Making Fiber Cross Sections

In the last fiber lab, we’ve observed hairs and fibers longitudinally In the last fiber lab, we’ve observed hairs and fibers longitudinally. As useful as a longitudinal view is, it doesn’t reveal all of the visible information about a specimen. To complete our microscopic examination, we need to prepare and view a transverse section (also called a cross-section) of the specimen.

The surface of a specimen is seldom perfectly transparent, a transverse section may reveal details about the internal structure of a specimen that are obscured in a longitudinal view.

It may be obvious from a longitudinal view that a fiber has been dyed, but the “end-on” view of a transverse section allows us to determine whether the dye is spread uniformly throughout the fiber or concentrated near the surface.

It can be difficult to determine cross-sectional shape of fibers from a longitudinal view. With a transverse section, it’s immediately obvious if the cross-section is circular, oval, oblate, kidney-shaped, triangular, lobed, irregular, or otherwise.

Producing transverse sections in a primitive lab is difficult Producing transverse sections in a primitive lab is difficult. It’s easy enough, of course, to cut a hair or fiber. The hard part is getting a thin transverse section mounted and correctly oriented on a slide. Several means can be used to obtain transverse sections of hairs and fibers, including the following:

Cross-section test slide method Professional forensics labs use special metal microscope slides designed for sectioning hair and fiber specimens made of 0.25 mm thick stainless steel, punctured with numerous 0.9 mm holes. In use, a fiber bundle is drawn through a hole using a short length of thin, soft copper wire (about 34 AWG) to pull the looped fiber bundle through the hole.

The fiber bundle can be bulked out to fill the hole by using known fibers. Using a scalpel, craft knife, or single-edge razor blade, the fiber bundle is sliced off flush with the top and bottom surfaces of the plate, leaving 0.25 mm thick fiber sections that can be viewed by reflected or transmitted light.

“Drillzilla” Method

Cork Method Use a high-quality, fine-grain cork section about 2.5 cm thick. Press a thin needle through the cork, using pliers if necessary, until the eye of the needle is just visible above the surface of the cork. Thread fiber specimen through the eye of the needle, and draw the needle all the way through the cork, stopping before you pull the fiber specimen completely free of the cork.

with a scalpel (or other sharp cutting instrument) to cut the fiber free from the needle, flush with the surface of the cork. Then use the scalpel to cut a thin section of the cork with the fiber specimen embedded. Problem: cork crumbles when we tried to cut thin sections.

Home-Made Microtome Microtome using bolt and wide nut. 1. Thread bolt 2-3 turns into the nut, leaving the remaining part of the nut to form a well. 2. Using forceps, hold the fiber vertically in that well as you drip molten candle wax into the well, filling it slightly overfull. 3. Allow wax to solidify; then use a scalpel, craft knife, or single-edge razor blade to trim off the excess wax flush with the face of the nut. 4. Discard first section, & turn the nut slightly to drive a thin section above the face of the nut. 5. Use the scalpel to slice off that thin section and mount it on a slide. Used with care, this method can produce several sections of 0.25 mm or thinner.

Drinking Straw Method 1. Cut a short (~ 3 cm) length of plastic drinking straw. 2. Lubricate inside surface with a very small amount of oil, tiny bit of glycerin or similar lubricant. 3. Place straw section vertically against a safe work surface, using bottom edge of the straw to pin one end of the fiber against the work surface. 4. Using forceps, hold the other end of the fiber as nearly vertical as possible centered in the straw and have your assistant drip candle wax into the straw until it is nearly full, being careful not to burn the fiber or yourself. 5. Allow the wax to harden for a minute or so, and then use a glass rod to slide the wax plug until the end is just past flush with the end of the straw. Use the scalpel to trim off the first section and discard it. Press the plug a bit further out of the straw and carefully use the scalpel to cut a section as thin as possible. glass stirring rod straw wax

Dripping Wax Method One of the easiest ways to make a fiber cross section is to drip melted candle wax onto a microscope slide until you have a mound built up about 3mm thick. Carefully embed the fiber into the wax before it hardens completely. Then drip more wax onto the fiber to build up the mound another 3mm or so. Once the mound hardens completely use a scalpel or razor knife to cut thin vertical sections through the wax.

After you have practiced each technique After you have practiced each technique. Select 2 of your single, most interesting, best prepared specimens to make into a permanent slide

Making a permanent mount slide is like making a wet mount, but we will use 2-3 drops of permanent mounting solution instead of one drop of water. Otherwise, follow the directions for making a wet mount.

Permanent slide assignment You will make 2 permanent reference slides. A reference slide is something that we can compare unknowns to, so that we can identify a sample.  Your slide should look something like this:  

http://blog.makezine.com/laboratory-611-make-transverse-sect