A method of manufacturing a sheathed electrical heater assembly comprising the steps of: molding a core element into a desired shape, the core element having an outer surface; heating the core element at an elevated temperature sufficient to release organic material from the core element; placing a heating element in communication with the core element; and, encapsulating the core, and the heating element in an insulation protection layer, whereby the heater assembly is formed. Additionally shown is a sheathed electrical heater assembly comprising: a core made of an organo-ceramic material; a heating element in communication with the core; an insulation protection layer encapsulating the core and heating element to produce the heater assembly, the protection layer comprising and organo-ceramic material; and, a sheath encapsulating the heater assembly, the sheath also comprising an organo-ceramic material.