These are Fire-Red, a red refractory low plasticity clay mined in Manitoba, Canada. Red fireclay are not common. The top bar on each set is fired to cone 10R, the iron imparts deep color but the matrix is still fairly pours. The next one down in each set is fired to cone 10 oxidation. The third one down is cone 8, the red color is holding (it shifts to brown between around cone 9).
In the ceramic industry, refractory materials are those that can withstand a high temperature without deforming or melting. Refractories are used to build and furnish kilns.