The body is Plainsman H450. Both have a black engobe (L3954N) applied to the insides and half way down the outside during leather hard stage (the insides are glazed with Ravenscrag GR10-C talc matte). The outer glaze on the left has 1% iron added to the base matte recipe. The one on the right has no iron. Notice how differnent the glazes are over the black engobe.
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.