In the Japanese pottery world, the word ‘tsuchiaji’, lit. ‘flavor of the clay’ is a much used term. In the case of Karatsu, even more so, though I wouldn’t be surprised if Japan’s long, rich history of unglazed ceramics is the main reason this term is so important.
Possibly the most distinctive characteristic of Karatsu ware is the clay, and one thing you might notice when looking at the ware is the lack of glaze on the foot. It’s probably safe to say that most potters (in the world) pick up pots and examine the foot first when viewing a pot, because it says a lot about the potter who made it. Most of the Karatsu potters and collectors I know do this, not just to view the carving job on the foot, but also to see the clay that was used.
The clays found in Saga prefecture, including the Kishidake area close to Karatsu, are usually quite large particle, non-plastic clays which contain a lot of sands. Lots of sandstone and coal throughout the area as well. Many potters (modern and historically) use sandstone in their clay body to seal the body, since many of the clays will not mature even at higher temperatures. Not only are many of the clays quite sandy, but in many cases the clay used was actually sandstone, not clay. One contemporary potter uses pounded sandstone for his clay body, and close examination of some old pots has shown that they too might be made of sandstone. One group of researchers travelled to Korea and found a place where similar practices had been utilized for hundreds of years (the name of the place escapes me now). These potters started with sandstone and pounded it in stamp mills. They then seived it into various particle sizes and created their clay body from that. Left over particle sized were used for other purposes such as glaze ingredients, making kiln furniture, etc… so that there was no waste, all of the sandstone was used.
Anyway, the most prized characteristic of many Karatsu clays is effect of ‘chirimenjiwa’ or crepe-like crinkling that occurs when the clay is trimmed soft. This effect is most obvious in the 3rd photo below, top left, in the 3 footrings made of a white, sandy Kishidake clay, from the Hobashira kiln.
In all of the photos below, the pots shown are quite old (400 years give or take). All of the clays have developed a patina over the years and are likely much darker than when they were first fired.