low fire glaze failed the vinegar test - questions for

updated wed 24 may 00

John Hesselberth on mon 22 may 00

ClayArt added

Hi Rachael and Anji,

I'm not an expert on low fire glazes, but I will take a shot anyway.
Hopefully Ron Roy or some of the other glaze-knowledgeable people will
look at this one also. This glaze is low on alumina and high in lithium
for an 04 glaze. Most of the other numbers seem to be OK; although I
think you normally see more sodium and/or potassium in a low fire glaze.
Since you also said it is matt, I am betting this glaze gets its mattness
from being just plain underfired. Underfired glazes are the most
unstable of all--if you haven't formed a good melt, then you can't expect
it to hold much of any colorant. And of course, as I've pointed out many
times, copper is the most difficult of all to hold. Anji, copper will
also make food taste bitter at 5-10 ppm (or mg/l). You probably got a
few days worth of copper when you drank that stale coffee. I'm betting
this one will leach well over 100 ppm in the standard leaching test from
what you describe. Rachael, I would label this glaze for NON FUNCTIONAL
WORK ONLY in big red letters and forget about it. I wouldn't even begin
to try to rescue it for functional work. I will put it in my collection
of unstable glaze recipes, though, and maybe I can do a better job of
understanding it someday.

This gives me an excuse to make a plea to those working with low fire
glazes. Please have some of them leach tested. Particularly if they
contain significant amounts of colorants like copper, cobalt, manganese,
nickel or chromium. So far, no one has shared any low fire results with
me. I suspect there are some really bad low fire glazes out there. I am
going to start testing some myself this summer, but I'd sure like to have
some others help scan the field to see how good or bad the situation
really is. As always, testing instructions can be found on my web site
at:

http://www.frogpondpottery.com/glazetest.html

John

Anji Henderson wrote:

>I too have used this glaze formulation. And found DO
>NOT put it on a coffee cup.. I used this cup for all
>of two hours.. I am a slow and long coffee-er.. So the
>first few sips were fine, then with out fail I got
>involved with the computer and it sat for a while..
>PeTuiee!!! It tasted horrible.. No doubt there is
>leaching of something.. I couldn't say I know it is
>bad for you but that taste, it can't be good.. What
>ever it is I would say that even with it being over
>clear or Majolica it is still there, just diluted. The
>person that made this glaze is not a functional
>potter, but did say to me it was food
>safe...???....But never said it was food functional..
>
>This is what I would like to know out of curiosity.. (
>I have already resigned these beautiful colors to non
>functional wares, and not vases (it leaks faster then
>commercial low fire glaze)
>
>--What is it in the glaze that is making it wrong?
>
>--Is there any thing that can fix it and keep the same
>colors?? (like more silica or something)
>
>--what happened to the color in Rachael's test??
>(looking for a more scientific like answer)
>
>--If there is one additive that is dangerous, what is
>it and is it because of percentage, or combination AND
>is it bad in all glazes??
>
>Gee, sorry Rachael I didn't mean to pick your post
>apart, but you hit a topic I have been curious about
>for a year, and you opened the door.. : )
>
>Anji
>
>
>
>
>
>--- Rachael Rice wrote:
>> We had been experimenting with low fire glazes at
>> school this semester.
>> I turned several white earthenware bowls which I
>> experimented with many
>> different combinations of commercial undergalzes &
>> glazes . A few of my
>> bowls came out beautiful but I didn't trust their
>> food safety so I put
>> vinegar in them and found that after just several
>> hours they had a white
>> ring around them and the beautifulness of the bottom
>> was gone.
>> Do I assume this is not a combination I can use on
>> functional pottery?
>> Here is what I did. I painted apple green
>> underglaze on the bowl and
>> then dipped it in a commercial Campbell's turquoise
>> and royal blue
>> glaze. I dipped the rim in Campbell's gloss black
>> glaze. Then I dipped
>> the bowl in a low fire copper matte glaze that a
>> teacher mixed up.
>> Here's the formula:
>>
>> Gerstely Borate 380 grams
>> Lithium 100 grams
>> NepSy 50 grams
>> Silica 420 grams
>> EPK 50 grams
>> Cu 20 grams
>>
>> It was fired to cone 04. We used this matte alone
>> on bowls and found
>> that oils from food stained the surface and decided
>> if we put it on over
>> a commercial gloss or majolica it sealed it better.
>> Any advise would be
>> appreciated.
>>
>> Rachael.
>>
>>
>__________________________________________________________________________
>____
>> Send postings to clayart@lsv.ceramics.org
>>
>> You may look at the archives for the list or change
>> your subscription
>> settings from http://www.ceramics.org/clayart/
>>
>> Moderator of the list is Mel Jacobson who may be
>> reached at melpots@pclink.com.
>
>
>__________________________________________________
>Do You Yahoo!?
>Send instant messages & get email alerts with Yahoo! Messenger.
>http://im.yahoo.com/
>
>__________________________________________________________________________
>____
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.


