Crazing

I was just browsing in clayart postings, have not actually subscribed
though. This interested me
I have seen out and about well named stores selling dinnerware with crazed
> glaze on the inside of bowls and mugs. Does any one have any reaction to
> this and does any one know how they can get away with it!? Since crazed
> glaze traps bacteria?? Is there something that I am missing?
Dear Claudia -
This is a very common misconception, and it is important to try to correct
it. Crazed glazes do not trap bacteria, and this is not a hygene issue.
For the last few millenia people have been eating off crazed dinnerware,
both highfire and earthenware, with no record of anyone being sickened by
bacteria trapped in the crazing. Crazing weakens the piece, especially in
highfire wares, and therefore is considered a flaw in the commercial china
industry. But then again, in some cases they use "crackle glazes" (glazes
designed to craze) on purpose for effect, since antique pottery (especially
earthenware) is often crazed, and thus many people associate crazing with a
patina of long use and great age in ceramics.
Best wishes -
- Vince
Vince Pitelka
Now while Vince has a point, I think the original person has a point too. I
do think it is about food safety, but not because the crazing collects
bacteria, but because it is a sign that it is not a good glaze.
Whats more if for instance it is a clear glaze over decoration the toxic
oxides could leach into food.
What I find more disturbing however is food vessels glazed where it looks
unsafe. For instance where it is a green glaze and there are black pools of
copper oxide in the bottom of bowls.
How seriously do you all take the issue of food safety?
annemarie
Reply to
Xtra News
In article , Xtra News writes
If you are talking earthenware, then provided the glaze is covering any decoration or has the oxide well mixed in, it will have effectively contained it during the melt stage. The exception to that rule is using Copper Oxide or Carbonate with a fritted lead based glaze, as the copper effectively cancels out the fritting process and the lead toxins will leach like anything especially with acidic foods or drinks. If you are talking stoneware then high Barium glazes are the no-no where food is concerned, particularly matt ones. The best rudimentary test is to lay a piece of lemon on a suspect glaze for 24 hours and check for discoloration. Other than that, have your work professionally tested.
Steve Bath UK
Reply to
Steve Mills
Yeah I got the lemon test from the cone 6 book, do you know it? I have seen so many glazes that look to me like barium glazes in food vessels. Plus the pools of copper oxide. Yeah I just think there are some potters out there who do not take the food safety thing seriously at all and it concerns me because first it is a health hazard and second because it reflects badly on all of us.
Reply to
Xtra News
Glazes craze because they do not "fit" the claybody - i.e., they have different expansion rates. It's perfectly possible to have a glaze that crazes on a particular clay body which is a "good glaze" in the sense that it won't leach. Of course, some crazed glazes may leach, but the two issues are not directly related.
I would add that it's not proper to copy a post from one forum to another without the author's permission. Strictly speaking, there are copyright issues, but more than that, it's just not fair to the author. In this situation, you questioned Vince's comments in a forum that he doesn't participate in. Vince is both a ceramics instructor and published author of a pottery book. He really knows his stuff.
deg
Reply to
Dewitt
Oh of course, bit pedantic there deg. I guess I should have said not a good glaze for that particular clay. I took it for granted that we all new that. Also crazing in a clear glaze over oxide, stain or underglaze decoration is not "safe". The glaze fired on a different clay or fired to a different temp may be a stable good glaze.
I consider myself flamed, I perhaps should have just talked about the discussion without cut and pasting. You could of course have moved on to talk about unsafe glazes, pools of oxides... I guess you were more interested in flaming.
Reply to
Xtra News
And here I thought my comment above was pretty darn gentle - much more so than Vince's reply would likely have been if he had run across his post copied from clayart to another forum. We can ask him if you like.
deg
Reply to
Dewitt
Whatever. Like I said I should have just talked about the discussion, rather than cut and paste, didn't think about upsetting people, just interested in the topic. Unsafe glazing concerns me. If Vince is very knowledgable, he would surely be concerned about unsafe glazing too.
