Does this teaware look safe?

[debunix]

There is another topic discussing the safety of raku glazes in teaware, but for those of us buying teaware without clear pedigrees, I thought it would be useful to have a separate topic to help identify characteristics of glazes that are more or less likely to be safe for daily use.

My chinatown tea shop has CA prop 65 warnings signs posted here and there, and specific warning is made about the decorative glazes on the outside of some of the ceramic teapots & cups, but most of the pottery has very little labelling besides a price tag. My favorite teacups are very small, with a glittery metallic look to the glaze on the upper portion of the inside and outside of the cup, and a thick translucent crazed glaze on the bottom of the inside:



Can anyone identify what type of glaze this is, and whether it is likely to have safety issues in daily use?

[JBaymore]

debunix,

You might have already seen my postings on ceramic toxicology in other threads here. A bit into "my field".

It is REALLY important to put this potential issue into some persepctive...... lest we get into a "Witch Hunt" mentality on the subject of glaze hazards. Probably 99 .9999999999% of the glazes on functional ceramic wares used in major countries are fine for use.

That being said.... there ARE concerns with some glazes. There are concerns other than the lead and cadmium that CA Prop 65 and the FDA laws deal with. Lead and cadmium are the only oxides currently regulated by the FDA at this point....so there is nobody looking at the other typical oxides to evalaute whether there are issues.

There is no real definitive way to just "look" at a glaze and know if it is going to leach oxides into food contents. Some nice glossy fused looking glazes are not all that stable, and some matt metallic gun-metally glazes are quite stable.

To a degree, you are depending on the "technical pedigree" of the potter or pottery manufacturer. The more technical education background they have, the more likely that there are no issues.

To indicate a bit how infrequent the issue is, let's take a quick look at what has to "happen" for there to be a concern:

To be of concern, a glaze first has to contain an oxide that has some detrimental properties to the human anatomy. Luckily the vast majority of the materials used for ceramic glazes do not have these kinds of problems. The list of potential "culprits" is rather small.

Second, the material has to be present in amounts that are a large enough percentage of the total composition to be of concern. Fortunately, many of the oxides of concern are colorants, and are typically used in small percentages.

Third, the composition of the glaze has to be such that the material is not bound well into the glassy matrix, allowing it to be released into the liquids from foodstuff.

Next, the PH of the liquids from the foodstuff directly impact the disolution of the questionable material out of the glass. The more strongly acidic or basic the liquid is, the "larger" the driving force for leaching is.

And then you have the duration of the contact between the liquid and the glaze. The longer the contact occurs, the more leaching will happen. So there is a huge difference between a cup of tea that is sitting in a cup for a few minutes to a half an hour...... and the orange juice stored in the pitcher in the fridge for a couple of days.

This is a very complex subject. There are no "quick" answers that are comprehensive. I spend a lot of time teaching college students about this subject. As they say, "you could write a book".

best,

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

[debunix]

I have not seen your other posts, and will go ahead and seek them out.

What particularly brought this up was the raku glaze discussion, where the point was made that low-fired glazes may be more likely to permit leaching of their contents, and when the content of a raku glaze is unknown, it might be safer to reserve the fancy raku glazed vessel for occasional rather than daily use. But when I call up images of raku wares on google, I see a lot of glazes that don't match my simplistic mental image of an iridescent finish. And I bet I'm not the only one who really hasn't much of a clue as to what is raku or not.

So I wondered: can someone who knows glazes look at a particular finish, and be able to guess with a fair degree of confidence that that finish style involves compound X or finish technique Y that may increase the risk of leaching compound x into your daily tea?

What I have no idea about is: are there 500 different ways to get the metallic glittery look, or the deep pool of translucent color in the bottom of the vessel, or that deep matt finish with a subtle shine; or are there actually just 10, of which 95% of inexpensive wares will probably have used the same one, which is by far simplest/cheapest? If the latter is the case, I'd be interested to know more about the one, because chances are good that's what I've got.

