases. The accuracy of PLM is strongly dependent on the skill and experience of the user. When used appropriately, PLM can detect materials (particularly in small amounts) that may go undetected by other, more advanced techniques (SEM, FTIR, XRF).

Manganese blue pigment, XPL, 400x

Linen reference, PPL, 200x

Details

What this technique measures: all pigments and fibers can be analyzed with PLM. These materials have specific characteristics under the microscope that can be recorded and/or measured to aid in identification. Even if a specific identification cannot be made, PLM is incredibly helpful in eliminating other possibilities that may be under consideration. For pigments, examination in both plane and cross-polarized light is crucial. Properties such as size, shape, color, fracture/crystallinity, refractive index (relative to the mounting medium), and pleochroism are observed in PPL. Properties including anisotropy (also referred to as birefringence), interference colors, type of extinction, and angle of extinction are observed in XPL. For fibers (animal, plant, synthetic), properties such as diameter, length, surface morphology, presence or absence of surface scales and scale pattern/size/shape are some of the many characteristics that can be observed in PPL. Properties such as birefringence, extinction, and sign of elongation are observed in XPL.
Limitations of this technique: The accuracy of PLM is strongly dependent on the skill and experience of the user. The technique takes much practice and study, as some characteristics can be much more subtle than those in reference libraries. Reference slides, although indispensable to PLM, are “pure” examples which is rarely the case in practice where contamination from dirt and aged binding media/yellowed varnishes are often present (collecting samples of, and using PLM to examine, soiling materials can be very helpful). Depending on the mineral source, method of manufacture, and age/deterioration, pigments can sometimes appear different from any references.
Can/how can this technique be made quantitative?: PLM is primarily a qualitative technique. Some analysts do use it semi-quantitatively, assessing the approximate percentage of a pigment in the field of view.

Samples

Is this technique non-destructive?: No. PLM is a destructive technique, but it should be noted that the amount of material necessary is almost invisible to the naked eye, and would fit on the head of a pin.
How invasive is this technique?: PLM is a minimally invasive technique and sample locations are typically invisible.
Minimum size of sample necessary for this technique: Again, the sample can fit on the head of a pin and may very well be invisible to the naked eye when mounted on the slide. Bigger is not better. If you have too much sample, mounting can be an issue. PLM samples must be small and well dispersed, because if there are too many “clumps” of particles and fibers the microscopist will be unable to observe the necessary properties.
Time to run one sample: PLM samples can be collected, mounted, and ready for microcopy in minutes.
Sample preparation methods: Samples for PLM are very small and should always be collected under magnification. For pigment identification, samples can be collected on the head of needle (often Tungsten) or scalpel blade wherever the target paint layer is exposed. If a painting is varnished, the varnish layer much be scraped through to reach the paint layer (varnish can be resaturated afterwards). The pigment particles are then dispersed on a clean glass microscope slide and mounted and coverslipped in a media with known refractive index (Cargille MeltMount RI 1.662, a thermoplastic media, is most typical in Conservation). For paper fiber microscopy, sampling is slightly more invasive, as the sample area (ideally an edge) must be dampened with a size 0 brush and the fibers gently pulled from the paper with microtweezers. For textile samples, samples are collected dry, and (most) fibers cannot be pulled (if in a weave), but must be cut with a scalpel or other blade, for removal. Samples should always be taken from inconspicuous locations to minimize damage. Typically paper and textile samples, once collected, are transferred to a clean glass slide and dispersed in a drop of deionized water. This water can be used as a temporary mounting media for microscopy. Staining with reactive agents (such as C-stain) may occur at this stage, after which the sample must be disposed of. For permanent mounting, the water is allowed to evaporate and the sample mounted in a permanent media such as Cargille MeltMount.

Applications

How is this technique used in the field?: To identify pigments, particles, and fibers in art and artifacts. Results can help date an artifact, aid with provenance, help understand original appearance, use, and current condition.
Risks associated with this technique: Cargille MeltMount must be heated for use. When in its molten state vapors should not be inhaled. Use fume hoods or extraction trunks.

Budgetary Considerations

Approximate cost to purchase equipment for this technique? : A new PLM may run as high as $50K. Used PLMs are widely available at much less expense ($5K or less). If purchasing a used system, one may wish to work with a local microscope-service (reach out to local hospitals and universities to ask who they use) to make sure the system purchased is appropriate for your needs, and which accessories are required.
Annual costs to maintain or run?: Beyond equipment expense, there is almost no annual cost for maintenance of a PLM. Consumables (slides, cover slips, Meltmount) are the primary expenses but cost is still very manageable compared to other techniques.
Sample analysis costs?: Labs vary but costs can range from $75-$150 per sample.

Nikon NiU biological microscope used for PLM at the Colonial Williamsburg Foundation, Materials Analysis Laboratory

Case Studies

Tooth powder: An 18th c. paper box of pink tooth cleaning powder in the collection of the Colonial Williamsburg Foundation (1950-593) purportedly contained ground coral, according to the original label on the container. However, instrumental analysis by another institution in 1989 using NMR, XRF, and XRD determined it contained not coral, but gypsum, and the reddish colorant could not be identified. Recent PLM analysis by CWF confirmed that the powder contained gypsum, and determined that the red pigment was an organic red lake precipitated onto particles of wheat starch. Excerpt of analysis report for tooth powder (1950-593), CWF.

Additional Information

Complementary Techniques: PLM is an excellent complement to a number of analytical techniques including FT-IR, XRF, SEM-EDS, and Raman spectroscopy.
Variations of this technique: Various filters and accessories (Chelsea filter, red wave plate) can be used to enhance the capabilities of the basic PLM.

References

Eastaugh, N., V. Walsh, T. Chaplin, R. Siddal (2004). The Pigment compendium: a dictionary and optical microscopy of historical pigments. Oxford: Elsevier Butterworth-Heinemann.
Feller, R. and M. Bayard (1986). “Terminology and Procedures Used in the Systematic Examination of Pigment Particles with the Polarizing Microscope.” In Artists’ Pigments: a handbook of their history and characteristics, volume 1. Feller, R., ed. National Gallery of Art, Washington D. C., 285-298.
Mayer, D. (2003). Polarizing Light Microscopy. In Analytical Techniques in Conservation, Getty Conservation Institute course workbook. Williamstown, MA.
McCrone, W. (1982). The Microscopical Identification of Artist’s Pigments. Journal of the International Institute of Conservation-Canadian Group. 7 (1&2), 11-34.
McCrone, W. (1979). Application of particle study in art and archaeology conservation and authentication. In The Particle Atlas, Edition Two, Vol. V: Light Microscopy Atlas and Techniques. Ann Arbor, 1402-1413.
McCrone, W., L. McCrone and J. G. Delly (1978). Polarized Light Microscopy. Ann Arbor.
McCrone Atlas of Microscopic Particles.
Olympus Microscopy Resource Center
ZEISS Microscopy Online Campus Interactive Tutorials
NIKON Microscopy U - Fundamental Concepts in Polarized Light Microscopy
Molecular Expressions Optical Microscopy Primer - Specialized Techniques - Polarized Light Microscopy
Hooke College: Permanent Slide Preparation

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