The TAPPI Standard Paper Materials Collection of the Library of Congress: A Photomicrographic Collection
Part I: Introduction & Materials and Methods
by Anne Downey, Presv Consv and Frank Hengemihle, Presv R&TD
INTRODUCTION
The Conservation Section of the Library of Congress, Preservation Division houses a collection of common book and paper materials. These materials have been used for microscopic work by Conservation for some time. The fiber samples were mounted on glass slides. As an outgrowth of this work a photography study followed.
Objectives: The photo micrographic collection was initiated for three reasons. First, a collection such as this can be useful to compare unknown fiber samples against a standardized reference, and thereby aid in the identification process. Second, this collection represents a visual atlas of processed papermaking fiber samples. Although there are photomicrographic references currently available to show standard fibers used in papermaking, there are none that address the processed fiber. Finally it is very useful to have both SEM and optical micrographs of each fiber sample. The two together present a more complete concept of the fiber in question: the optically generated image illuminates the internal morphology of the fiber, while the SEM image presents the external topography.
MATERIALS AND METHODS
Scanning Electron Microscopy: Each of the 70 Tappi paper samples were mounted individually on aluminum pin mounts. A thin slice of the sample was attached to the mount using double sided carbon tape. The mounted samples were sputter-coated with gold. The coating was done with a Polaron Autocoater E5200 at 2 milliamps for 100 seconds to yield a 300nm coating. The samples were viewed with a Cambridge S-200. The Accelerating Voltage was 10EKV, the coarse focus set to 10 and the Resolution between 2 and 3. The working distance was 15mm. Photographs and negatives were produced at magnifications of approximately 200X and 500X. A videotape of each sample was produced. The video has 1 minute each at 250X, 500X and 1000X and is available from the Preservation Office.
SEM Photography: A site was first selected to photograph. An optimal site included all cell types of the fiber and was also one which had a narrow depth of field for maximal focus. Problems with a build up of electron charge randomly across the sample restricted choice of the site to be photographed. After site selection, the image on the monitor was fine-tuned by adjusting the signal level and the contrast. The image visible on the monitor rarely replicated the image produced on the photograph: there were deviations in contrast and brightness that were difficult to predict and accommodate for . The resulting photographs therefore vary in these aspects.
To produce the photographs, 4x5 Polaroid 55 (positive/negative) Film was loaded in a Polaroid 545 Land Film holder which interfaced with the microscope. The image was scanned onto the negative while the microscope was in Vis 1 scanning mode. The positive and negative were then processed following the manufacturers' recommendations.
Optical Microscopy: Permanent microscope slides of the 70 Tappi fiber samples were made. The fibers were mounted using Cargille meltmount (n = 1.662), with an attempt to well disperse the fibers. To view and photograph the mounted fibers, an Olympus Vanox Polarizing light microscope was used. Fibers were viewed and photographed with both the 10x and 20x objectives. While the 10x objective was in place, the aperture diaphragm was set to 0.2 For the 20x objective, the aperture diaphragm setting was 0.4. For all viewing and photography, the voltage output was 5.
Optical Photomicrography: 35 mm color reversal slide film (Ektachrome 64 T Tungsten film) was first used to photograph the fiber samples. Three shots of each fiber were taken: one with the microscope analyzer in and set at 45 degrees crossed polars, one with 75 degrees crossed polars. A separate microscope slide was prepared of each fiber by staining the fiber with Graff's "C" stain. The color produced by the stain can be useful to confirm the identity of a fiber and also can indicate the processing used. The stain is also useful as it emphasizes the morphological characteristics of the fiber, thereby aiding in a visual identification.