Maxine krasnow on fri 2 dec 05
I have a Cone 6 glaze (Queens), it oatmeal coloured and shiny and has Barium in it. Is there a home test I can do to test that the glaze is food safe. many thanks Maxine
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Ron Roy on sun 4 dec 05
Hi Maxine,
Send me the recipe and I may be able to tell you if there is likely to be a
problem - I may even be able to reformulate it without the barium.
RR
>I have a Cone 6 glaze (Queens), it oatmeal coloured and shiny and has
>Barium in it. Is there a home test I can do to test that the glaze is
>food safe. many thanks Maxine
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on mon 5 dec 05
Hi Rose,
Better to talk in terms of durable and stable rather than food safe - the
term become meaningless because it is used in so many situations.
There is an interesting discussion in Ceramic Glazes - by Taylor and Bull
on the subject.
It seems that - as the crystals form they rob the surrounding glaze of
silica - so while the crystals are silica enriched and more stable - the
surrounding glazes is can be silica starved and not stable.
It is also true that as the amount of colourant is increased - it is harder
to keep it in the glaze under acidic conditions - this is particularly so
with copper. Even a stable glaze can be destabilized by adding enough of a
colouring oxide for instance.
I do think you can afford to have glazes tested by the way - about $30 a shot.
In the end there may be some crystalline glazes that may be stable under
food - the question becomes - which ones are.
RR
> This is not about Barium, but has to do with glaze safety. I have one potter
>friend who thinks that crystalline glazes are PROBABLY food safe and thinks I
>should operate as though they are. I have another potter friend that thinks
>that crystalline glazes should NEVER be used on food containers because of the
>heavy metal content. I have always thought the latter was correct. What is
>your opinion? Right now, lab testing is a little out of my budget ( I don't
>advise use for food to begin with, so why test) . Just curious.
>
>Rose Downs
>Terra Madre
>www.terramadre.com
>Princeton, TX 75407-9734
>469-450-5480
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Rose on mon 5 dec 05
Ron,
This is not about Barium, but has to do with glaze safety. I have one potter
friend who thinks that crystalline glazes are PROBABLY food safe and thinks I
should operate as though they are. I have another potter friend that thinks
that crystalline glazes should NEVER be used on food containers because of the
heavy metal content. I have always thought the latter was correct. What is
your opinion? Right now, lab testing is a little out of my budget ( I don't
advise use for food to begin with, so why test) . Just curious.
Rose Downs
Terra Madre
www.terramadre.com
Princeton, TX 75407-9734
469-450-5480
Michael Wendt on mon 5 dec 05
Rose,
The cost for release testing is really quite low. Contact Alfred Analytical
to get the current pricing.
I use chun glazes which contain barium so I wanted to make sure I was not
producing pots that might poison people. Warning them not to use a bowl for
food would not be enough in a court of law since bowls change hands as yard
sales items, donations to thrift shops, gifts and inheritances. Best to be
safe.
One interesting side note: I got a copy of the EPA list for allowable levels
of various metals in drinking water more as a reference for relative
toxicity and found that barium is tolerated at twice the level of copper in
drinking water.
The copper pipes in homes release copper into the water so constant low
level exposure is likely.
Manganese, selenium and other metallics are all present in food supplements
like Ensure and in multivitamins like Centrum.
As a result, many people may be close to a toxic reaction that could be
triggered by one more addition.
At least look into testing
Oh, yeah... all the glazes I had tested fell far below the allowable level
of release even for drinking water standards
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Rose wrote:
Ron,
This is not about Barium, but has to do with glaze safety. I have one
potter
friend who thinks that crystalline glazes are PROBABLY food safe and thinks
I
should operate as though they are. I have another potter friend that thinks
that crystalline glazes should NEVER be used on food containers because of
the
heavy metal content. I have always thought the latter was correct. What is
your opinion? Right now, lab testing is a little out of my budget ( I don't
advise use for food to begin with, so why test) . Just curious.
Rose Downs
Terra Madre
www.terramadre.com
Princeton, TX 75407-9734
469-450-5480
John Hesselberth on mon 5 dec 05
Hi Rose,
Sorry, but there is not answer to your question without testing. What
'heavy metal' are you worried about? Are you using lead or cadmium in
your glazes?? If you are talking about zinc or copper or cobalt and
a 'typical' crystalline glaze, there is virtually no data available
on the durability of these glazes. I have tested a couple and they
had surprisingly little leaching of zinc; however with the low level
of alumina typically in these glazes I would expect problems with
some of them. I just don't have enough data to take an position and
I'm not aware that anyone has done much with testing their durability.
You could vinegar test them and at least get a partial answer about
their durability. If the color fades or the surface changes in sheen
after 3 days soaking in ordinary household vinegar they are not very
durable. They still may leach more than you are comfortable with though.
Regards,
John
Who was disappointed to learn the rock music use of 'heavy metal'
has become the primary definition of the term in at least one
dictionary. Fortunately 2 others still listed the chemical use of the
term (metals with sp. gr. over 5) as primary.
On Dec 5, 2005, at 8:56 AM, Rose wrote:
> Ron,
>
> This is not about Barium, but has to do with glaze safety. I
> have one potter
> friend who thinks that crystalline glazes are PROBABLY food safe
> and thinks I
> should operate as though they are. I have another potter friend
> that thinks
> that crystalline glazes should NEVER be used on food containers
> because of the
> heavy metal content. I have always thought the latter was
> correct. What is
> your opinion? Right now, lab testing is a little out of my budget
> ( I don't
> advise use for food to begin with, so why test) . Just curious.
Rose on tue 6 dec 05
Ron,
Thanks for the reply. I don't make functional ware. I have never seen a
need to test because of this fact. I have been of the same opinion as you
concerning the relocation of silica and resulting stability of the non- crystal
portion of the glaze. Glaze testing is completely out of the question right
now. My husband was laid off over two years ago and last week, we reached the
end of the rope. I am waiting for the power company to come out and turn off
the power.
Thanks,
Rose
At 11:30 PM 12/5/2005 -0500, you wrote:
>Hi Rose,
>
>Better to talk in terms of durable and stable rather than food safe - the
>term become meaningless because it is used in so many situations.
>
>There is an interesting discussion in Ceramic Glazes - by Taylor and Bull
>on the subject.
>
>It seems that - as the crystals form they rob the surrounding glaze of
>silica - so while the crystals are silica enriched and more stable - the
>surrounding glazes is can be silica starved and not stable.
>
>It is also true that as the amount of colourant is increased - it is harder
>to keep it in the glaze under acidic conditions - this is particularly so
>with copper. Even a stable glaze can be destabilized by adding enough of a
>colouring oxide for instance.
>
>I do think you can afford to have glazes tested by the way - about $30 a shot.
>
>In the end there may be some crystalline glazes that may be stable under
>food - the question becomes - which ones are.
>
>RR
Terra Madre
www.terramadre.com
Princeton, TX 75407-9734
469-450-5480
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