I'd hope that others will also post some interesting finishes, and that we might all learn enough to be more confident picking things up in local shops, or online, where the seller's first language may not be the same as ours, or where a used item's provenance is entirely unknown.

[JBaymore]

debunix,

With a lot of ceramic experience and technical training/study there are some "telltale signs" that can be recognized for certain things. But this requires a LOT of experience to spot correctly. Not the typical provenence of the layperson (ceramic-wise ).

One example here......... CERTAIN formulations of lead glazes are pretty easy to spot. They have a certain shade of yellowish cast, a certain type of surface quality, and can cause certain characteristic color renditions with certain colorants. These glazes will be fired onto a low fire body, usually redware of some sort, often fully or partially covered with a white slip under the glaze.

BUT....... other lead glazes can be idestinguishable (visually) from non lead bearing glazes. Some commercial dinnerware products have some lead in the glazes (and will not pass the CA Prop 65 laws but will pass the FDA laws). The only way to tell with those kinds of glazes is to use a labrotory leaching test to see if there is lead release. And even THAT is not definitive proof that there is no lead there. To really know the composition of the glaze you'd have to use far more sensitive and destructive lab testing. Or have access to the glaze recipe itself.

Additionally, may potters really LOVE the look of that lovely (but rather toxic) high lead glaze. So they go out of their way to formulate a glaze to emulate that lovely surface and yellowish cast of early lead glazed wares without using lead. Getting it exact is hard to do...... impossibkle really...... but you can come close. So that glaze you might see that LOOKS like a high lead glaze...... may not actually BE a high lead glazed piece.

Another example of the difficulty of recognizing this stuff for the non-technically trained ceramist....... if you have a blood red glaze on a piece, it is possible that it contains cadmium, a potentially toxic substance. But it is also possible that red is caused by the combination of iron oxide in just the correct formulation in a heavily lead based glaze ("Kutani Red"). So there is a a different toxic substance to think about. But it is also possible that the blood red is caused by copper fired in a certain way (reduction) in a non lead bearing glaze. Nothing really too toxic there. And it is also possible that it is a newer "high tech" colorant that utilizes a zirconium oxide "shell" formed around a little spec of cadmium that is refered to as an "encapsulated stain". Supposedly barring cadmium release if you believe the manufacturer's information. It is also possible that red is from a non-encapsulated cadmium compound in a lead glaze. A potential double whammy there.

SUBTLE visual clues from the above surfaces and color shades along with an analysis of the overall piece that it is fired onto will let a ceramist start to narrow down the possiblity.

What the actual level of hazard to the user is from any of the above is not easily ascertained without sophisticated testing. For example, a little thin pattern painting of Kutani Red on the inside of a cup has that lead bearing glaze only on a small fraction of the overall surface. The lead release into the contents will be a function of not only the lead release factor from the glaze formulation, but the surface area in contact with the leachate. So while the glaze itself may have a high lead release, the amount of the glaze is so small as to render the release into the cup below an area for concern.

I could easily and offhandedly say here that you should be concerend about all metallic, semi-matte to matte gunmetal looking glazes. Many times they are not "good glass" and can leach stuff. SOme of them contain manganese in high concentrations.... a bit of a potentially nasty mateial to ingest much of. But that would not really be correct and could start a thinking process in readers of this posting that ALL glazes like this are "bad". THAT would be a serious dis-service to both the potters of the world and the consumers of fine pottery. Yes, some of those kinds of glazes are of concern about leaching. But not ALL of them are.

I teach my students that the correct answer to any technical question in ceramics is, "It depends". There are so many variables involved in the formulation and firing of cermic wares that you need to be like a doctor gathering information on symptoms and test results in making a definitive diagnosis. The "Mark One Eyeball Test" (visual inspection) alone is not really a good indicator of all of this stuff.

