These are 9" x 4.5" x 1" Firebricks 
Nowadays they are called heavy and dense  Firebricks but old masters still call them fire clay bricks just because they are made of simple fireclay (which actually is the most ordinary mud.) Fire clay can be easily located out in the nature but it must containing the right refractory properties, suitable content ratio of silica and alumina. Some shops call these bricks fireplace bricks. They are used for instance for building cooking chamber in wood fired ovens, for creating fireplaces, all sorts of fire boxes and wood heaters’ lining,