I’ve read of several ways to clean the stains and hard water deposits from pots. They included scrubbing them with a scouring block, CLR, baking soda, soaking them in rain water, and even burying them for a few months.
This pot, a 40-year old Heian Kouso came to me with considerable calcium buildup, and while I wanted to preserve the patina it was developing, the buildup was distracting.
First up, CLR, a cleaning solution formulated to remove calcium, lime, and rust.
Next, a Sand-flex block, used to clean tools. My concern with using this product was that the grit would etch the glaze, and/or buff away any signs of weathering.
After some online searching, I decided to try a paste of baking soda and vinegar. It was allowed to remain on the pot for three days. At first it was hopeful:
So, on November 9, 2013, I buried the pot in my growing bed and gave it a few months to work.
Liberated on 6/14/14 to check results after 7 months in the ground:
So a year to the day later, here is how it looks on 11/9/14:
Bonsai Pot Unearthed:
It’s a bit hard to tell by the photos, but some pitting and inner-rim glaze chips that show up white are still white, but the calcium deposits that would show when in use were nearly gone after one year in the ground. I would not hesitate to use this pot for a local show at this point.
I think the results were better in the warmer months, but cannot conclude whether it was ground temperature or more frequent watering that contributed to those results.