Green clay yixing toxic?

[shyrabbit]

John,
Thanks again, yes I agree, It's important to make us all aware.
Michael

[Herb_Master]

Hojo states that green clay eliminates taste.

What does Hojo say about Yellow Division Clay - I have a collection of these that I use for Anxi oolongs ?

[isaac]

There can be metal salt (good or bad) added to yixing to manipulate green/blue color. Good examples were made by some yixing artist (very few of them though, such as Zhu Dan).

Hi Gingkoseto,

My teapot was made by Zhu Dan. It is a lovely pear shape green teapot with four matching teacups. I like it for the workmanship and the beauty of the curves.

May I know the difference between good examples and bad examples please?

[isaac]

If the copper is in the porous clay body formulation and not in a fully fused glaze, it is reasonably likely to have some leaching issues.

Hi John,

Thanks for your informative post.

I gather that the copper in the teapot will leach? If so, is tea or water a sufficient solvent such that over time the copper will be reduced tremendously?

[JBaymore]

If the copper is in the porous clay body formulation and not in a fully fused glaze, it is reasonably likely to have some leaching issues.

Hi John,

Thanks for your informative post.

I gather that the copper in the teapot will leach? If so, is tea or water a sufficient solvent such that over time the copper will be reduced tremendously?

isaac,

Unfortunately, shy of minorly destructive lab testing (which I'd LOVE to see the results from in this case), we cannot say "will" with certainty.....only that it "may". My original thoughts are based upon TYPICAL ceramic technical information. For example:

There are limited sources of ceramic materials that can produce the green type colors in this shade range; copper being the cheapest and most common. Copper as a colorant does not like to "stay" dissolved in a silica based glassy matrix. Metals bonded into crystalline materials are even less prone to "stay put". Clay bodies, even stonewares, are not heavily glassified, with a large preponderance of the material still in the crystaline form. Any copper present in the raw materials of the clay body will, at best, only be partially disolved into the glassy phase of the body; some will be involved in "staining" crystalline formations in the body. Yixing pots are known (reknowned) for their unique porosity, indicating two things; some likely underfiring of the body (hence more non-glassy phase), and increased surface area which will exacerbate any possible leaching issues there might be. Copper (and lots of other stuff) can easily leach out into water and stuff like teas.

The problem here is that there are HUGE numbers of variables involved in the ceramic technical process end of things. We unfortunately have NO idea of the technical expertise nor controls utilized in the production of just about any wares. We also do not know the genesis of the particular clay body. And "high firing" is absolutely NO guarantee when it comes to potential leaching issues.

So........ all I can say is that it might leach copper. IF there is even copper present. For all we know they are using something else and it might not even be a "fired in" colorant. Which itself we do not know for sure..... This is where working with a trusted gallery is important.

The whole leaching subject is a terribly complex thing to understand.

Yes, on a technical level if copper is leaching out, over time the amount of available copper to leach out will go down. Basic chemistry . We are not creating matter in the clay body. The big unknown is the amount of unstable copper that is present to start with, what exact form it is in, and the concentration that it releases into a given PH solvent in a given amount of time.

I would not assume that by repeated "washings" of hot water or tea that after a certain amount of time the copper would of course be "gone". It could possibly take a LONG time to accomplish this. It is an unknown, lacking any lab data.

All of this above being said....... everyone reading this thread...... please see all of the various threads on ceramic ware toxicity questions and such in the TeaChat forum sections to gain more of an appropriate overview of the whole subject. I've posted a lot on the subject, as have others. While there certainly are potential issues.... in the overall scheme of things .... they are generally SMALL. Both in severity and in frequency.

That is not to say that this does not happen................ but as I have mentioned before in threads, as a professional potter I can get $2 mil of product liability coverage for well under $1000 a year. That should tell you something.

best,

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

PS: Anyone want to donate a green Yixing teapot for a lab test? Then we'd have some REAL data to go on rather than speculating. But remember that would ony be for the specific clay and firing techniques used on the particular pot.

[gingkoseto]

There can be metal salt (good or bad) added to yixing to manipulate green/blue color. Good examples were made by some yixing artist (very few of them though, such as Zhu Dan).

Hi Gingkoseto,

My teapot was made by Zhu Dan. It is a lovely pear shape green teapot with four matching teacups. I like it for the workmanship and the beauty of the curves.

May I know the difference between good examples and bad examples please?

You got a Zhu Dan teapot! It must be very expensive! Pictures please!
The bad examples I spoke of is simply adding chemical dye to the clay to manipulate color and make it look prettier. This may happen a lot more on red clay than green clay. But when real artists add color to the clay for artistic purposes, the color is supposed to be safe.

[gingkoseto]

PS: Anyone want to donate a green Yixing teapot for a lab test? Then we'd have some REAL data to go on rather than speculating. But remember that would ony be for the specific clay and firing techniques used on the particular pot.

