This is not a substitute. It is the original Barnard (a.k.a Blackbird) slip clay. Supplies are limited as it is no longer being mined. Glazes calling for Barnard are typically brown or black owing to the concentration of iron and manganese oxides.
For more information on Barnard Slip Clay, see the Digitalfire.com Reference Library of ceramic materials.
Different Raw Materials ordered in full bag qty (20 kg to 100 lb) will be combined at checkout for a volume discount if applicable.