Crazing

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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



Re: Crazing
ospam.annemariebutler.com> writes
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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
--
Steve Mills
Bath
UK

Re: Crazing

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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.



Re: Crazing
On Fri, 11 Nov 2005 12:04:35 +1300, "Xtra News"

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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

Re: Crazing

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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.

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  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.



Re: Crazing
On Mon, 14 Nov 2005 07:41:08 +1300, "Xtra News"

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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

Re: Crazing

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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.



Re: Crazing
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


Re: Crazing
wrote:

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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

Re: Crazing

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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
http://ceramic-materials.com/cermat/education/271.html
and this one by the same person
http://ceramic-materials.com/cermat/education/144.html

Run 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
http://www.lenntech.com/Periodic-chart-elements/Cu-en.htm

http://www.jtbaker.com/msds/englishhtml/m0715.htm

These 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.



Re: Crazing
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


Re: Crazing

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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.
http://www.thebigidea.co.nz/article.php?sid=2982&mode=&order=0



Re: Crazing
On Fri, 25 Nov 2005 14:30:35 +1300, "Xtra News"

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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

Re: Crazing

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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.



Re: Crazing

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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

Xtra News wrote:

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
  <title></title>
</head>
<body>
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 &nbsp;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.... <br>
Russell Andavall<br>
<br>
Xtra News wrote:<br>
  <pre wrap="">"Dewitt" <a class="moz-txt-link-rfc2396E"
wrote in message
<a class="moz-txt-link-freetext"
  </pre>
  <blockquote type="cite">
    <pre wrap="">On 22 Nov 2005 13:16:59 -0800, "plodder" <a
class="moz-txt-link-rfc2396E"
wrote:

    </pre>
    <blockquote type="cite">
      <pre wrap="">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?
      </pre>
    </blockquote>
    <pre wrap="">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
    </pre>
  </blockquote>
  <pre wrap=""><!---->
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
<a class="moz-txt-link-freetext"
href="http://ceramic-materials.com/cermat/education/271.html ">http://ceramic-materials.com/cermat/education/271.html </a>
and this one by the same person
<a class="moz-txt-link-freetext"
href="http://ceramic-materials.com/cermat/education/144.html ">http://ceramic-materials.com/cermat/education/144.html </a>

Run 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
<a class="moz-txt-link-freetext"
href="http://www.lenntech.com/Periodic-chart-elements/Cu-en.htm ">http://www.lenntech.com/Periodic-chart-elements/Cu-en.htm </a>

<a class="moz-txt-link-freetext"
href="http://www.jtbaker.com/msds/englishhtml/m0715.htm ">http://www.jtbaker.com/msds/englishhtml/m0715.htm </a>

These 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.


  </pre>
</blockquote>
<br>
</body>
</html>

--------------020909050208010303060807--


Re: Crazing
On Tue, 29 Nov 2005 07:24:29 GMT, Russell Andavall

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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 http://ceramic-materials.com/cermat/education/155.html
and http://www.ceramicstoday.com/articles/080999.htm

And 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 danger
is problem the greatest for potters who don't know how to handle it
properly, though the leaching issue should not be dismissed.  More
info at:
http://ceramic-materials.com/cermat/education/162.html

deg

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