production of microwave energy provides a yield of about 1 2 is of the order of 0 2 kwh kg for a product having a water content of 2 which means that the energy consumption is about 5 times less than that required when heating is provided by hot air alone the invention relates to an apparatus for the heat treatment of a product based on mineral fibers containing a binder applied in the form of an aqueous solution and capable of being hardened by a polymerisation and or polycondensation reaction comprising a metal container impervious to microwaves and equipped with a conveyor device at least one source of ultra high frequency radiation directed to the said product and a source of hot gas connected to means for directing the said gas to the surface of the product the power provided by the said gas representing 1 4 to 1 2 of the power from the microwaves if the product to be treated is in the form of separate objects the conveyor device for the said objects preferably comprises means for rotating the objects about themselves on an axis set at an angle other than zero to the direction of the microwaves preferably an angle of 90 u00b0 according to one preferred embodiment of the treatment of objects in the form of a cylinder of revolution the conveyor device for the objects and the means for rotation together constitute a supporting table which is optionally displaceable and is permeable to microwaves and pushing devices such as fingers or rods which act to roll the cylindrical objects on the support according to.