Polarized Light Microscopy

Our Conservation Department recently purchased a Zeiss polarized light microscope-”the best microscope on campus” according to the specialist who set it up for us, and who is knowledgeable about the other scopes in use at Penn. Having the nicest equipment around isn’t familiar territory for conservation labs, so we’re enjoying having this status, but more importantly, having such a nice piece of equipment to use.

Our new microscope installed in the Artifact Lab

Polarized light microscopy (PLM) is used for examination of specimens in many types of laboratories, including biology and geology labs. In conservation, we use PLM for identification of minute fragments from objects-anything from pigment particles to wood fragments to textile fibers. We also use this technique to examine corrosion products, salts, and other materials found on artifacts-all of this work helps us better understand what the objects are made of, their condition, and ultimately provides important information for making conservation treatment decisions.

For example, our Conservation Fellow Tessa de Alarcon, who is conducting a year-long condition survey of Penn Museum artifacts from Kourion, Cyprus, has been using PLM to examine salts present in ceramic vessels from this collection. Tessa is desalinating the ceramics to remove the salts, which likely accumulated in the ceramics in the burial environment and will cause damage if not removed. To confirm which salts are present, she removed samples of the salts and examined them under the microscope. Here is an image of one of the salt samples, which shows that there are 2 different types of salts present-nitrates and sulfates.

Magnified image of 2 types of salts present on a ceramic vessel from Kourion (400X magnification).

You can read more about Tessa’s work with the Kourion collection (and view a cool video clip!) here on the Penn Museum blog.

In the Artifact Lab, one of the first ways that I’ve used our new microscope is to examine fibers from a thread that detached from the fabric wrappings of the falcon mummy I described in a previous blogpost. Fortunately for me (but unfortunately for the poor falcon mummy!) there are lots of detached threads that were available to sample for examination under the microscope. Here is a magnified image of one of these threads:

A small detached thread from the falcon mummy’s wrappings (40X magnification). I noted that the thread has an “S” twist and the fibers are shiny.

Using our binocular microscope, I put a drop of water on the thread and teased out several individual fibers from the thread on a glass slide, and then covered the fibers with a cover slip.

This image shows all of the tiny fibers from the larger thread-it is important to examine these fibers individually in order to identify what type of textile the falcon mummy is wrapped in (40X magnification).

Once the slide was prepared, I mounted it on the polarized light microscope and examined it at 50, 100 and 200X magnification.

Fiber from falcon mummy textile wrappings (200X magnification)

Under such powerful magnification, it is possible to see features such as a very small lumen (central cavity) and nodes along the length of the fiber. These features are characteristic of flax fibers, and comparing my sample with known references (including in this great Fiber Reference Image Library), it was immediately clear that this is what it is. Flax is used to make linen, and since the majority of ancient Egyptian textiles are linen, I already had a good idea that this is what was used to make the falcon mummy-but this proves it!

You can see from this work that PLM is a very useful technique, but it also is important to have an idea about what the possibilities are for what your sample-background research and close examination before microscopy is essential.