Abstract: Impregnation of a porous carbon body involving deposition of carbon from a as can be effected by establishing a thermal gradient across the body such that carbon is deposited at a boundary which advances through the body. The present furnace is for use in such a process and comprises a heater element with which, in use, the body is in contact and a cooled surface facing the element, characterized in that the latter surface is covered with a layer of thermal insulating material to reduce the thermal gradient across the body. This improves the economy of the process. The insulating material is suitably fibrous, eg graphite felt, and is protected by a layer of relatively low permeability, eg graphite sheet, to prevent deposition of carbon which would reduce its insulation properties.