The only way to know for sure is to have a laboratory test for leeching. Make a cup, glaze and fire it, then send it to a laboratory that can test for cobal leeching. Two frequently cited laboratories in the U.S. are: Alfred Analytical Laboratory, 4964 kenyon Road, Alfred Station, NY, 14803 and Brandywine Science Center, Inc., 204 Line Road, Kennett Square, PA, 19348. Call them and ask about how to submit, costs, etc.
Alfred Analytical is closed.