Is there a way to slow the formation of rust on the kama?
Having bought extremely rusted kettle off eBay, I have had to face this problem. An answer comes from the cast iron cookware people in other contexts. To remove the existing rust, scrub out the flakes with water first, then you fill the kettle with cleaning vinegar (6% acetic acid) and let sit for several days. Don’t get vinegar on the outside of the kettle. The vinegar will solubalize the rust into this horrid stinking foam (use gloves and do outside). Rinse well, scrubbing with a hard bristle brush. This will get rid of all the obvious big flakes of rust and most of the fine grain rust. The next phase of treatment depends on how brave you are. If you want to be conservative, you boil several changes of bancha (cheap tea), each time boiling until the water turns black (about 2 – 3 hours) and rinsing before repeating. This acidifies (via tannic acid) the remaining rust and fixes it (mostly) in place. Then boil about 3 or more changes of clear water until the resulting water following a 1 hour boil is acceptably clear. You are good to go with normal maintenance then. If you are brave and want to prevent the rust from reforming, then you need to paint the inside with a high temperature curing ceramic to seal the iron away from the water – I used POR15 and cured in the oven at 400 degrees F for 4 hours, allowing it to cool in the oven overnight. Then a couple of clear water boils and again, you are good to go. The kettle I did this treatment on still makes clear water with no discernable taste changes from the source water a year later. YMMV. – elmar
The best way to treat rust is to get it off completely by a specialist in Japan.
Have them put on a coating of “sabi dome” a rust suppressant. Then after each use, rinse out the kama carefully with hot water, dab the inside with a dry cloth and get it as dry as possible. Place the “dried” kama on the still warm but extinguished fuuro or ro, being careful that not even the tiniest piece of red coal was left in the ash, even if stirred. The ambient heat from the ash will help to further dry the kama. Leave the kama UNCOVERED for at least 36 to 48 hours or rust will return.
Never leave a kama or other metal utensil covered until at least 36 to 48 hours have passed. If you cannot get ambient heat under a kama to get it dry after use, leave it uncovered for even longer, possibly 72 or more hours, to assure it does not rust.
If is already rusted inside, I am not certain what an amateur might do without causing damage to the kama.
I wrote this also under the ‘kama maintenance’ question:
If the kama is rusty, my teacher told me several methods to get rid of the rust:
According to a kama-maker: boil water in the kama and prepare some bancha tea in a separate tea pot. When the water in the kama comes to a boil, add the bancha tea and boil for one more minute. The water will become black. Let cool and leave overnight. Repeat if needed. Finish the process by boiling water only in the kama several times.
According to koyamaen: fill the kama for 2/3 with water and add one table spoon of baking soda. Leave for half a day. Finish by boiling water only in the kama.
Another way: boil water with cornstarch in the kama.
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