The method avoids the disadvantages of classic methods. One examines under the microscope the paper ashes obtained by calcination at a temperature that does not change the physical state of the fibers (150°C at most) in the presence of activated oxygen in an electromagnetic field. Diagrams of the LTA 600 Tracerlab incinerator, counting of the fibers, calculalion of their volume and weight and evaluation of the glass density are discussed. -- AATA