John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com

"Pots, like other forms of art, are human expressions: pleasure, pain or
indifference before them depends upon their natures, and their natures
are inevitably projections of the minds of their creators." Bernard
Leach, A Potter's Book.

Anji Henderson on tue 23 may 00

ClayArt added

<> glaze gets its mattness
> from being just plain underfired. >>
You may have something here.... With the few things I
used this glaze for the mug was actually the only
thing of mine that looked vitrifyed, and I am sure
that that kiln was ummm way over fired (oops)... the
rest were a dry and/or crateriee' looking blue.. But
the mug being over fired and the glaze LOOKING cured
apparently did not help the situation any.. As I said
the coffee may have set for an hour or two and it was
disgusting.. Although, the mug is a pretty brush
holder in my studio .. : )

<> few days worth of copper when you drank that stale
> coffee. >>
So I am gona die right?? Hehehehehe.. It was a year
ago.. I think if I was going to die it would have
happened by now.. BUT.. This is an issue since this
glaze is a staple in two campus' in our area.. Umm
campuses that have lot's of beginers.. What if I had
never heard of leaching, and happened to buy a new
brand of cheap coffee?? I could have just thought the
coffe was a bad brand and the coffee was bitter.....
Then I (and I am sure others are as stubron and cheap
as I) would have just dealt with the bitter coffee...
Wow, how much leaching could occur (of course you cant
answer this w/o testing)?? And how detramental could
it be??

