These fired bars are the L4410P low temperature clay body (it replaces the traditional 50% talc with 40% dolomite and 10% nepheline). These bars are fired from cone 5 down to cone 06 (top to bottom). The body contains 4% red iron oxide, this would normally be enough to produce a bright red fired color. But clearly, the dolomite is killing its development. A better option is to use the L4170 plastic terra cotta (or its L4170B casting version).
Iron Oxide Red
Red iron oxide is the most common colorant used in ceramic bodies and glazes. As a powder, it is available in red, yellow, black and other colors.
An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes.