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low fire glaze failed the vinegar test - questions for clayart

updated tue 23 may 00


Anji Henderson on mon 22 may 00


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.. : )


--- 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.
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