There are two types of firing, depending directly on the environment in which the product is fired - reduction and oxidation firing.
Oxidation firing is the standard type of firing which will produce white or red (depending on the colour of the clay you use) pottery. If you choose the reduction firing (temperature of 900 degrees), you will need to caulk absolutely all the furnace mouths, so that the clay products made with their own hands, languished without any air access.
When reducing firing in the furnace, the shards will take on a black or gray colour. All this is due to iron oxide passing into oxide, which is capable of forming a so-called spinel. It is this spinel that gives the pottery its dark or even black colour. The intensity of the hue often depends on the temperature used in the firing process and the quality of the glaze.
If excessive reductive gases are produced during the firing process, you will get a very beautiful black colour with silver as graphitization of the surface of the shard will occur due to the decomposition of complex types of hydrocarbons. If we consider the significance of such a shingle in technical terms, it will be interesting because of its excellent thermal conductivity.
We will try to characterize the processes that take place in the product when a reduction firing is chosen. We can state with certainty the fact that at this point the reduction of iron oxides and iron itself takes place - it is like the same processes that take place inside a blast furnace. The wood carbon is burned down to carbon dioxide and the carbon dioxide can in turn be reduced to carbon monoxide. However, the reactions described can only take place if there is a temperature regime that is close to that of the blast furnace. If a lower temperature is set, the iron oxide may lose some oxygen and the reduction process will not take place. If you require more precise information about the results of the reduction firing, this can be found out by precise chemical analysis of the already reduced product.