Reply to
Xtra News
Whilst there are some good points on both sides of your discussion (and particularly the one about cutting and pasting as Vince's comments may also be read 'out of context' and misconstrued), I would like to pose some questions about the hygene aspect of crazing. Why does industry consider crazing a defect? Isn't any surface that is more difficult to thoroughly clean a minor health hazard? Also we should perhaps consider that the imune system of 21C man is not as readily able to throw off some infections as his predecessors, who probably built theirs up by eating off crazed pottery? During my years in catering, cracked or crazed plates would be immediately disposed of but that may have been because they were considered unsightly. Andy
Reply to
plodder
On 22 Nov 2005 13:16:59 -0800, "plodder" wrote:
The primary reasons that industry considers crazing a defect are that a piece with a crazed glaze is substantially weaker than a piece with a glaze that "fits" properly and crazing is generally considered a visual defect. Personally, I doubt that crazing presents a significant hygiene issue and have never seen any research to indicate it does, but eating off crazed and stained dinnerware is certainly unappealing.
deg
Reply to
Dewitt
Well I decided to do a bit more looking into this, I will not cut and paste anybody, perish the though :o) been there, but I will post some sites, OK This one is fairly interesting
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this one by the same person
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out of time, there were lots more sites. These two are not bad.It is something I feel strongly about that I think many potters do not seem as concerned as they should be IMO. Food safety with glazes is an issue. Talking of the past and how human kind survived is one thing, but we know that lots of people died of lead poisoning and today I trust that people are not using unsafe lead glazes.Take a look at these
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are just two glaze ingredients, one usually thought of as unsafe, the other as more safe, but you will see that we should take these things seriously. This is of course particularly important for us as potters, and I trust everyone wears masks and wet wipes to clean up etc etc.
Reply to
Xtra News
Hi Xtra News, all very interesting. I think the items you unearthed tend to direct us towards sensible caution and precautions but for the sake of 'the public at large' those who sell their wares for use with food must be even more cautious. Enough said, I hope. Andy
Reply to
plodder
Well, I'm not sure what exactly you looked into. Only one of the links you posted directly relates to food safety and crazing. The others relate to food safety and leaching of toxic materials and/or potter exposure to toxic materials. As I said before and I'll say again, crazing and leaching are two separate issues. You can have a glaze that crazes that doesn't leach significant amounts of toxic materials and a glaze that leaches that doesn't craze. Or you can have a glaze that does both or neither.
The question that was asked and the question that I answered related to crazing. Both the issue of crazing and the issue of leaching / material toxicity are important, but it is not helpful to jumble up the two.
deg
Reply to
Dewitt
Yeah, probably. Been away apologies for not getting back sooner. I saw the Portage Ceramic awards in Auckland while away, only briefly we were running out of time. Its inspiring to see good work though. I am sure that I have seen it online for previous years. This one won't be online yet because it is still open but here is an article about it and one persons work.
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Reply to
Xtra News
Well it is a complex issue, but as mentioned before if you have underglaze decoration and a crazed glaze over I believe that it could be a hazard for food safety. I think it is an issue that we all need to keep vigilant about because it is important, both unsafe glazes, or crazed glazes, esp over underglaze decoration.
Reply to
Xtra News
Lets bring up the barium and manganese dioxide issues for food safety again... these sites here say that Manganese dioxide is poisonous on the macro gram level.... as in over a thousand grams!!!! Are you going to absorb hundreds of grams of Manganese dioxide from your glazed cup? Hundreds, tens? More than likely you would have trouble measuring the amount of manganese dioxide leached from a glaze. The same is true of barium. Its poisonous on the macro gram level. I hate this. Its irresponsible not to read the fricking articles about these substances and then start this hysterical nonsense about food safety again. Barium is not lead. Lead is very dangerous. It is poisonous on a microgram level and leaches substantially from a glaze. Lead should not be used in our studios. I would be more concerned about the effects of smoking tobacco in a studio than any of the chemicals that we use. The effects upon the potter of absorbing copper through our skin; the effects of dust and fiber from kaowool on our lungs... Food safety from crazing is really low on a list of what to worry about.... Russell Andavall
Reply to
Russell Andavall
This primary concern with manganese is not leaching, but the exposure to manganese fumes by the potter during the firing process. Over time, exposure can result in Parkinson's disease like symptoms. For more info see
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while the dangers of barium may not be as extreme as some believe,it is something that potters need to be aware of. Again, the dangeris problem the greatest for potters who don't know how to handle itproperly, though the leaching issue should not be dismissed. Moreinfo at:
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Reply to
Dewitt

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