Madara Karatsu. Madara refers to the glaze, usually a high silica white which reacts with the clay to produce a mottled effect, with various shades of white, cream, yellow, brown, and subtle optical blues. Most modern potters use rice straw ash as the main ingredient in their madara glaze, but other types of high silica ash are sometimes used as well.
We don’t know exactly what was in these glazes traditionally, there are probably various combination of glaze ingredients and clay bodies that would yield the madara effect, and I suspect that the potters of old just used what was on hand, which would explain why some of the old kilns are known for specific types of pots. Hobashira-gama, one of the Kishidake Ko-garatsu kilns was probably the best known for its Madara Karatsu ware.
Hakuo is one of my favorite tea bowls, and a sort of collection of ideal Madara Karatsu traits. The glaze is not too thin, not too thick, the clay is vitrified nicely, but quite sandy so that it reacts well with the glaze. Below is another piece of a tea bowl from Hobashira-gama. This is a shard that was given to me by a friend.
You can see the mottling , including the blue spots, probably from ash that settled and melted in the bowl.
There is a lot of sand in this clay body. You can see here that the glaze is not that thick. Many modern potters apply the rice straw glaze thicker, or too thick (often the case with me), resulting in a white glaze. Too thin and you end up with clear.
When you touch this shard you can feel that it is well vitrified. It does not absorb water. There is a lot of speculation about what the body really is. Recent examination of shards like these under an electron microscope indicate that the clay body is not really clay at all, but rather powdered stone. When people talk about the technology brought to Japan by Korean potters, they always mention the climbing kiln, and glazes. Perhaps more importantly, they also brought over the technology of processing stone into a usable body for making pots. They had this technology for processing porcelain stone, but did not have a source immediately, and made pots for many years before discovering the porcelain stone deposit at Izumiyama, Arita. Suitable types of feldspathic sandstone probably served as alternative ‘clay’ bodies in the meantime.