Chagama question about interior rust (image)

[JBaymore]

Chajin,

I am thinking of buying the chagama pictured below. I want it as an additional one to the ones I have. As a "backup" so to speak...and for some variety. It is old. The exterior is in rather good shape. The interior has the level of rust indicatd in this picture. The price I pay will depend on the thoughts on the interior though.

InsideChagamaRust.jpg (28.36 KiB) Viewed 1579 times

I am aware that a certain amount of rust is important to improving the flavor of matcha for Chanoyu. What I don't know is exactly how much rust qualifies as "too much rust". While I have experience with Chado.... I don't have DEEP experience with it. So I haven't seen a lot of older chagama in use,...and had any sensei tell me, "OK...see this kettle,.... this is getting too rusty to use." I don't have the practical associations of long experience with multiple chagama to evaluate the situation enough.

Opinions on what you see here please. From what I THINK I know , I think it might be too rusty. Your thoughts?

best,

...................john

[Intuit]

Rust is a very interesting material, chemically. It can act as oxidant and reductant. Japan's lacustrine and riverine supply sources are for the most part (with a nod to abundant mineral-y geothermal sources) very soft due to natural filtration. The Japanese use their iron kettles for 'sweeting' soft water, through the addition of small amounts of ferric iron to form iron chloride and organic iron complexes. At low concentrations, these can be generally sensed as tasting sweet.

Ferrous iron in cooking water can cause vegetables to blacken.

There comes a point when there is too much ferrous iron available within the rust layer. This is described by a plot of total iron in boiled water versus perceived aesthetic quality. Once the concentration measured iron passes through the top of a hyperbolic curve, you find that excess ferrous iron is disagreeable, forming tea stains on cups and teapot, and giving the water a sour taste. Soft water has little buffering capacity. At very low iron concentrations, it tastes 'off' due to dissolved gas acidity, and at high iron concetrations, water can taste sour, metallic or bitter.

In a newer kettle, the iron just below the exposed iron rust acts as a reductant to any ferrous iron in the water, working like a sacrificial metal. When the rust layer becomes thicker, the ferric iron level drops off and the disagreeable ferrous iron dominates.

The other issue seen with cast iron vessels with considerable rust is obviously corrosion weakening and wall failure.

I've roughly described the chemistry in several threads on kettles and water quality.

[JBaymore]

Thanks for the detailed info, Intuit. Good stuff. I've read your other writings on the water quality stuff too.

The real question here now is whether I blow thru the top of the hyperbola in the amount of time that the water will be boiling in intimate contact with that amount of rusty iron in the photo in the interior of the chagama.

For all intents and purposes, here is my water analysis.........

http://www.monadnockwater.hk/EN/EN_our_water_3.htm

My water comes from the exact same aquifer as that water. And I live literally right next door . I could just about throw a rock and hit their buildings.

best,

......................john

[Intuit]

Better test results report.
http://www.monadnockwater.com/testanalysis.htm

Obviously, it's very soft water: you live near a large natural bog area. You would certainly benefit from use of an unlined cast iron kettle. If you purchase it, you can manually remove the loose rust with a rotary brush or have it sand-blasted, assuming that corrosion pitting isn't extensive.

If this is an expensive vessel, pass it up. You can find others that are less rusted. You travel to Japan annually. Buy it in person, where you can visually inspect it or ask a fellow potter or tea buff whom you trust to find one and have it shipped.

If it's inexpensive, it's a toss-up John.

[JBaymore]

Thanks for the further opinion on this, Intuit. I certainly value that highly........ with your background . Thanks.

Hopefully it will be "cheap" (a relative term, I know). So I might take the "pig in the poke". We'll see.

I likely will be back in Japan in May...... so I'll be looking for another one then for sure. I have learned that looking for such stuff...... ubnless you want to pay top dollar..... you have to take it when you find it.

It actually will be a "business expense".

This one might end up doing a bit of "double duty". I am considering it for part of an upcoming exhibition I am having in January. I will show cha-furo that I make, displayed on a layout in the gallery that is a little "in character" (tatami, all of the other ceramic teawares I make sort of in correct positions, then other stuff like chasen, chasaku, and so on also there, etc.) , and I need one chagama for that which is "decent" to use in that display. I don't want to put my good primary chagama out in that setting.....just in case. Ditto my other good chadogu. I won't actually sell this chagama..... just use it as an "acccessory" for display.

So.... now I keep thinking about it a bit. And see if I can get the peson to take what I am willing to offer based on the "risk" factor in not being able to handle the piece myself right now.

best,

....................john

[Intuit]

Buy it, but only if the seller will provide you with a couple of additional photos, up close images of the underside/bottom. If you don't see significant rust on the outside, you are good to go. You can count on having to do some minor restoration work.

If it's for an exhibition, that's a business cost,isn't it?

I faced a similar dilemma, with respect to nearly impossible assessment of corrosion to a kettle I had found on a Japanese auction site. The auctioneers had about a dozen of these kettles, and were careful to espouse their use for 'display only'. The kettle I wanted had the least visible rust of the lot, but the photos didn't dwell on interior condition - for obvious reasons.

I took a pass on it. There will be other opportunities, down the road.