From now on let's keep an eye on the "broken pot" thread on teachat and the "heartbreak" thread of teadrunk (http://teadrunk.org/viewtopic.php?id=67)

[entropyembrace]

Very interesting posts John

May I ask what kind of educational background you have? It seems like you know a lot more chemistry than is normally associated with fine arts - ceramics programs and have access to analytical chemistry equipment.

I´m asking because I´m a chemistry student and I really enjoy ceramics as a hobby when I get a chance to take pottery workshops...the way you seem to combine analytical chemistry with the art of ceramics and pottery is very intriguing...but not something I´ve seen any University programs focusing on.

[JBaymore]

Entropye,

Besides being an active studio artist, I'm a professor of ceramics, and have been teaching glaze chemistry and kiln design for a long, long time. Wrote (and sold) some of the very first PC based molecular glaze calculation software back in the early 80s. Have taught those subjects (and others) at Mass College of Art and Boston Univeristy in undergrad and grad level courses, and now am at New Hampshire Instute of Art. First got involved in the health and safety part of things back at Massart in the 70s when I was on the H+S Committee and we brought in real occupational health people to fix up the studio environment. (eye opener!) Was a technical consultant for what was the largest ceramics supply company in the US at the time, Cutter/Eagle Ceramics, for a number of years. And yeah... you clearly caught that I have a serious "technical side" underneath the artist mask . Right and left brain are kinda' balanced . Although an "art major" in high school, I started out college life intending to be a marine biologist....... but found clay early on in my college studies. That was the end of that ! (It was only after deciding to be a potter that I discovered that I had a family history in the potteries in the Trenton, NJ area back into the mid 1800s.) Back when I was at studying at UMass (Amherst) the technical side of ceramics also was stressed far more than it typically is today. Early on I also had the pleasure of working with a colleague, Charles Abbott, at Massart..... one of the notable glaze gurus of the day coming out of Alfred. Learned a bunch from him.

I continue to study the subject however and wherever I can. My forays into Japan are always a help. If I keep my mind and my work alive and fresh... hopefully I can inspire at least some students to really think and create.

best,

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

PS: Forgot to hit this part....

Some simple stuff I have. Real analytic stuff is via real commercial testing labs. Such testing is routine if you know what to ask for. Hummmmmmm.... as a chem major you might have access to such stuff via you university. Wit theh interest in ceramics... that might be a nice side line to look into.

Oh... and our program at NHIA includes this kind of training as a part of required ceramic materials and kiln design and operation courses.

[Tead Off]

John, I want to ask you a question about porosity. You claim Yixing are known for their porosity. Do you know this from testing or because you have read this somewhere? As a potter, you probably understand a lot more about clay than a tea drinker. As a tea drinker, I understand that porosity is determined by firing temp, ie., the higher the firing temp, the less porosity. Earthenware, stoneware, porcelain, each one less porous as the firing temp increases.

From what I've read, many yixing as well as banko kyusu are fired at ranges of 1200c or more. Can these pots really be porous? Of course there is always some porosity, never zero porosity. But, where do we draw the line at calling something porous vs. non-porous (or little porosity)?

[isaac]

...

Hello John,

It was a very informative post you gave. Thank you.

[isaac]

You got a Zhu Dan teapot! It must be very expensive! Pictures please!
The bad examples I spoke of is simply adding chemical dye to the clay to manipulate color and make it look prettier. This may happen a lot more on red clay than green clay. But when real artists add color to the clay for artistic purposes, the color is supposed to be safe.

I am getting my camera in working order at the moment and will post once it is in working order.

Of course if it does not look nice, it is probably due to all the copper that had been leached out by repetitive washings

[JBaymore]

Teadoff,

I have never actually formally tested any Yixing pieces........ that is why the "anyone have a pot we can test" comment above. Nothing beats solid real-time data. I have seen, handled, and used them for tea. I don't collect them. I have read it. And in simple handling, it just seems apparent to me from the nature of the pieces.

I could be completely wrong too.

As a tea drinker, I understand that porosity is determined by firing temp, ie., the higher the firing temp, the less porosity. Earthenware, stoneware, porcelain, each one less porous as the firing temp increases.

Sorry to say that this is something that "everyone knows" but it is simply not exactly true. True porcelain is for sure vitreous, but there are so-called "porcelain" bodies out there that do not exhibit true glassy translucency and zero percent absorbtion. Stonewares can have a range of porosity. And some earthenwares can be pretty "tight".

Porosity in a clay body is mainly a function of, among some other things, the degree of vitrification of that clay body. That vitrification is a function of the percentage of glassy phase relative to the crystalline material within the body. The glassy stuff sort of "glues" everything together, if you will.

And in high fire ware, some crystalline material actually "grows" within and into the glassy stuff too. In fact, in the case of high fire glazed wares, the body actually grows little "extensions" of crystalline material out into the glaze layer at the interface, and the glaze eats a bit into the body there also. (That is part of why high fired wares tend to be stronger and more durable than most lower fired wares.)

Think of a clay body sort of like being fiberglass....... composed of a soft woven cloth and a liquid resin. The more fully the cloth is impregnated with the resin, the more "solid" and strong and impervious it becomes.