Anji





--- John Hesselberth wrote:
> Hi Rachael and Anji,
>
> I'm not an expert on low fire glazes, but I will
> take a shot anyway.
> Hopefully Ron Roy or some of the other
> glaze-knowledgeable people will
> look at this one also. This glaze is low on alumina
> and high in lithium
> for an 04 glaze. Most of the other numbers seem to
> be OK; although I
> think you normally see more sodium and/or potassium
> in a low fire glaze.
> Since you also said it is matt, I am betting this
> glaze gets its mattness
> from being just plain underfired. Underfired glazes
> are the most
> unstable of all--if you haven't formed a good melt,
> then you can't expect
> it to hold much of any colorant. And of course, as
> I've pointed out many
> times, copper is the most difficult of all to hold.
> Anji, copper will
> also make food taste bitter at 5-10 ppm (or mg/l).
> You probably got a
> few days worth of copper when you drank that stale
> coffee. I'm betting
> this one will leach well over 100 ppm in the
> standard leaching test from
> what you describe. Rachael, I would label this
> glaze for NON FUNCTIONAL
> WORK ONLY in big red letters and forget about it. I
> wouldn't even begin
> to try to rescue it for functional work. I will put
> it in my collection
> of unstable glaze recipes, though, and maybe I can
> do a better job of
> understanding it someday.
>
> This gives me an excuse to make a plea to those
> working with low fire
> glazes. Please have some of them leach tested.
> Particularly if they
> contain significant amounts of colorants like
> copper, cobalt, manganese,
> nickel or chromium. So far, no one has shared any
> low fire results with
> me. I suspect there are some really bad low fire
> glazes out there. I am
> going to start testing some myself this summer, but
> I'd sure like to have
> some others help scan the field to see how good or
> bad the situation
> really is. As always, testing instructions can be
> found on my web site
> at:
>
> http://www.frogpondpottery.com/glazetest.html
>
> John
>
> Anji Henderson wrote:
>
> >I too have used this glaze formulation. And found
> DO
> >NOT put it on a coffee cup.. I used this cup for
> all
> >of two hours.. I am a slow and long coffee-er.. So
> the
> >first few sips were fine, then with out fail I got
> >involved with the computer and it sat for a while..
> >PeTuiee!!! It tasted horrible.. No doubt there is
> >leaching of something.. I couldn't say I know it is
> >bad for you but that taste, it can't be good.. What
> >ever it is I would say that even with it being over
> >clear or Majolica it is still there, just diluted.
> The
> >person that made this glaze is not a functional
> >potter, but did say to me it was food
> >safe...???....But never said it was food
> functional..
> >
> >This is what I would like to know out of
> curiosity.. (
> >I have already resigned these beautiful colors to
> non
> >functional wares, and not vases (it leaks faster
> then
> >commercial low fire glaze)
> >
> >--What is it in the glaze that is making it wrong?
> >
> >--Is there any thing that can fix it and keep the
> same
> >colors?? (like more silica or something)
> >
> >--what happened to the color in Rachael's test??
> >(looking for a more scientific like answer)
> >
> >--If there is one additive that is dangerous, what
> is
> >it and is it because of percentage, or combination
> AND
> >is it bad in all glazes??
> >
> >Gee, sorry Rachael I didn't mean to pick your post
> >apart, but you hit a topic I have been curious
> about
> >for a year, and you opened the door.. : )
> >
> >Anji
> >
> >
> >
> >
> >
> >--- Rachael Rice wrote:
> >> We had been experimenting with low fire glazes at
> >> school this semester.
> >> I turned several white earthenware bowls which I
> >> experimented with many
> >> different combinations of commercial undergalzes
> &
> >> glazes . A few of my
> >> bowls came out beautiful but I didn't trust their
> >> food safety so I put
> >> vinegar in them and found that after just several
> >> hours they had a white
> >> ring around them and the beautifulness of the
> bottom
> >> was gone.
> >> Do I assume this is not a combination I can use
> on
> >> functional pottery?
> >> Here is what I did. I painted apple green
> >> underglaze on the bowl and
> >> then dipped it in a commercial Campbell's
> turquoise
> >> and royal blue
> >> glaze. I dipped the rim in Campbell's gloss black
> >> glaze. Then I dipped
> >> the bowl in a low fire copper matte glaze that a
> >> teacher mixed up.
> >> Here's the formula:
> >>
> >> Gerstely Borate 380 grams
> >> Lithium 100 grams
> >> NepSy 50 grams
> >> Silica 420 grams
> >> EPK 50 grams
> >> Cu 20 grams
> >>
> >> It was fired to cone 04. We used this matte
> alone
> >> on bowls and found
> >> that oils from food stained the surface and
> decided
> >> if we put it on over
> >> a commercial gloss or majolica it sealed it
> better.
> >> Any advise would be
> >> appreciated.
> >>
> >> Rachael.
> >>
> >>
>
>__________________________________________________________________________
> >____
> >> Send postings to clayart@lsv.ceramics.org
> >>
> >> You may look at the archives for the list or
> change
> >> your subscription
> >> settings from http://www.ceramics.org/clayart/
> >>
> >> Moderator of the list is Mel Jacobson who may be
> >> reached at melpots@pclink.com.
> >
> >
> >__________________________________________________
> >Do You Yahoo!?
> >Send instant messages & get email alerts with
> Yahoo! Messenger.
> >http://im.yahoo.com/
> >
>
>__________________________________________________________________________
> >____
> >Send postings to clayart@lsv.ceramics.org
> >
> >You may look at the archives for the list or change
> your subscription
> >settings from http://www.ceramics.org/clayart/
> >
> >Moderator of the list is Mel Jacobson who may be
> reached at
> >melpots@pclink.com.
>
>
> John Hesselberth
> Frog Pond Pottery
> P.O. Box 88
> Pocopson, PA 19366 USA
>
=== message truncated ===


__________________________________________________
Do You Yahoo!?
Send instant messages & get email alerts with Yahoo! Messenger.
http://im.yahoo.com/