An alternative method of detecting dispersion staining colors for the determination of asbestos fibers in bulk materials.

Asbestiform mineral particles can be detected and identified by means of a polarized light microscope, making use of the dispersion staining technique. In order to obtain dispersion staining (central stop), che rays which pass undeviated through liquid and specimen must be stopped. A microscope objective can be fitted with a device which can insert a central stop on the objective back focal plane to prevent undeviated rays from reaching the objective. Alternatively, the sample can be observed in a conventional dark field set-up, that is, it can be illuminated by a hollow cone of light of greater aperture than the microscope objective so that only rays which are strongly deviated can enter the objective. The first method, proposed by McCrone and widely used in the USA, allows very bright and defined colors to be obtained but at the same time entails low resolution. The second method, recommended by italian regulations as a method that can be used in asbestos characterization, produces less defined and fainter colors. By suitably modifying the angular aperture of the illuminating cone and regulating the distance between condenser and specimen, good results were achieved: on the one hand a good image resolution were retained, on the other, the colors observed were bright and well defined. The detection of these colors can represent a fundamental step in the characterization of the specimen under analysis.