Hair is chemically stable especially when compared to other physiological materials such as blood, semen, or any other body fluid.  Because hair is strongly resistant from decomposition, its property makes hair a nearly ideal type of physical evidence.  As you can see in the figures on the both side, a strand of hair has three layers: cuticle, cortex, and medulla.  The outermost covering, cuticle consists of hard overlapping scales that point toward tips end.  The cortex contains pigment granules that give hair its color.  The medulla is a hollow tube within the hair, which may be present or absent.  Depending on the hair, this hollow tube is continuous, or fragmented.  The examiner compares above factors: color coarseness, granule distribution, hair diameter, and presence or absence of a medulla. 

    Forensic science investigators determine the medullary index of hair which is the diameter of the medulla relative to the diameter of the hair, expressed as a fraction.

Humans have a medullary index of less than 1/3, and the medullary index of animals is 1/2 or greater. 

For preparation of slide, dry mount and wet mount are implemented. 

The dry mount is convenient in terms of preparation. However, degree of curl and twist cannot be observed due to constraints in the mounting process.  It may be useful before the wet mounting to study the exterior texture and the overall color of the hair.  Usually, several hairs are placed in parallel on slide, so that their texture and color can be compared easily.  The dry mounting technique is nothing fancier than the words it describes.  Samples can be fixed with melted Kronig cement, but it is unnecessarily if the cover slip can be fixed firmly on the slide. 

The wet mount is essential in hair analysis because of the refractive index of Canada Balsam – a special resin used to prepare slides – is close to that of the keratin in the hair.  If the dry mount provides a view of texture and color, wet mount provides the interior structure of the hair, such as inclusion and pigment granules.  Because the refractive index of mounting medium plays the most significant role in viewing internal details, it is suggested that if other reagents than Canada Balsam is used, it is chosen carefully.

In the wet mounting technique, aligning hair in parallel to each other is not required, because it is very difficult to do that with the addition of mounting medium.  Loop and figure “8” shapes are more common to carry out this preparation.  However, there are significant drawbacks on this method.  Once long hair has been mounted, it is difficult to compare a particular feature along the shaft with its distance from the root.  In addition, image quality may deteriorate if long hair overlaps with each other.  This deterioration is induced by spherical aberration, but this will not be explained in detail.  Anyone who has an interest to learn more about spherical aberration, they can visit Olympus Microscopy Resource Center where spherical aberration is covered in detail. 

The parallel mounting technique is a method for affixing the samples to the slide before the cover slip is affixed.  Usually, the selection of adhesive is not important if it is accommodated with the dry mount.  However, mounting medium should not dissolve the adhesive in the wet mounting.  Gum tragacanth, gum Arabic, vinyl acetate glue, and cyanoacrylate (also known as “crazy glue”) are typical adhesive in use today.  Tacking points are just at the border of the end of the cover slip.

Now that the preparation of slides has been covered, what is next?  What are we suppose to look for in the prepared slides?  First, there should not be any confusion between hair and fabric fiber.  The easiest way to differentiate between hairs and fibers is by using a light microscope.  A PLM (polarized light microscope) can be useful but not necessary.  A PLM is more useful when you need to tell the difference among types of fibers: such as cotton, silk, rayon, polyester, nylon, and wool, etc.  Remember, the wool comes from a sheep and  it is also a type of animal hair.  It is simple to distinguish between natural and synthetic fibers, and other animal hairs under the microscope.  When you look at hair under the microscope, it is possible to see various structures that are different among similar types of hair, and among different classes of hair.  Fibers and hairs are very different when they are viewed under the microscope. 

Gross features such as color, texture, degree of curl or wave are examined with the unaided eye and low power microscope.  Specific characteristic of hair by race are described in the table below.