Oil spot glazes were first developed in 12th Century China, and highly prized at the time. When Japanese monks came over to China, they brought back ceramic tea bowls and other items, with the black glaze named after the mountain by the monastery - Tenmoku. In time they also learned how to produce their own tenmoku and oil spot glazes, highly revered in the tea ceremony.
Creating an oil spot glaze is a highly technical challenge, but the basis is iron - as it heats up, red iron oxide (Fe2O3, or rust) becomes unstable and, at about 1230º C, decomposes to black magnetite (Fe3O4) and then to FeO (also black). But what makes the spots? Well as the iron decomposes it gives off oxygen, so bubbles rise through the glaze to the surface. Now we need to get the bubbles to take some iron to the surface. With the right glaze proportions, the glaze reacts with the clay and forms an intermediate layer, so the glaze layer itself is thinner. If the glaze solution is saturated with iron, it then becomes supersaturated, and the excess iron is carried to the surface by the oxygen, and when the bubble breaks the surface it leaves a little pool of pure iron oxide, which then crystallises. The glaze also needs to be applied very thickly to make enough big bubbles for the spots to be visible, and the right consistency that the surface iron forms a smooth surface but doesn't spread out too far. Hold the kiln at the right temperature for a number of hours to give enough time for the bubbles to get to the surface, and for the crystals to grow. Varying the glaze composition can also change the colour of the spots from black to rust brown. Quite clever, those Chinese!
All of these pieces are food and dishwasher safe.