Abstract: A furnace bottom construction comprises refractory blocks resting atop a seal in turn resting on the top surface of a layer of ramming material in which are embedded cooling ducts resting on a concrete foundation. The seal is composed of thin, fluid-impervious, flexible, ductile sheet material deformed to follow the contours and indentations of the top surface of the ramming material. The bottom surface of the seal and the top surface of the ramming material are in intimate contact along substantially the entire area of said top surface, with substantially no air gaps between the two.