Guide to the Identification of Black Particles Under the Microscope

Guide to the Identification of Black Particles

This is part of a guide to the identification of particles seen in samples of free particles. It is designed to allow access to files using any significant feature noticed by the microscopist rather than requiring a specific set of measured parameters. Often the most significant feature of a particle is unique to that particle type and so is not a useful diagnostic element for a broader Key for particles in general. This guide proposes an approach to the identification of black particles using the light microscope. This is a more "how to" guide rather than a key to black particles. A key is provided in another file (Click here for the Black Particle Key.).

Not all Opaque Particles are Black

Check to see if the particle is black by going to full crossed linear Polarized light. Paint spheres will show a dull grayish color when viewed this way. Hemetite spheres will appear dark red. Conductive spheres will show a bright halo. These include magnetite spheres and metal arc spheres. Magnetite is black but the metal arc spheres will appear the color of the original methal

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Shape and Size in Important

Cenospheres from liquid fueled burners and diesel engines are spheres. Shoe wear, tire wear, etc. are all rough ellipsoids or bent rough ellipsoids, provided that their minimum diameter exceeds 10 micrometers. Charred wood and plant material tends to have sharp, smooth edges and will often exhibit cell structure visible with reflected light. Toners may be spherical or irregular in shape but they cluster around the same size in a given sample. Soots vary in shape depending on the amount of hydrocarbons present. Carbon fibers are straight fibers with diameters ranging from about 4 to 10 micrometers. Charred clothing and paper fiber tends to be much less uniform in diameter and is generally curved.

Reflectivity is Important

The reflectivity of the particle is a function of the relative refractive index of the particle and the mounting medium and the texture of the surface. If the surface texture is rough on the order of a micrometer or less then reflection is attenuated. If the texture allows for larger flat areas in appropriate orientations then the texture controls the distribution of the image of the illuminator and the Intensity of the image of the illuminator is controlled by the refractive index.

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