To make this post more useful, I am going to test various methods that have been suggested to me on my kobiwako shibo that has recently developed a tea stain ring from extended infusions with a natural baozhong:
Methods suggested to me so far includes:
- Citric acid either in powder form or lemon peel (by @@maple. This should be a relatively gentle method. There is some risk of the citric acid being absorbed by the clay and impacting the seasoning, but it should be relatively mild
- Ground walnut shell powder with either finger or magic eraser (from my tea master). This should have very low risk of absorption but possibly some risk of eroding the natural texture of the the surface
- Tea seed powder. This is supposed to be a traditional way. No idea where to get this material so won't try it but am listing it here.
- Spot application with baking soda. May damage seasoning. I will try only if no other choice
- spot application with sodium percarbonate. Will definitely impact seasoning. Final backup
citric acid. Made a saturated solution of citric acid. Using a qtip soaked in the solution, I cleaned the upper right quadrant. As a control, I attempted to clean the lower area with a water soaked qtip. The difference is clear. The qtip with citric acid turned brown and the stain came off readily.
Walnut powder: wetted the pot and rubbed rest of stained region with my thumb along with powder. This took great effort, but still worked.
Conclusion: both citric acid spot clean and walnut powder rub works. Walnut powder takes more effort while there is some risk of citric acid absorption.