Abstract: Foam glass developed under vacuum is continuously formed into a slab while being cladded on its major sides and longitudinal edges with sheet metal. The metal claddings or facings for the two major sides of the slab are held under tension during the solidifying of the slab body. End plates are inserted transversely through the product at specified points during its movement and sections of the continuous product are cut to length, whereby the sections are prestressed partly as a result of compression along two axes caused by differing coefficients of expansion of the cladding and foam glass body and partly to atmospheric compression of the product on a third axis perpendicular to its major sides. Modified forms of the product eliminate the metal cladding or skin entirely or provide the same as a cast-on coating without mechanical prestressing.