Paul Herman on wed 26 feb 03
Hello Ron,
I'm curious about your 'no way' statement below.
You wrote: if you even overload any glaze with enough colouring oxides
it
> will start leaching
I ask: Leaching what?
It seems to me that if the glaze and colorants in question contain no
toxic materials like copper, barium, etc., then it would still be "food
safe". Is cobalt a material of potential concern? I thought it was one
of the non-toxic colorants. Or is it only safe when used in small
amounts, like 1% or less?
An example would be: If I paint Iron or Rutile on top of a ^11 celadon
glaze (which contains feldspar, silica, whiting, and kaolin + 1% Iron)
would it make it NOT "food safe"? And if so, how? I think unstable
glazes may leach toxics, but only if they contain toxics.
Thanks and Best wishes,
Paul Herman
Great Basin Pottery
423-725 Scott Road
Doyle, California 96109 US
potter@psln.com
----------
>From: Ron Roy
> Hi M
>
> No way - if you even overload any glaze with enough colouring oxides it
> will start leaching - best to take the suggestion of putting it under the
> glaze.
>
> A better way is to mix the cobalt with the glaze - in the ratio that gives
> you the colour you want and put it over - or even better - under.
>
> There is a potential problem here - cobalt is a strong flux at cone 6 and
> will tend to bleed unless the glaze is quite stiff.
>
> Now I am thinking - is it just a wash - a very thin watery solution - and
> only on the out side - maybe that would be OK - especially if combined with
> a good stable glaze (read high in SiO2 and Al2O3.)
>
> RR
>
>>If you paint cobalt oxide over a food safe glaze, is the glaze still food
>>safe?
>>
>>thanks,
>>M
>
Paul Herman on thu 27 feb 03
Hello John,
The original post asked about the "food safeness" of cobalt decoration
on glaze.
Of course I don't want my pots to fade, but I have not noticed this
phenomenon in my glazes. I might add that my pottery display lives
outside in the blazing desert sun, and pots are torture tested with
tomatoes, lemons and vinegar in the bachelor potter kitchen. However,
they are fired at ^11, hence great stability, hardness, etc. by virtue
of high SiO2 and Al2O3 content.
I am curious about the fading mugs you have. What kind of glaze, pigment
and firing temp?
Best wishes,
Paul Herman
Great Basin Pottery
423-725 Scott Road
Doyle, California 96109 US
potter@psln.com
----------
>From: John Hesselberth
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: Oxides over glaze RR
>Date: Thu, Feb 27, 2003, 12:40 PM
>
> Hi Paul,
>
> Regardless of whether or not it is "food safe" -- whatever that means --
> do you really want functional pottery that is not durable and that fades
> over time? When the colorants leach out the color changes. I have some
> mugs like that that I purchased from another potter. They really gross
> me out. I only use them now as a demonstration of how bad glazes can be
> if not properly formulated.
>
> Regards,
>
> John
John Hesselberth on thu 27 feb 03
Hi Paul,
Regardless of whether or not it is "food safe" -- whatever that means --
do you really want functional pottery that is not durable and that fades
over time? When the colorants leach out the color changes. I have some
mugs like that that I purchased from another potter. They really gross
me out. I only use them now as a demonstration of how bad glazes can be
if not properly formulated.
Regards,
John
On Wednesday, February 26, 2003, at 05:02 PM, Paul Herman wrote:
> It seems to me that if the glaze and colorants in question contain no
> toxic materials like copper, barium, etc., then it would still be "food
> safe".
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Hesselberth on fri 28 feb 03
Hi Paul,
I don't know the recipe, but they were glazed and then fired in a salt
kiln to cone 10-11. Colorants were probably copper and manganese. They
have faded only on the inside when the salt glaze seems much lighter
and, of course, where acidic, hot coffee sat in them on a daily basis
until we noticed the fading. They were among our favorite mugs until
then.
