Sand-bed
The sand-bed of a cupola is the sand rammed on the bottom, on which the molten iron rests after it has been melted above and fallen down through the fuel thereon. The old or black sand off the floor, that which has been slightly burned, is the best to use for this purpose; being free from clay, it does not bake hard, and by using it a little drier than ordinary moulding-sand it may be rammed well down to a solid bed without fear of danger from blowing or boiling--a too frequent occurrence when the sand used for this purpose is close and too damp. The thickness of the sand-bed may always be increased when it is desired to reduce the depth from the tuyeres down, but there should never be less than 2 1/2 inches over the bottom plate. A slight down grade towards the tapping-hole is necessary to run all the iron off, but avoid too much slope, as it increases the pressure at the tapping-hole, making it more difficult to insert the bott. See CUPOLA; SPOUT; BREAST-HOLE; BOTT.