Abstract: Materials which become carbonized at high temperatures, such as wood, cellulose, household refuse, used tires, plastics, tar sand, oil shale, and the like, are subjected to thermal treatment, i.e., pyrolysis or gasification, in an electric shaft furnace having top and bottom electrodes. The material is charged into the furnace at the top and continuously moved downward through the furnace so slowly that as a result of heat transfer from the hotter parts of the furnace, it is carbonized to an extent which renders it electrically conductive by the time it comes in contact with those parts of the electrodes by means of which electric current is conducted through the material. The material charged at the top of the furnace can be caused to move downward by withdrawing suitable amounts of solid material from the bottom of the furnace, for example, or by gasifying the material with steam.