The present invention relates to a device for hardening foundry cores of a sand-containing molding material, wherein the core, for its hardening, is subjected in a core molding tool to a catalyst vapor/carrier gas mixture and subsequently to a pressurized air stream, each at a predetermined pressure and a predetermined temperature, wherein a heating and mixing stage, which can be upstream of the core molding tool, is connected to a container containing an organic catalyst in liquid form and to a pressurized air source, wherein the device does not have a preheater, so that the liquid organic catalyst and the pressurized air are heated together in the heating and mixing stage, and the device has a separate flushing line, wherein a first cutoff valve is arranged in the line to the heating and mixing stage, which is closed at the beginning of the flushing, and a second cutoff valve is arranged in the flushing line, which is open at the beginning of the flushing.
