Abstract: A perlite expander control system is disclosed in which a source of and detector for penetrating radiation are used to determine the density of an expanded perlite product flowing through a conduit. The density of the perlite product will be inversely proportional to the amount of penetrating radiation transmitted to the detector. A signal generated by the detector in response to the amount of transmitted radiation received is processed in appropriate circuitry to control the feed of raw perlite ore to the expander, thus resulting in the correct amount of ore being fed to the expander per unit time to result in obtaining the desired degree of perlite expansion. Typically beta, gamma or X radiation of various energies may be used as the penetrating radiation.