Joan & Tom Woodward on tue 18 may 99
Oh Wise Ones of the List,
I am rising from the depths of despondency after unloading the kiln filled =
with
pots bearing what I thought were reliable glazes (having tested from the =
same
batch) only to find I had a load of serious UGLIES. So, I ask for your help
with three questions:
First, I used a glaze which I think is called GCX:
Kon F-4 Feldspar 48.4
Zinc oxide 29.5
Whiting 9.4
Flint 13.8
Cobalt carb .5
Copper carb 4.0
Rutile 5.0
I fired Cone 5 (oxidation) with a 15 minute soak at temperature. Clay was =
Idaho
Buff, which has a fair amount of grog. But I'd tested this glaze on it =
before
with good results. Witness cone was bent to shelf. Came out with major
craters, particularly inside. I could call it my moon bowl.
The bowl has some nice underglazing work on the outside, so I think it's =
worth
an attempt to save.
I'm wondering if I put a clear over it and refired, whether the capacity of =
the
clear to run would possible cause the glaze on the craters to move too. Any
thoughts on this or other suggestions? Also, I've read/been told that the =
soak
should be at temperature=3B alternatively that it should be 200 degrees =
down from
temperature. Is there a right answer?
On a different topic, a kind person replied to one of my posts about finding=
a
cost effective clear glaze by recommending the Duncan food safe glaze. I =
used
that when I was taking classes, but got nervous because of the lead in the
glaze. It's Duncan GL 612. Says =22This glaze when fired to cone 06 or =
hotter
complies with FDA guidelines for lead and cadmium release.=22 Also says =
=22Conforms
to ASTM 0-4236.=22 I wish I could say that I always trusted what the =
government
told me, but it ain't so. Is it safe?
Finally, I recently got some propylene glycol from the good folks at Clay =
Art
Center in Seattle to help make a dipping glaze brushable. Can I use that in=
any
old glaze if I want to use it for brushing?
I'll really appreciate any help on any of my questions. Hope it=3Bs not too=
much
for a Monday morning=21
Joan, slightly waterlogged from a weekend kayaking symposium in rare Alaska
gorgeous weather.
Barney Adams on wed 19 may 99
I've been able to clear up some pin-holes by
applying another glaze with good melt over the original
and re-firing. I used corn sryup applying a heavier amount
to the pin hole before dipping. The just re-fired in with
another normal glaze load.
Barney
Joan & Tom Woodward wrote:
>
> ----------------------------Original message----------------------------
> Oh Wise Ones of the List,
>
> I am rising from the depths of despondency after unloading the kiln filled wit
> pots bearing what I thought were reliable glazes (having tested from the same
> batch) only to find I had a load of serious UGLIES. So, I ask for your help
> with three questions:
>
> First, I used a glaze which I think is called GCX:
>
> Kon F-4 Feldspar 48.4
> Zinc oxide 29.5
> Whiting 9.4
> Flint 13.8
>
> Cobalt carb .5
> Copper carb 4.0
> Rutile 5.0
>
> I fired Cone 5 (oxidation) with a 15 minute soak at temperature. Clay was Ida
> Buff, which has a fair amount of grog. But I'd tested this glaze on it before
> with good results. Witness cone was bent to shelf. Came out with major
> craters, particularly inside. I could call it my moon bowl.
> The bowl has some nice underglazing work on the outside, so I think it's worth
> an attempt to save.
>
> I'm wondering if I put a clear over it and refired, whether the capacity of th
> clear to run would possible cause the glaze on the craters to move too. Any
> thoughts on this or other suggestions? Also, I've read/been told that the soa
> should be at temperature; alternatively that it should be 200 degrees down fr
> temperature. Is there a right answer?
>
> On a different topic, a kind person replied to one of my posts about finding a
> cost effective clear glaze by recommending the Duncan food safe glaze. I used
> that when I was taking classes, but got nervous because of the lead in the
> glaze. It's Duncan GL 612. Says "This glaze when fired to cone 06 or hotter
> complies with FDA guidelines for lead and cadmium release." Also says "Confor
> to ASTM 0-4236." I wish I could say that I always trusted what the government
> told me, but it ain't so. Is it safe?
