“Baking trays” are actually slabs for lining stainless baking trays. They are made of a refractory material designed for the highest possible thermal mass. In other words they retain a lot of heat. If you place pastry directly onto hot steel it burns, especially the edges. Baking trays distribute the heat evenly and regulate the speed of heat flow into the pastry. The density is very high, about 2.8kg/litre. Baking trays have a layer of fiberglass scrim in the middle, so that if they crack they do not break. They are also used in electric pizza ovens. The surface is porous and can absorb fats. The top surface on which you bake is smooth and the bottom surface is rough.
The sizes are 15mm, 20mm and 25mm thick. Slabs are cast to order, to fit your oven, so there is no cutting and wastage. 25mm slabs are mainly used as pizza oven floor tiles, and 500mm x 500mm tiles are kept in stock. 50mm tiles are used as Coke Wharf Tiles, which get covered with yellow hot burning coke which then gets quenched with water. The thermal shock resistance spec is 900°C into water 30times without breaking. 900°C is the maximum continuous service temperature, but the material can withstand 2000°C for the short retention periods of hermit reactions, because crucibles of this material can absorb and dissipate so much heat.
The color is dark brown and the top surface is smooth enough to show reflections. The particles are coarse. Baking trays can be trimmed or cut with difficulty with an angle grinder, but easily cut if you use a diamond blade.