There was a large-scale survey in Tokoname (one of the biggest teapot production regions) last year. Over 80% of the public and tea-experts tasted that the tea brewed from the Tokoname-teapot is more delicious than the teas brewed from teapots made by other materials. The other teapots are made from porcelain, glass and aluminum. 84% of the public (63% of the experts) thought that the tea brewed from Tokoname-teapot has more umami. Majority found more bitterness on the teas from teapots made of the other materials. Many people who tasted the tea from the Tokoname pot said that it was richer and mellower than the others. In fact, according to a scientific analysis by a taste-sensor machine, the Tokoname teapot can enhance the richness of the flavor of teas compared to the other teapots. For the teas brewed from the glass and aluminum teapots, the bitterness level exceeds the level of umami.
This chart is from Saron No.37 by Nihoncha instructor association.
The report of the survey didn’t state the specific reason why Tokoname pot has the great effect on brewing green tea. But, it is generally said yakishime of Tokoname teapots affect the taste. Yakishime is a kind of baking method which is no glaze is applied in high temperature baking. Yakishime teapots are porous ceramics, which has a little textured surface. The porous ceramic adsorbs catechin of tea and makes mild-bitterness tea.
Anyhow, the survey is showing the fact of that many people actually said the tea from Tokoname pot is better.