Abstract: Herbaceous plant material is processed by heating it to within a specific temperature range for a predetermined period of time, within a novel processing apparatus. The plant material is held in an atmosphere whose oxygen content is controlled to convert a chemical found in the material into a more useful form without significant degradation taking place during the conversion process. In the instance wherein the herbaceous material is the plant substance cannabis, the material is heated in an inert atmosphere in accordance with the inventive method to carry out a step of decarboxylation which changes cannabinolic acid present in cannabis into the psychoactive drug or chemical delta-9 Tetrahydrocannabinol. Control of decarboxylation in accordance with the invention prevents destructive pyrolysis and degradative oxidation of the delta-9 tetrahydrocannabinol.