The word refractory stands for a material or mixture of materials that can withstand high temperatures, without reacting with other materials with which they comes into contact.
They have the uncanny ability to absorb a lot of heat and sell it at baking preparation by replicating what a professional oven does (such as wood oven or fan oven).
One refractory stone alone is able to abundantly improve baked goods but the maximum is achieved with two, one thicker under and a finer above. In this way, it sends an enormous amount of heat to the bread, the one below does the same thing but, being in direct contact with the product and being easy to cool, it should be thicker. Between the two stones there must be plenty of space, so that the firing chamber is possibly the most similar to that of a professional fan oven.
First of all, there is to say that you can find stones and ceramic materials, such as porcelain stoneware tiles, or soapstone. For the first type you have to pay attention, as they are not intended for food use and may not be suitable, although if it is cooked to the ideal temperature (1200° C and above), the materials of which it is composed will stabilized a lot (vitrification), also the glazing surface is safe, but in any case the porcelain is still a very fragile material (it is not heat-resistant), in addition it misses a component that confers resistance to high temperatures and the thermal expansion (the chamotte).
Another widely used material is soapstone, although it is a material that goes very well, is not able to absorb the moisture in the bakery products while they are cooking, as the refractory or Obsidian or slate does.
In light of what said above it is obvious that the most suitable material is the refractory stone but in order to do its job well, it must be thin, no larger than 1 centimeter, max 1.5. To make sure that it warms up quickly it is good to put it at least 10 minutes on the bottom of the oven and then another 10 minutes under the grill, or heat it on the stove.