An incinerator hearth includes a bottom formed of a refractory slab. The slab has a plurality of upwardly-extending projections which define a serpentine channel. A conduit is placed within the channel. The conduit is made of steel or another material capable of withstanding high temperatures. The height of the conduit is slightly lower than the height of the projections. The upper surface of the conduit and the upper surfaces of the projections together define the combustion surface of the hearth. A heat transfer fluid is pumped through the conduit, and this fluid prevents the combustion surface of the hearth from becoming too hot. Thus, slag formed in the hearth tends not to adhere to the combustion surface, and can be easily removed without damaging that surface and without requiring that the incinerator be cooled down and manually cleaned. A control system regulates the temperature of the fluid by directing a variable proportion of the heat transfer fluid through a heat exchanger, in response to the sensed temperature of the liquid.