Notes from Aremco: (see aremco_purity.txt): Feb 17, 2017 1)      The ceramic contains 95+% Aluminum Oxide, 2.0-2.8% CaO, 1.5-2.0% SiO2, <0.1%Na2O & <0.1Fe2O3    2)      During production, the CaO reacted with the alumina that resulted in slightly brownish circles parallel to the wall, but this will not affect the usage of the part. >> Valley Cottage, NY   10989 >> Tel - 845-268-0039 ext. 307 >> Fax - 845-268-0041 >> www.aremco.com