This is a guest post written by Amanda Satorius, Preservation Science Specialist in the Preservation Research and Testing Division. Her work includes completing historical pigment and paper production research, as well as expanding and preserving the Cultural Heritage Analytical Reference Material (CHARM) collection. She is also part of PRTD’s “Go Team” of scientists that use non-invasive portable techniques to improve understanding of collection materials.
In the world of preservation, it is often the smallest parts of an object that provide scientists with the answers to the challenging questions we face. And as you may have already read on other blog posts material identification is key. Understanding what a collection item is made of allows for successful conservation, storage, and display. A tool that is part of just about every laboratory of scientific or material study, including Preservation Research and Testing Division (PRTD), is the microscope. They are frequently used with various light sources to examine the surface of an area of interest or area of damage, like close-up examination of things such as paper fibers, pigments, leather grain patterns, textile weave patterns or even the surface texture of ceramics to name a few.
However, it is not always possible to analyze an object on a standard microscope set-up. Sometimes it is just too big to fit underneath a microscope or too fragile to move or support on a microscope stage. In these cases it is imperative that we have portable options that can be hand-held or supported above an object.
Luckily, there are many small handheld portable microscope options on the market that are affordable and range from a low tech, almost webcam quality, to more high-tech options that are almost as powerful as a benchtop. Below you can see three different portable USB-microscope options PRTD uses depending on the object in question and some examples of images and videos.
The first microscope is a basic handheld model that has a 5 megapixel camera and uses white reflected light for images and videos. It has a magnification range from 20-200x, but can also be mounted onto an adjustable stage. This microscope is perfect for quickly transporting to and setting up at any location to get high quality microscopic images of any size or condition object, like 3-D objects, large format books and documents, and items on display. Great for getting more detail (Figure 2) on small scale regions of interest, like a painted illustration or a wax cylinder showing surface contamination, that can help guide further analysis.
The next slightly more advanced option is a handheld model that has a 3 megapixel camera with white, ultraviolet (UV), and infrared (IR) light options for image and video capture. It has a magnification range of 40-120x, and can be mounted onto an adjustable arm. It also has several accessories, including one that can be used for calibration of the magnification, a foot control pedal, and a filter wheel that can filter out specific wavelengths of light for more targeted microscopic analysis. This microscope is great for doing in-depth microscopic analysis with control over light source and magnification allowing for more chemical information to be gathered, which guides further testing. In Figure 3 below, analyzing a reference sample allows us to identify if a paper has optical brighteners by fluorescence in UV light, as well as chemical characteristics of specific pigments, like the blue that will disappear in IR light, but the green pigments remain visible and even turn black. This indicates that the green pigments are likely mineral based as they absorb IR light, but the blue pigment is not.
The final portable microscope available is not a handheld model, but rather a USB digital microscope with a large field of view (8” x 11”) that is mounted on an adjustable tetrapod (four-legged stand) and can be easily broken down and transported. It comes with a 3 megapixel camera and a high resolution macro lens. Accessories include LED white light sources, IR filter, a calibration wedge, and the ability to have raking light for texture analysis. This portable USB-microscope is most similar to benchtop microscope set-up, with higher magnification and standard light options and the ability for calibration. The large, adjustable working distance allows for analysis of smaller scale 3-D objects and documents, but also creates a large focal range for overview and close-up images. Below (Figure 4) you can see how great a magnification change you can get with this microscope with two images of the book plate stamp of the eagle. Further, this set-up is great for setting up raking light to provide surface texture details as seen in the text on the book paper substrate.
As you can see, even though not all microscopes are created equal, they all have their uses and can provide needed information to guide further research in order to answer those tough preservation questions!
Thank you so much for this short overview of using microscopes in preservation analysis. I’m a new subscriber and in awe of the care and attention to minute details go into your work.
With much appreciation for your invaluable efforts preserving history.