See those black cracks, large and small, long and short, connected or not connected. See this intense red, smoothed by an adjacent pale white-grey. See those black spots and shiny almost golden dots.
This is a close view of the surface of a pot glazed following a Japanese technique called Raku.
The primary material of the pot is stoneware with a large amount of large grog. The shiny dots at the surface arise from these grog particles, millimeter sized chunks of fired clay. The more the grog amount, the more the clay looks and feels like sand. Don’t imagine using the pottery wheel with that type of clay, it will burn your hands very quickly. However, using grog has the great advantage that you can make thick parts of 1 cm with very little risk of crack formation during drying. Indeed, if you have a crack starting in your clay, it will stop as soon as it meets the hard grog particle. Also, for the same reason, the pot after firing is more resistant to thermal shocks than a pot without grog.
After bisque firing at 950 °C, the glazing was done following the Raku technique. The glazes, here white and red, were applied as normal glazes, using a pinsel and covering the surface. After quick drying, the pot is placed in a kiln, consisting mostly of a metallic vessel (like a bin) with an exhaust pipe or chimney of circa 40 cm, and with a chamber for the wood. I invite you to check this website to see how it looks like: www.raku.ch. I put small pieces of wood and lit them into fire. When the flames are coming out of the chimney, I stopped adding wood and let it burn for a while. At that point, the temperature inside the kiln is of roughly 1000°C.
That’s the moment to open the kiln, and take out the pots using a long metallic pair of pliers. The pot is really warm and is bright yellow-orange-red. I directly placed it in a pile of wood chips that was prepared before and covered it with more wood chips. There is some smoke but no fire. If fire starts, you just need to add more wood chips to stop it. This process creates a reducing atmosphere around the pot: all the potential air and oxygen is burnt. The consequence of a reducing atmosphere is for example that carbon does not vanishes but stays in the form of ashes. If you put a ceramic pot without glaze in a reducing atmosphere at high temperature, it will become black.
In addition to the change of atmosphere between the kiln and the wood chips, the temperature drops dramatically from 1000 °C to air temperature. Such a thermal shock leads to the formation of cracks in the glaze. The cracks uncover the stoneware, which then become black because of the atmosphere. Now you also have the justification of the choice of the clay: having a lot of grog permits this thermal shock without breaking the pot.
It is very difficult if not impossible to predict the size of the cracks and where they will appears. Each pot is unique!