be sufficiently inert so that the surface of the substrate that it contacts is not degraded nor otherwise adversely affected i have discovered that a suitable fluid is polyphenyl ether referring now to fig4 there is depicted by dashed lines a substrate temperature profile 46 consisting of a heat phase 48 where the substrate temperature increases at a rapid rate a hold phase 50 where the substrate temperature is constant for a short time and finally a cooling phase 52 where the substrate temperature is decreased rapidly in order to achieve this substrate temperature profile the fluid temperature profile 54 is maintained in container 12 by a mechanism to be described since there is a time lag between the substrate temperature and the fluid temperature the profile 54 depicting fluid temperature therefore is several degrees higher than the substrate temperature as depicted by profile 46 heating or cooling the substrate with liquid is much faster and more efficient than cooling with air or gas as a heating medium or radiant heat fig5 provides a vivid comparison between a substrate profile 46 using a liquid as a heating and cooling medium and profile 56 using heated air or inert gas in a belt furnace for similar ceramic substrates experimental measurements on 90 mm square substrates have shown heat transfer efficiency in oil heating to be 40 times.