I knew that clay can change how tea tastes, so I looked into how that works. There are several factors that affect how the flavor changes.

1) Deoxidized firing and oxidized firing of pottery

A teapot can be fired either with an oxidized or a deoxidized method. In short, these refer to pottery that is fired with a perfect combustion by infusing it with air, and pottery that is fired with an imperfect combustion by largely restricting it from contacting air. Pottery is made up of minerals, and those minerals are oxidized when it is fired with an oxidized method, and conversely, those minerals are deoxidized when it is fired with a deoxidized method.

In the case of clay that is thought to be the same type, whether the clay is fired with an oxidized or a deoxidized method will cause a difference in the taste and scent of the tea. Up to now, I took oxidized pottery from both Sado Island and Tokoname and compared them to deoxidized pottery. Although there were differences in the degree of strength of the variations, between the oxidized and deoxidized firing methods, I observed the same tendency in variations regardless of the clay.

A characteristic with oxidized firing is that the aroma is more readily enhanced. The tendency is that the scent easily spreads inside one's mouth, so it is best suited for oolong tea and black tea.

In the case of deoxidized firing, the scent does not stand out as much compared to oxidized firing. In exchange, a richer flavor can be experienced. I think that would be best suited for pu'er tea and green tea. Additionally, people who place more of an emphasis on flavor over scent apparently also make oolong tea in teapots made from this clay.

2) Firing temperature

In general, it is assumed that the more minerals in the clay, the more effect that will have on the water. Nevertheless, if made in a teapot that is 100% iron, that does not mean that the flavor of the tea will be milder. Although the amount of minerals contained in the clay is important, the exterior structure of the clay is also an essential component. The larger the surface area of the clay is porous, the more the flavor of the water will change. Also, grains of clay that are finer will have a stronger effect on the water. In practice, if the same type of clay is fired at different temperatures, the lower the temperature, the more that the flavor of the water will change. The higher the temperature it is fired at, the more the degree of change to the flavor diminishes. If fired at a high temperature, that will cause more of the grains to melt and vitrify, and the vitrified portions are unlikely to have that much of an effect on the flavor of the water.

3) Types of minerals in the clay

The types of minerals in clay change the flavor of the water in different ways. For example, iron, tin, titanium, and gold particles will give it a milder taste. In comparison, copper, zinc, aluminum, and silver particles will cause the water to have a bitter taste.

While researching the above, I remembered that the manager of a teaware shop told me that a Tenmoku teacup will give the tea a milder taste. Tenmoku refers to teacups made with a deoxidized firing method that is covered in a glaze that has a lot of iron, so I believe that property is similar to an iron kettle or a vermilion stoneware teapot. The glaze on Tenmoku pottery has a lot of iron, giving it a darker color. Tenmoku from China's Song dynasty was made from clay in the northern regions that have a lot of iron, giving the clay a black color.

Going from this theory, celadon pottery is made with a deoxidized firing method, and the natural glaze made from wood ashes has a lot of minerals, so that will make the water taste more delicious.

I immediately tried to see if using a Tenmoku teacup would bring about a change in the flavor of the tea. Most teacups used for green tea are covered in a vitrified glaze, so in line with this theory, that would mean there would not be the sort of the change in flavor observed from unglazed pottery.

I tried several times to compare tea drunk from a teacup used for green tea and a glass cup, but to be honest, my tongue could not pick up on a difference in the flavor. In the end, if someone wants there to be a change in the flavor, I think Mumyoi ware, or a vintage Tokoname ware or Yixing ware teapot should be used.