Abstract: A structure defining an upwardly opening tray is provided and initially filled with a hardenable mixture compactable to at least a reasonable degree and a grid is thereafter introduced into the tray and depressed downwardly into the hardenable mixture to a level spaced above the bottom of the tray and with upper portions of the grid projecting above the mixture. A second hardenable mixture is thereafter introduced into the tray within the openings formed by the grid and on top of the first mixture to a level substantially flush with the upper surfaces of the grid. A male die including coplanar spaced die members snugly receivable within the voids or spaces defined by the grid is positioned with the spaced die members in registry with the openings or spaces defined by the grid and forced into the spaces or openings so as to compress both mixtures within the tray.