Moss wrote:"Adding iron oxides do nothing but change the color. "
Not true. Adding oxides modify the interface between the clay and the interior. If that iron is in glass formers, then it acts as a colorant and forms a glaze, if it is NOT in enough glass former to create a glaze, then the iron oxide is simply iron on the inside of the pot which is what you want for this test.
Iron in the clay and iron applied to the outside are identical chemically. If the clay is totally vitrified, no water is going to enter it anyway. The chemical changes in the water from ionization or whatever are going to occur in the interface between the clay and the water. So if you modify the interface of the interior to be iron, you are going to get the best possible interface between the iron and the water. Bet you a dollar on that.
If you add iron oxide thickly enough and reduce, you end up with iron on the inside of the pot. Just like a tetsubin.
If the desirable mineral is not iron, then it won't matter of course, but if it is iron, that will do the trick.
Now regards other clays, the combination of firing and maturity may be the key. If the iron is only part of the issue, then a clay that is reduced, but not fired totally to maturity, it may remain porous enough to create more surface area for the water to exchange with the other minerals in the clay.
According to Hojo's analysis, this is what he says on the subject:
'100% of Tokoname red clay is made of mixing iron oxide. The melting temperature of mechanically ground iron particle is much lower than naturally existing iron granule. Usually they start melting at around 800 degree C, while natural iron granule can stand until 1300 degree C.'
I asked him this question in regard to the difference between Tokoname clay and banko clay. I then asked him about Sado red clay.
'In fact, tokoname red clay and sado red clay is almost the same in terms of mineral composition. With the modern techinich, people can easily analyze the mineral compornent in clay. The major difference is particle structure. The natural red clay contains crystalized iron granule which is very solid and in round shape (less surface area), while artificially mixed red clay contains iron oxide which was mechanically ground from iron. The particle shape and size of mechanically ground iron is very uneven which has very big surface area. It is just like the relationship between diamond and carbon powder. Due to the big surface area and none-crystal form, mineral can not stand until high temperature baking. It starts melting at 1150degree. As for Sado red clay, they can stand until 1350 degree. With higher baking temperature, clay shurinks a lot and drastically increases its surface area.'
Moss, this is why there is so much doubt about the current yixing wares. They have oxides mixed into the clay to color them but do nothing to the effect they have on water. Pure, high iron clay is the way to go, IMO.
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