無釉の磁器作品です。
It is an unglazed porcelain work.
 
吸水性のないつちなので茶渋やシミは表面的な汚れです。スポンジで洗うと綺麗になります。もしスポンジで取れない茶渋が付いたら市販の台所用漂白剤で漂白できます。
Since it is non-absorbent earthenware, tea stains or stains are superficial dirt. They can be cleaned by washing with a sponge. If there are tea stains that cannot be removed with a sponge, you can bleach them with commercially available kitchen bleach.