Abstract: Particulate material, hereinafter called sand, is used in a sand article, such as a mold article, for foundry purposes such as a green sand mold and sand core consisting of sand and a binder and is hardened in a molding box by sucking a catalyst gas from a catalyst gas source through the molding box and the sand article. The molding box is continuously exposed to the action of a reduced pressure source and normally evacuated to the atmosphere. When the pressure has been reduced to a predetermined pressure less than atmospheric pressure, the catalyst gas is briefly supplied to the molding box under slight increase of the absolute pressure, whereupon the molding box is evacuated to the catalyst gas source until the pressure therein is reduced to the predetermined pressure to recycle the unused catalyst gas leaving the molding box to the catalyst gas source.