These bars were fired to cone 5. Until we looked at the stained version of the body the umber version did look pretty dark in color. The unstained stoneware already has about 2% iron but that does not seem to assist the umber in developing color nearly as much as it does the stain. Even at only 4%, the stained version is still darker. The umber generates some gases of decomposition at cone 6 (affecting glazes), limiting its use there, with the stain that is not an issue.