Background:
Asbestos and artificial mineral (ceramic, glass and rock) fibers are used as thermal insulating materials; both are agents harmful to human health. The current exposure limit values for respirable ceramic fibers are twice sharper than for glass- and rock fibers. Fibrous materials used for a long time at elevated temperatures wear out and require replacement. The aim of the study was to establish physical characteristics enabling identification of the types of thermally destroyed man-made mineral fibers. No such data are available in the literature.

Material and Methods:
Bulk samples of thermal insulating fibrous material were investigated in this study. One sample was unknown. Microscopic determination of artificial mineral fibers was performed by assessment of a number of physical properties, such as crystallinity and refractive index. Crystallinity of fibrous materials was estimated using an X-ray diffractometer. Morphological and optical observations were achieved with a phase contrast and a polarized light microscope. The refractive index of fibers was assessed by observing relief and Becke line. It was evaluated by using immersion liquids controlled with an Abbe refractometer.

Results:
Compared to new man-made mineral fibers, those which had been used at high temperature for long time performance changed color and gloss, and their refractive indices were increased. So was their fragility.

Conclusions:
Optical properties alone may not be sufficient to distinguish between non-crystalline glass or ceramic man-made fibers. Identification requires significant operator training and experience