>
> Finally, I recently got some propylene glycol from the good folks at Clay Art
> Center in Seattle to help make a dipping glaze brushable. Can I use that in a
> old glaze if I want to use it for brushing?
>
> I'll really appreciate any help on any of my questions. Hope it;s not too muc
> for a Monday morning!
>
> Joan, slightly waterlogged from a weekend kayaking symposium in rare Alaska
> gorgeous weather.
Paul Lewing on wed 19 may 99
Joan,
I think, without doing ana analysis of your glaze, just a quick look,
that the zinc may be part of your cratering problem. That's an awful
lot of zinc oxide, which at high concentrations can cause that. And the
5% rutile isn't helping any. I think if you reduced the zinc, you might
even have a glossier glaze. I recently invented a zinc base for myself,
and with about 11% ZnO in the recipe, it has .45 moles of ZnO in the
formula, which is above the set of limits I use by quite a bit. You
have almost 30% here, so I'd start a test with about 8 g.instead of the
29 and add it back in maybe 4 g. increments up to the original 29 and
see if that would produce a workable solution.
Good luck, Paul Lewing, Seattle
David Hewitt on thu 20 may 99
Pin holing arises because gas bubbles are produced from decomposition of
the glaze and also the clay body. these bubbles increase in size as the
melt becomes more fluid, float to the surface and burst. Given
sufficient time at the glaze firing temperature they will, hopefully,
heal over.
If the gases are coming from the clay body, this can sometimes be cured
by a slightly higher biscuit firing temperature.
If the gases are from the glaze, then a longer soak at the top firing
temperature may be the answer, together with a slower cooling for the
first 100 degrees down if this is possible.
Glaze thickness can also play a part. The thicker the glaze the more the
gases have to push their way through before they burst. A too thin
glaze, however, can give insufficient glaze for the pin holes to heal
over.
I have found that changing from a grogged buff stoneware to a white
smooth stoneware has overcome this problem, but then the colour response
changes.
I hope that some of these points helps
In message , Joan & Tom Woodward writes
>----------------------------Original message----------------------------
>Oh Wise Ones of the List,
>
>I am rising from the depths of despondency after unloading the kiln filled =
>with
>pots bearing what I thought were reliable glazes (having tested from the =
>same
>batch) only to find I had a load of serious UGLIES. So, I ask for your help
>with three questions:
>
>First, I used a glaze which I think is called GCX:
>
>Kon F-4 Feldspar 48.4
>Zinc oxide 29.5
>Whiting 9.4
>Flint 13.8
>
>Cobalt carb .5
>Copper carb 4.0
>Rutile 5.0
>
>I fired Cone 5 (oxidation) with a 15 minute soak at temperature. Clay was =
>Idaho
>Buff, which has a fair amount of grog. But I'd tested this glaze on it =
>before
>with good results. Witness cone was bent to shelf. Came out with major
>craters, particularly inside. I could call it my moon bowl.
>The bowl has some nice underglazing work on the outside, so I think it's =
>worth
>an attempt to save.
>
>I'm wondering if I put a clear over it and refired, whether the capacity of =
>the
>clear to run would possible cause the glaze on the craters to move too. Any
>thoughts on this or other suggestions? Also, I've read/been told that the =
>soak
>should be at temperature=3B alternatively that it should be 200 degrees =
>down from
>temperature. Is there a right answer?
>
>On a different topic, a kind person replied to one of my posts about finding=
> a
>cost effective clear glaze by recommending the Duncan food safe glaze. I =
>used
>that when I was taking classes, but got nervous because of the lead in the
>glaze. It's Duncan GL 612. Says =22This glaze when fired to cone 06 or =
>hotter
>complies with FDA guidelines for lead and cadmium release.=22 Also says =
>=22Conforms
>to ASTM 0-4236.=22 I wish I could say that I always trusted what the =
>government
>told me, but it ain't so. Is it safe?
>
>Finally, I recently got some propylene glycol from the good folks at Clay =
>Art
>Center in Seattle to help make a dipping glaze brushable. Can I use that in=
> any
>old glaze if I want to use it for brushing?
>
>I'll really appreciate any help on any of my questions. Hope it=3Bs not too=
> much
>for a Monday morning=21
>
>Joan, slightly waterlogged from a weekend kayaking symposium in rare Alaska
>gorgeous weather.
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
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