Regards,
John
On Friday, February 28, 2003, at 01:23 AM, Paul Herman wrote:
> I am curious about the fading mugs you have. What kind of glaze, pigment
> and firing temp?
http://www.frogpondpottery.com
http://www.masteringglazes.com
Edouard Bastarache on sat 1 mar 03
Hello Paul,
if you want more information on cobalt go to:
http://www.digitalfire.com/education/toxicity/
and click on cobalt.
Later,
"Ils sont fous ces Quebecois"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://perso.wanadoo.fr/smart2000/index.htm
Ron Roy on sat 1 mar 03
Hi Paul,
It depends on how you translate "food safe" When I see that question framed
in that way my reaction is on a broad level.
I think - he is asking if the glaze will be affected by acid or alkaline
conditions.
Yes it will - and anyone using that ware will eventually notice it - in the
way the glaze looks or perhaps in the way it affects the taste of food.
I don't know if Cobalt is a health hazard - it depends on a number of
factors - how much is in the local water, the individual reaction to cobalt
- how much of a load does any individual have already.
Let me use myself as an example - I store iron - it's called
haemochromatosis - one of the most commonly inherited diseases in NA. So if
you start painting iron on the outside of a glaze - which is in contact
with the food I am eating - then it is toxic to me.
My point is - we are not supposed to be providing trace elements with out
glazes - it is our job - as pottery makers - to make stable glazes for
those parts of our ware that come in contact with food.
My second point is - what do we do to the sales of hand made pottery when
we use glaze that change in use? I think it will be better for all of us if
we develop a conservative attitude to this subject.
To address your leaching question - leaching what? - the answer is
everything in the glaze.
You are right - if there are no toxics in the glaze then why worry. My
answer to that is - it depends how much and we don't know everything about
toxicity - a very difficult subject to address. We do know that cancer is
right up there as a killer - and we do know there are more chemicals in the
inviroment now than there ever have been - and many scientists see a
connection there.
I'm just saying - if you paint cobalt on top of a glaze - even if the glaze
started off stable you may upset the balance - and I suggested two ways to
minimize the effect.
RR
>I'm curious about your 'no way' statement below.
>
>You wrote: if you even overload any glaze with enough colouring oxides
>it
>> will start leaching
>
>I ask: Leaching what?
>
>It seems to me that if the glaze and colorants in question contain no
>toxic materials like copper, barium, etc., then it would still be "food
>safe". Is cobalt a material of potential concern? I thought it was one
>of the non-toxic colorants. Or is it only safe when used in small
>amounts, like 1% or less?
>
>An example would be: If I paint Iron or Rutile on top of a ^11 celadon
>glaze (which contains feldspar, silica, whiting, and kaolin + 1% Iron)
>would it make it NOT "food safe"? And if so, how? I think unstable
>glazes may leach toxics, but only if they contain toxics.
>
>Thanks and Best wishes,
>
>Paul Herman
>Great Basin Pottery
>423-725 Scott Road
>Doyle, California 96109 US
>potter@psln.com
>
>----------
>>From: Ron Roy
>> Hi M
>>
>> No way - if you even overload any glaze with enough colouring oxides it
>> will start leaching - best to take the suggestion of putting it under the
>> glaze.
>>
>> A better way is to mix the cobalt with the glaze - in the ratio that gives
>> you the colour you want and put it over - or even better - under.
>>
>> There is a potential problem here - cobalt is a strong flux at cone 6 and
>> will tend to bleed unless the glaze is quite stiff.
>>
>> Now I am thinking - is it just a wash - a very thin watery solution - and
>> only on the out side - maybe that would be OK - especially if combined with
>> a good stable glaze (read high in SiO2 and Al2O3.)
>>
>> RR
>>
>>>If you paint cobalt oxide over a food safe glaze, is the glaze still food
>>>safe?
>>>
>>>thanks,
>>>M
>>
>
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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
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