There is an obvious need to develop a burn-out procedure for dental investments which is faster and less expensive but still produces successful results. Microwave heating of dental investments could be such a procedure. In a solid, the heat buildup caused by microwave irradiation is due to absorption throughout the interior rather than to heat conduction from the surface as in conventional burn-out ovens. Thus the temperature rise should occur much more rapidly and uniformly, expecially for poor thermal conductors such as dental investments. Preliminary experiments have shown that the temperature rise in dental investments is indeed rapid but that temperatures high enough for dental casting are not always obtained. Using a conventional microwave oven, temperatures above 500 degrees C could be obtained only by positioning the investment at the aperture where the microwaves entered; but such high temperature could be obtained only in gypsum bonded investments (and not in phosphate bonded ones), were not always reliably obtained, and were sometimes associated with uncontrolled, uneven heating. Such preliminary experiments illustrate that microwaves can be used to burn-out dental investments but that practical utilization of the technique will require better control of the process. The rate and uniformity of heating is a function of the composition of the investment (both bonding material and special additives), of the total power delivered to the oven, and of the distribution of that power near the investment. All of these are controllable. The primary purposes of this study are to determine (1) the effects of total incident power and of power distribution, (2) the effects of various additives (such as magnetite particles or carbon fibers) on the temperature rise of dental investments irradiated by microwaves of frequency 2450 MHz, and (3) the effects these microwaves may have on the physical properties of the resultant investment. This study should provide the information necessary to design more adequate ovens and suitably modified investment materials to permit the rapid and efficient burnout of dental invetments by microwaves.
