Hanne Bjorklund on mon 17 may 99
A letter to the editor in NZ Claynews, (March/April 1999) voiced concern
over Barium in two popular blue club glazes. The writer states further:
> "The fact that both glazes are fired to 1250c with one hour soak is no
guarantee there is no metal release. The type of flux and other
ingredients in the glaze and the type of clay the
glaze is used on are also critical. As stated in Potters Beware (revised
1996) under 'Hazards to Customers': - "Barium can be leached from high as
well as low fired glazes if not made and fired completely". This means the
glaze formula must fit the clay perfectly and be mature under specific
firing conditions. These vary if oxidized or reduced. The only absolute
guarantee the glaze is safe is to have it tested by a certified laboratory.
"<
I agree with the last sentence , but I have always believed that Barium is
NEVER safe on pottery intended for food. Is there a clever Clayart'er who
would like to comment on whether there are some circumstances, as outlined
in the letter, that could render Barium safe?
HANNE
bjorklund@clear.net.nz
Ron Roy on wed 26 may 99
The question is - what is a safe level of barium leaching out of glazes
into food. I don't know the answer to that - seems to me it's not a simple
question - and neither are the answers.
How much barium does it take to kill a one pound rat? How much barium to
kill a one pound fetus? We know how much it takes to kill a rat - well we
could find out. But how do we find out about the other situation.
Lets say we can come up with a safe PPM for Barium - some take the
allowable limits for water as a safe level - if the government says it's
safe it must be - right? - well maybe is the answer here I think.
Then the question becomes - can I make a glaze that releases less then the
allowable - yes I can - but there is a limit - If you are going to start
with 30% Barium Carb. in the glaze it may be impossible to stay under the
allowable. If you have less than 10% Barium it can be done I am sure -
especially at the higher temperatures. But if there is less than 10% barium
in a glaze - whats it there for - not much influence on colour or surface -
none if its a white glaze - why not replace it with Calcium or strontium -
or both?
Why is it there in the first place? - who ever made the glaze simply didn't
understand the issue or the mechanics of glaze making - probably used it
because it was there. Probably did a triaxal line blend and picked the
glaze that looked the best - no idea of durability or expansion even
entered the process. It's the way it was and the way it is - but it's
getting better.
RR
>----------------------------Original message----------------------------
>A letter to the editor in NZ Claynews, (March/April 1999) voiced concern
>over Barium in two popular blue club glazes. The writer states further:
>
>> "The fact that both glazes are fired to 1250c with one hour soak is no
>guarantee there is no metal release. The type of flux and other
>ingredients in the glaze and the type of clay the
>glaze is used on are also critical. As stated in Potters Beware (revised
>1996) under 'Hazards to Customers': - "Barium can be leached from high as
>well as low fired glazes if not made and fired completely". This means the
>glaze formula must fit the clay perfectly and be mature under specific
>firing conditions. These vary if oxidized or reduced. The only absolute
>guarantee the glaze is safe is to have it tested by a certified laboratory.
>"<
>
>I agree with the last sentence , but I have always believed that Barium is
>NEVER safe on pottery intended for food. Is there a clever Clayart'er who
>would like to comment on whether there are some circumstances, as outlined
>in the letter, that could render Barium safe?
>
>HANNE
>bjorklund@clear.net.nz
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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