This invention provides a stepped heating cycle for the pre-treatment of phenolic microballoons prior to carbonization thereof, wherein the heating cycle comprises the steps of sequentially: gradually elevating the temperature of the microballoons to a temperature in the range 100.degree. C. 170.degree. C.; holding the microballoons at the elevated temperature for 1 24 hours; and gradually cooling the microballoons. This invention also provides a heat-dissipation reactor (11, 21, 31) which comprises a walled reaction chamber having a bottom and no top, the reaction chamber being fitted with high thermal conductivity inserts. When used in accordance with this invention (61), the volume within the walls of the reaction chamber is charged with phenolic resin microballoons. In a preferred embodiment, the reaction chamber (11, 21) is subdivided into a plurality of subchambers by a vertical grid of aluminum plates (19, 29). In this embodiment, about half or more of the wall area of each subchamber comprises aluminum and a top edge (17) of the aluminum wall material communicates with atmosphere above the reaction chamber.