Abstract: A serial flow of a variable-boiling point volatile fluid is established through the wall of a casting mold and through a heater and condenser associated therewith by means of a pump. The condenser includes a lower portion in the path of flow of the volatile liquid, and an upper portion through which an auxiliary conduit extends, such conduit conveying a variable flow of coolant through the upper condenser portion. Changes in vapor pressure of the volatile liquid as the temperature in the mold changes are detected in the upper portion of the condenser by a suitable sensing device, and the output indication of the latter is employed to control a regulating valve for the auxiliary conduit to vary the flow of coolant in the auxiliary conduit. Such variations in flow, in turn, serve to adjust the vapor pressure, and thus the boiling point, of the volatile liquid and to thereby help compensate for temperature changes in the casting mold.