I ordered the 9.2 mm binconvex yunzi stones from Yellow Mountains. One of the stones was cracked almost perfectly along the midline. When I looked at the two halves, it looked like there was a hole in the center of the stone filled with some powder.

I was wondering if anyone knows that the powder is. I am curious especially considering the company's past history of making stones with lead content.

As far as I know, Yunzi is sort of a glass-like material. Various materials (I think the precise make up is secret) are mixed in a powder and melted and then cooled. Sounds like what you are describing is a stone that did not get melted fully.

Lead was one of the components (for weight, I guess), but more recently they are switching over to lead-free formulations. So if your stones were advertised as lead-free it should be OK (since the surface and centre should be the same make-up).

Biconvex is also more recent and more difficult than single-convex (because that is a simple matter of dripping onto a sheet), so maybe your stones were made when they did not get the exact time/temperature right.

If you can get a high-powered flashlight and look at the stones with the light shining through you can probably see the powders as a dark spot in the middle and any inconsistencies (e.g. bubbles) because Yunzi has a degree of translucency. You might see a lot of small imperfections because I think they were still trying to sort out the problem with the new formulation/shape (probably more in the black stones because they are less noticeable normally).

For normal play it shouldn't be a big issue. I hope to get my hands on a set sometime

lead oxide lowers the melting point and make the molten glass easier to work with
http://en.wikipedia.org/wiki/Lead_glass

Oh, cool. Thanks for the information, it's very interesting

Here's an interesting and related story.

I recently received my set of Yunzi 9.2mm biconvex stones in the mail. I ordered from the very helpful YM Imports. Following the included instructions, I rinsed, dried, and oiled all of the stones. Anxious to put my new set to use, I set my Go board out and proceeded to play through a recorded game. When it was time for bed, I left the game out, and in fact left it there all day the next day while I was at work.

When I came home, I was again excited to play through another recorded game with my new equipment. When I went to inspect the board, however, I noticed something very strange. One of the white stones was lying on the board, rather pathetically, in three separate broken pieces. There was a white chalky powder both under and near the stone. What's more, the four or five stones immediately surrounding the fractured stone had been shifted back as if moved by the force of a miniature explosion!

Click here for a picture of the stone.

No one had touched the stones or board since I had left them the night before. With curiosity and some trepidation, I picked up the stone and noticed three things. First, the interior of a Yunzi stone is very smooth and shiny, like obsidian or glass. Second, there was a cavity in this particular stone. When I picked up one half of the broken stone, a white powder fell from the cavity onto the board. The other half contained the same powder, which stayed in place. Finally, I noticed that there was a line running from the cavity to the top of the stone.

My best guess as to what happened is that, due to the imperfections in this particular stone, moisture was able to enter the cavity. Perhaps do to temperature change in my house (I live near Chicago and keep the heat lower during the day compared to the evening), the stone exploded when the internal pressure became too much.

Luckily, none of the other stones suffered a similar fate. I am very happy with the Yunzi stones, and happy that YM Imports included 10-15 extra stones of each color. In any case, I thought my story strange enough to share. I hoped you enjoyed.

That's a very interesting story. Actually, the discovery of a stone split in half that I mentioned happened right after returning from a trip to a go club on a cold evening. The stones were sitting in my car for a while, so maybe the temperature change made one of the stones explode.

Interesting. I've accidentally chipped more than my fair share of Yunzis (the tile floor at starbucks is not kind to dropped stones), but never observed anything like that. Must be rare!

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FYI, I contacted Hebsacker Verlag, the best-known (to me) German distributor of Go stuff, and asked them re: their yunzi stones and lead oxide, b/c I’m interested to purchase such but don’t want something poisonous in my fingers (except for the coughs), and they told me that the mixture has been changed a few years ago.

Greetz, Tom

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I heard about the powder from someone reviewing a go board. They said in the packaging (I don't know how they organize it since I still don't have a go board) it had some powder. I would assume it's from the stone if it was only a little bit on the crack line. And I would assume all stones of the same type have that (with what they're made up of, but it probably doesn't matter if it's double or single)

The following stones have been tested and do have lead:
Double Convex, New Yunzi, size 32 9.2mm
Old Yunzi, all sizes

The following stones have tested negative to lead content:
Single Convex, New Yunzi, all sizes
Double Convex, New Yunzi, size 38 and 42, 10.7mm and 11.9mm

You will notice that that's information from way back in 2008 on chiyodad's blog. I'm relatively sure that there's no longer lead content from Yellow Mountain yunzi now.

Back when I was still studying, I had a modest go shop that sold Yunzi, among other things (closed it down after graduating to get a 'real' job).

There was a bit of a scare about the lead content in Yunzi back then, but it's long since been dealt with. I'd be very surprised if your Yellow Mountain stones weren't lead free.

It does sound like at least one of your stones was undercooked though.

It's not that uncommon for one or two to break occasionally. The glass is very hard (which means it's also brittle) and if you slam the stones down, drop them on hard surfaces or regularly take them to go club and back (where they can be jolted against one another inside the bowls) you'll eventually have a few broken ones.

Usually when they break there's no powder. I used to get broken ones in every shipment from China - probably because because of rough handling and sea freight. The inside is normally very smooth, shiny, translucent glass and it's very unusual to see powder.

When you get new Yunzi, they'll usually be covered in a fine powder, which is a residue from the manufacturing, and most people wash the stones before using them because of that. However that's a different, finer powder to what you might see when one breaks.

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Go Game Guru: Learn Go | How to Get Better at Go | Go Game Shop | Go Boards | Baduk TV

I found this on a website called chiyodad.blogspot.com

YMI conducted its own tests and found surface-accessible lead on samples of their Yunzi.

YMI brought this matter up with the Yunnan Weiqi Company. The YWC received two new official lab results of the most recently produced Yunzi, confirming 0.19% lead in the white stones and 0% in the black. They were rather baffled because they had taken lead out of the formulation in early 2006. The lead had occurred somewhere in their supply chain and they eventually narrowed it down to their zinc supplier. The zinc supplied for use in the Yunzi contained lead. The YWC will be switching zinc suppliers and will also try making Yunzi without zinc.


In the meantime, YMI is working to put together an exchange program for their customers who have concerns about surface-accessible lead. Obviously, because the YWC is still sorting-out its sourcing, it may take as long as a month before YMI has lead-free Yunzi available. Pong Yen at YMI has advised that if anybody is immediately concerned and does not want to wait until then, they can send in their set of Yunzi stones (without their bowls) and YMI will send them a set of marble stones as a replacement.

If you want to test it get a lead check for about 6$ anywhere.

Just curious, did you even try to read the two posts above you?

For high powered flashlight you need good stones
and also need dark powders for bubble