High firing (1250 - 1300 C+) is not a guarantee of vitirfication of a clay body. It makes it far easier to achieve, but it does not assure that. The 1250 - 1300 C firing range that potters tend to use for "high fire" wares is no casual "accident" or even "mental design" . It happens to be the natural firing range for vitrification that many naturally occuring, highly malleable stoneware clays exhibit without any real modifications. It is sort of determined by "god". Or you might say the "Gamakamisamma" (kiln god). Above this range, and those natural clays deform in the kiln. Below this and they exhibit more porosity.

But thre are many clays that do NOT fully vitrify at that range.

For example, all that high fired ware has to be fired in a kiln of some sort that stands up to the temperatures ("heat work", actually) that high fired wares are fired to. Those kilns are frequently (always historically) made of bricks of certain types of clay(s). Clay that does NOT vitrify at those firing ranges. It is therefore called "refractory"....... it resists significant permanent changes when repeatedly exposed to heat energy. Otherwise the kilns structures would quickly "melt" over repeated firings. Not what potters want to have happen.

So I can have a clay body that at 1200 or 1300 C actually is what potters call "open". It is relatively absorbent. It does not contain the amount of fluxing material that is needed to "glue it together" well. (Many Japanese clay bodies exhibit this characteristic... it is what helps give the pieces so much "character".) I can also have a clay body that at only 900 C has the correct fluxes added to the clay materials that cause it to be quite vitreous at that lower firing range. Industry has made very sucessful low fire vitreous clay bodies; many commercial dinnerwares are of this type.

Then also relating to potential porosity, there is the issue of particle size distribution and something called "packing". If the distribution of the raw clay particles in the body is such that there are not enough little tiny fine bits to fill in the spaces left between the big bits nestled up against each other, that leaves little "gaps" in the structure of the material. These gaps sometimes do not "fill in" with glassy phase material. This leaves little holes that form a "porosity" because they are little holes between solid glassy material.

These "holes" also increase the surface area of the outer part of the clay body wall, due to the roughness and the exposure of more of the normally internal wall to the outer surface. So if there is going to be any interaction between the clay surface's chemistry and the contents of a pot, this makes more clay wall in actual contact with the liquid.

From my casual observations of Yixing clay, I'd say the "typical" characteristics of the body are a combination of those two factors. I think it is a less vitreous than 0% apparent porosity, and I think the particle distribution is such that it also leaves "holes" in the clay.

Again as I said above... I could be completely wrong about the nature of the body. Yixing is not some sort of ware that I have spent much time formally studying.

It is even possible that there is no copper in the green Yixing stuff. There is a kinda' rare green color rendition that cobalt will exhibit (more typically producing blues and purples) in the presence of high levels of Al2O3 and TiO2. If the Yixing clays contain significant titania, I already know they will contain alumina... it is a constant in any clays. So it could be. I believe that I have read that it is copper in the green Yixing, and I think someone even mentioned that here in this thread quoting from a vendor's site, but lacking detailed tests or definitive information from an actual Yixing source...... maybe it is not copper. Personally, I would EXPECT is it copper from the nature of the pieces....... but maybe not.

Ceramics, from the technical side, is a crazy complicated mess . But as Mr. Spock would say, "Fascinating!".

best,

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

[Tead Off]

Teadoff,

I have never actually formally tested any Yixing pieces........ that is why the "anyone have a pot we can test" comment above. Nothing beats solid real-time data. I have seen, handled, and used them for tea. I don't collect them. I have read it. And in simple handling, it just seems apparent to me from the nature of the pieces.

I could be completely wrong too.

As a tea drinker, I understand that porosity is determined by firing temp, ie., the higher the firing temp, the less porosity. Earthenware, stoneware, porcelain, each one less porous as the firing temp increases.

Sorry to say that this is something that "everyone knows" but it is simply not exactly true. True porcelain is for sure vitreous, but there are so-called "porcelain" bodies out there that do not exhibit true glassy translucency and zero percent absorbtion. Stonewares can have a range of porosity. And some earthenwares can be pretty "tight".

John, thanks for the detailed post. Green yixing clay is not what most tea drinkers use. That may have something to do with the copper in it for all I know or there may be other reasons the predominant clays, red and purple, far outnumber the green and yellow clays of yixing. But, concerning yixing teapots and porosity, there is no blanket statement that is going to cover the topic. Too many variables exist in the clays, the processing of the clays, the kilns, and, the firing target. As you point out with porcelain bodies, not all vitrify.

For tea drinkers, the iron content of yixing clay is what makes them so desirable, producing delicious teas much the same way as a tetsubin affects the water. Green clay would not have this same affect due to its lack of high iron content and this may be one of the reasons it is not spoken about in the same way the red and purple pots are. This is all aside from its possible toxic metal content.

[entropyembrace]

Thank you for the detailed reply John, it was very interesting

I don´t think toxicity would be the issue with green yixing if the green is coming from copper...like John said copper is not very toxic and unless you have Wilson´s Disease your body shouldn´t have problems eliminating it from your system.