will edwards on tue 21 nov 00
Hello,
No way to fully describe the in's and outs regarding toxins being release=
d at
^10 but be assured they can come out at any cone. First getting heavy met=
als
like cadmium or softer ones like lead and copper out or cut down to a lev=
el
that is known to stay in. (tested parameters)
I don't confess to be a guru at all but I did own my own manufacturing
business that made colorants for artists and thus I did learn some things=
=2E
Find the best guide-lines through sources like Hamers and see what the li=
mits
are for glazes in the categories you would use.
Take and do a saturation level on several pieces you normally would use =
maybe
3% copper in. Use 5% or more and see how well it comes back from the lab =
based
on the formulae.
Gun metal glazes I noticed have been used on some items for food service.=
I
generally would be worried over that since many times it is an over-suppl=
y of
copper or Manganese or both.
I understand that they are people writing articles on such like John H an=
d Ron
R. but some people will not ever see these articles in print and thus wor=
d of
mouth is important. (Hint - post them on your web sites) Then I could tel=
l
everyone where to look! Tell those you know that certain information is
available to them but never trust a recipe without fully understanding th=
e TLV
and LD40 and point of saturation and the unity limits. I still have a tim=
e
with limits because so many have their minds made up which works best.
I still use boron as a flux in unity but hear I shouldn't do that. RO col=
umns
and all that go's with this business can really create havoc. About the t=
ime
you understand one thing they will be two more that pops up!
John would you agree that sometimes a higher firing temperature could eve=
n
reverse the potential of a glaze to allow for toxic releases due to break=
down
or stressors of improperly formulated materials. Lets say a glaze that ca=
n
take ^8 but fired to ^10 etc..
William Edwards
Opelika Arts Center
____________________________________________________________________
Get free email and a permanent address at http://www.netaddress.com/?N=3D=
1
John Hesselberth on wed 22 nov 00
will edwards wrote:
>John would you agree that sometimes a higher firing temperature could even
>reverse the potential of a glaze to allow for toxic releases due to breakdown
>or stressors of improperly formulated materials. Lets say a glaze that can
>take ^8 but fired to ^10 etc..
Hi Will,
I have seen no evidence that firing higher will increase leaching. That
doesn't mean it can't happen under some circumstances; just that I have
never seen it in a lot of testing.
There are 2 basic reasons a glazes leaches: 1) it's poorly formulated
and/or 2) it is underfired. If it is poorly formulated it will leach no
matter what temperature it is fired to. If it is underfired leaching
will go down as firing temperature is increased and then, from what I
have seen, leaching will level off. I have purposely overfired a glaze
by 3 or 4 cones and seen essentially level leaching numbers. A glaze
can, of course, can be poorly formulated AND underfired. If that is the
case leaching will decrease until the glaze is fired to maturity and
leaching will then level off at a high level.
Regards, John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
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