Examination & Documentation

Scanning electron microscopy (SEM) is a technique used to examine the surfaces of objects. It works by sending a narrow beam of electrons toward a specimen. When these electrons hit the surface, they interact with it in two main ways. Some electrons bounce back toward the beam, which are called backscatter electrons, while others get knocked out of the sample, known as secondary electrons. 
To create an image, the electron beam is moved across the surface of the sample in a pattern. Detectors measure the number of backscatter and secondary electrons that are produced, allowing scientists to build a detailed picture of the surface. However, this process must be done in a vacuum because air could scatter the electrons, ruining the results. This means that the air in the chamber has to be almost completely removed before good images can be taken.
One challenge with using SEM is that it requires the specimen to be dry and conductive. Wet materials can lose moisture inside the vacuum, resulting in distortions such as warping or cracking. Additionally, non-conductive materials can accumulate electron charges, causing a loss of signal. To solve this, many organic specimens are coated with a thin layer of carbon or gold.
Fortunately, advancements in SEM technology have produced "environmental" chambers that don’t require such a high vacuum. These chambers allow for some air inside, which helps preserve moisture in certain samples and reduces the issues with electron accumulation. For instance, at the Field Museum, there’s a 50 cm diameter environmental chamber that can examine objects up to 25 cm wide without needing to coat them.
Another interesting aspect of SEM is that the interaction between the electron beam and the sample also creates soft X-rays. These X-rays can be analyzed using a technique called Energy Dispersive X-ray Spectroscopy (EDS or EDX). The Field Museum's SEM features an EDS detector, which enables scientists to identify and quantify the chemical elements present on the sample's surface.

X-ray fluorescence (XRF) is a type of analysis that provides information about the elements in a sample. When a sample is exposed to an X-ray beam, the atoms in the sample respond by emitting X-rays of different energies. Because each element gives off a unique pattern of emitted X-rays, scientists can use this information to figure out which elements are in the sample.

In the past, most XRF machines were large, sealed units that required scientists to take samples away from museum objects for testing. However, now there are portable XRF instruments that can be brought directly to the object, allowing for surface analysis without the need to remove any material. Although this makes it easier to test objects, the measurements may not be as precise.

Fourier transform infrared spectroscopy, or FTIR for short, is a tool that conservators use to determine the molecular structures present in various materials. By understanding these structures, they can often identify the materials used to create an object. FTIR is particularly useful for analyzing and identifying organic compounds, like resins, starches, and proteins, which are commonly found in ethnographic objects.
At the Field Museum, conservators have access to a DATR-FTIR unit. To conduct an analysis, a small sample, approximately 0.5 mm in size, is pressed onto a diamond stage. Then, infrared light shines through the diamond at an angle. Some of this light is absorbed by the sample, while the rest is reflected back to the detector. By studying the resulting absorption spectrum, conservators can compare it to reference spectra to identify the material or examine individual peaks to determine the specific molecules responsible for the pattern. The Field Museum's Economic Botany collection is a valuable resource, providing a diverse range of sample materials to enhance its reference library.
One example of an object they have analyzed is an ostrich shell cup, which dates back to around 2500 BCE from the site of Kish in Iraq. This cup is made from an ostrich shell, with the top cut off and a pottery flange glued on that is coated with a brown-black resin. Small triangular pieces of nacre are added to the resin for decoration.
Researchers wondered whether the brown-black resin was a type of bitumen, a natural oil product, or a plant-derived resin, such as pine pitch. Although both materials have similar elemental compositions, FTIR can distinguish between them. The FTIR spectrum for natural asphaltum is quite different from that of pine pitch. By removing a tiny fragment of the resin from the cup's rim and analyzing it with FTIR, the resulting spectrum showed a close resemblance to that of asphaltum, indicating that it was likely the material used.