Molding sands used by the metal casting industry are pro-
duced by mixing virgin silica sand with clay or organic chem-
ical binders. The most widely used sand binders are bentonite
clays (sodium bentonite and calcium bentonite), and these
molding sands are known as green sands. Carbonaceous addi-
tives, such as crushed bituminous coal, are added to green
sands to produce a reducing atmosphere during casting, while
cellulosic additives (e.g. wood flour, cob flour, pecan shells,
oat hulls, and rice hulls) are used to absorb moisture and pre-
vent expansion defects. In addition to the clay-bonded molding
sands, several types of chemical binders or resins are used to
make both molds and cores. Cores are used with both clay-
and chemically bonded molding sands when a casting requires