What’s in Yixing clay?

So, what’s in Yixing clay anyway?

Last year I got in touch with Professor NH Cheung of the Department of Physics of Hong Kong Baptist University, because they have this technique that they have been using to do spectroscopy on various things – forensic analysis on ink toners, for example, among others. They can use it to figure out what elements are present in any given substance without causing damage to the material itself. Well, what better than this to test if Yixing clay has lead or not? After all, that’s what everyone’s worried about, it seems, and this method seems infinitely better than the rather dodgy lead test kits that you can buy. So we got in touch, and Professor Cheung’s PhD student (soon to be Dr) Bruno Cai conducted the tests. You can read the full report here: 2012.12.31 Report of PLEAF analysis on yixing tea pots-1

I asked them to take samples from both the lid and the base of the pot, so as to get a more general sense of whether there are differences. I also gave them two pots – identical ones from, presumably, the same batch, which are here listed as “sample 1 and sample 2″. I thought it might be interesting to test to see if they share similar characteristics. In case you’re wondering, they’re the same as these ones:

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The quick summary is – no lead (Pb). Among the elements present are: Aluminium, Calcium, Iron, Magnesium, Silicon, and Titanium. We don’t know in what quantity they are there, but that’s a start.

We might try to do more tests on different pots. They have also done tests on yixing ware previously – some reddish yixing cups, to be precise. The signatures are a bit different. This could get interesting.


Comments

What’s in Yixing clay? — 19 Comments

  1. Thank you for this post and sharing your results: i used one of those pots earlier today to drink some Shui Xian! Nice to know they’re Pb-free!

  2. Very nice to see something like this done. However before I believe it I have one important comment on their work. In analytical chemistry everyone learns the most important things to pay attention to for reported numbers are the reproducibility and the detection limit. They discuss the reproducibility but never the detection limit of the technique they use (it is not a common technique). Since the while point of this is detecting something it should be included. Is it possible for them to get detection limits somehow? Possibly having a potter make some small samples with known amounts of lead contamination to make a calibration/detection limit set or some other method.

  3. I can see the value of this sort of testing for lead, but I wouldn’t imagine that anyone has ever identified lead in any unglazed ceramics, including wares made with Zisha clay. Lead was used (and still is used, to a lesser extent in items not intended for food/beverage use) as flux to facilitate the flow of low-temperature firing of glazes, and to impact the glaze color. It would never be mixed with the clay itself since it would have the opposite chemical reaction to what the ceramicist would want during firing. Historically, it has always been the glazes that leached lead into food and drink from ceramic ware, not the base clay.

    Even in the absence of historical information regarding usage, I think it’s safe to assume that there couldn’t be lead in Yixing wares because they’re fired at 1100-1180°C (according to Seven Cups: http://www.sevencups.com/tea-culture/yixing-teaware/making-yixing-teaware/), and the melting point of lead is 327.5°C. That discrepancy would cause a problem with the structural integrity of the pot. Basically, the qualities and attributes of lead are ideal for use in low temperature secondary firing of glazed ceramics (or would be if it weren’t for the toxicity), but inappropriate in combination with clays.

    • I personally never thought for a moment that there’s lead in yixing clay, but lots of people seem worried for reasons I don’t understand. Granted, China is full of dodgy products, but lead in yixing clay is probably not among the problems.

      • I don’t understand that irrational fear either. I’ve seen plenty of crappy fake Yixing clay teawares that I wouldn’t be willing to trust in use, but they typically don’t look like high quality Yixing teapots at a distance of less than 2 feet. (I’m not saying that I can identify counterfeit Yixing teapots by simply looking at them, but I can identify really obvious poor quality tea wares.)

        Do you think it is possible to use this same PLEAF analysis to identify the specific clay mine that produced the material for particular pots? With the increasing scarcity of genuine clay from the Yixing mines, it seems like that would be quite useful.

    • Um… that’s not quite true.

      Egyptians used clays that had lead. The clay was very high in salts as well, which also lowered the melting point, leading to a self glazing kind of clay.

      Lots of early glazes had lead, and it was used in food because they didn’t know lead would kill you until the 19th century. Pewter is a perfect example of using lead for consumption, as well as the piping used to carry water, in Rome. Early canned food was sealed with lead, and made people crazy.

      As for the melting point of lead.. that has nothing to do with the temp something melts at. The famous tang dynasty 3 color glazes from china were high lead glazes, but those don’t melt until like 1700 F. And those were used for food as well as decorative ware. Glazes.. are just clays that have melted. You can use a cone 08 clay as a glaze on cone 10 pottery. The brown glazed jugs of the early us settlers was just a clay from a river bank in Albany NY.

      Having lead in something though, doesn’t mean it can get out. There are lead glazes that are not dangerous, but the line is fine so we lump them all into ..”better safe than sorry”.

      Leaded crystal glassware for example, has lead. But it’s trapped in the silicon matrix, so it’s safe to use with food/drink.

      The lead test kits… don’t test for lead presence. They test for lead leaching. It’s usually an acid, which accelerates lead dissolution. If it doesn’t leave the silicon matrix of a glass, glaze, or the clay body, even if it’s there, it’s safe to use with food because it stays in the object.

      Cobalt is similar. Cobalt is bad for you too, but it’s used extensively for deep blues in glass, and pottery. The blue/white ware made famous in the ming dynasty is cobalt, but it doesn’t leach out, so it’s safe for food. You’ve probably eaten off it at chinese restaurants, or drunk from teacups like this. But, it’s not hard to make a cobalt glaze that is not safe to eat/drink from.

      All clay has aluminum, and silica. Anything orange/brown/yellow and most things black have iron, which almost always has some titanium in the mix, because they occur together in nature. The silica is almost always from feldspars, which also always have calcium, and or sodium. Alumina is also usually tainted with some magnesium. So.. really, I could have told you those things were in the brown pots just by looking at it. I wouldn’t be surprised to see trace manganese either.. because you usually get some with natural iron as well.

      If you’re worried about lead getting in your tea, the leaching test is what you want. If it doesn’t leach into the test solution, it’s not going to leach into the tea, even if it is present.

      • I am unfamiliar with ancient Egyptian ceramics, but the “self glazing” technique you described that used lead (and salt) would produce a very different surface on the object, and my basic point was that ceramic pieces that are unglazed – like Yixing teapots – have an appearance that would not be possible if there were lead in the clay.

        • I hate to be contrary, but your assertion is dangerous to accept at face value.
          As a flux, lead lowers the melting point.
          As long as you don’t exceed that melting point, it will not look any different than clay without lead.

          It’s like looking at a snowman, and looking at an icicle. Snowman looks matte, icicle looks shiny. But they are both chemically the same. You can not rely on appearance for composition

          • I understand your point. Have you ever seen any ceramic pieces that were fired at a low enough temperature for the lead to have no impact on the surface appearance? I recognize that my points are largely theoretical since I’ve never seen any ceramic teapots that disprove what I understand to be true about Yixing teapot manufacture, so I would be very interested in seeing contradictory examples.

  4. Here are some interesting articles I have read regarding studies of what Yixing teapots are made of:

    J. Wu et al. An analysis of the chemical composition, performance and structure of China Yixing Zisha pottery from 1573 A.D. to 1911 A.D., Ceramics International (2012)

    Sun Jing Ruan Meiling. Microstructure and properties of yixing zisha ware. China Ceramics (1993)

    The first one was particularly interesting.

    SFLouis

  5. Got a firsthand look at the composition of one of these pots today: broke the handle off mine! Definitely good clay. Managed to fix it, but it isn’t pretty!

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