An apparatus for the heat treatment of a product includes a metallic heat transfer assembly for transmitting heat either directly to the product or to a mould containing the product. The heat transfer assembly comprises a plurality of separate parts which are joined together either permanently or in an operationally separable manner. Each part includes a gas distribution passageway, the passageways together defining a closed loop path for receiving and circulating around the heat transfer assembly hot permanent gas from a hot gas source. One or more heat transfer passageways formed in the heat transfer assembly itself, or between the heat transfer assembly and the mould, communicate with the closed loop path in the heat transfer assembly so as to receive hot gas therefrom and lead this gas to an exhaust outlet to the atmosphere.