I've mentioned before that I can give out an address for a testing lab that will test for the release of any oxide from a glaze. This is done by the labratory standard for FDA testing, and you can compare the resuilts to the FDA and CA Prop 65 standards for lead and cadmium. And to the EPA drinking water standards for other materials. It is minimally destructive testing, but they will need the exact piece shipped to them. It will cost you about $30 for the basic testing fee, and about $30 per oxide you have them test for. So a test for both lead and cadmium release will cost you about $90, plus two way shipping and insurance on the piece.

Again I want to stress here that the frequency of this actually being an issue is not very high. In the US potters can get product liability insurance for this and all other issue VERY cheaply. If it were a big problem,...... I couldn't buy $2 mil. of coverage for well under $1000 a year.

best,

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

[debunix]

Clearly this is way more complicated than I had hoped.

Thank you for all of the insight.

[GreenwoodStudio]

Hi Debunix-
As John has mentioned it's pretty impossible to tell from the photo if this glaze would leach anything harmful. I wanted to chime in however and tell you that the glittery glaze that's on the this cup is something called an Adventurine glaze. I'm a bit obsessed with this type of glaze and at one point spent over a year testing variables to achieve results that I was satisfied with. Good examples just fascinate me. I never used lead in my glazes but often the best examples do have lead in them in some form. I don't say this to scare you, and have no way to really know what your glazes contain or if they leach anything harmful for that matter. I'll just say that if that were my cup, I probably couldn't help myself and would use it, it's beautiful!!

[Maitre_Tea]

I've seen these types of cups being sold in Taiwan, specifically in fairly high traffic tea shops, so the stuff is probably safe. If they're the same type of glaze, than it should be safe for drinking tea out of.

[Artizen]

Here,Here Jbaymore and thank you for putting the facts on the table.
I agree that too much worry is put into this issue. I also work professionally in clay, though not as a potter anymore but rather as a sculptor. I was in collage 35 years ago though taking pottery classes when this issue first raised it's ugly head.We followed what happened with great interest. I think that quite a bit of the hype in the beginning concerned politics. China and Mexico were starting to really move into the American marketplace with cheap dinnerware. This upset many american manufacturers. Claims started to be made that foreign wares were unsafe because of toxic levels. When the matter was looked into it started to backfire. It turned out that a very popular american dinnerware known as Fiestaware (the old...the new is safe) contained high lead levels in their reds, yellows, and oranges. Suddenly there was a huge scare in the public sector concerning glaze safety. Later on with the introduction of the microwave any metallic glazes became a no no.
In government articles we read at the time one of the testing process really stood out. It seems that they fed rats a Cup! of lead a Day! After two weeks the rats died. Personally I often wondered if a rat's stomach was even large enough to hold a cup of anything. At the time I thought that it seemed a strange benchmark. Though I totally agree that staying away from any toxins is a good thing.
Even then it was concluded that dinnerware containing some lead was not that dangerous as long as it was not subjected to heat. Also it was advised that drinking vessels be avoided because of skin contact.
Lead in glazes act as a flux (allow them to flow) so that minerals that normally could not be used at earthenware temps were now available to the potter so certain colors could be achieved at lower temps. It really is more complicated but lets not get into it here. But now we have safe substitutes and most formulas available to potters are labeled either food safe or not. For the lay person I would say that most earthenware produced today in this country is food safe. Professional potters are reminded constantly of this by their clients asking "is this really safe"? So they tend to ere on the side of caution. When you get into the higher temp ware such as stoneware it was never really much of an issue even 32 years ago.
That being said I would be cautious of Raku ware (the process....not the maker) and ware exhibiting a large amount of crazing in the glaze. I caution not because of toxic glazes (though some raku glazes do have lead) but because crazing equals cracks. Food can become trapped and bacteria can grow. Again this is more of an issue with more porous clays such as earthenware or raku than stoneware.

[dougan]

And don't we all like to drink our (higly acidic) Champagne and other bubblies out of crystal glasses which contain a minimum of 24% PbO...lead oxide? Just a thought.....

[debunix]

Probably most of us in this conference drink a heck of a lot more tea out of our favorite teacups than we do champagne out of lead crystal glassware.

I doubt I have ever used a true lead crystal glass, period.