Snail Scott on sat 26 nov 11
On Nov 26, 2011, at 7:14 PM, Deborah Thuman wrote:
> I made bowls. I used commercial underglazes on the inside of the
> bowls. Bisque fired to ^04....
> Glazed inside of bowls with clear glossy glaze....The insides of... =3D
bowls are spotty and
> look as if there is no glaze on them...
Many commercial underglazes (a.k.a. engobes) are
actually fairly vitrified after a high bisque like ^04 -
enough to trap, or to exclude, moisture. If you are=3D20
relying on the absorbency of the normal bisque-fired=3D20
earthenware to get the glaze thickness correct, the=3D20
semi-vitreous engobe layer can prevent that. When=3D20
dipping, for which absorbency is everything, it is a=3D20
real issue. Even when brushing, it can have a noticeable=3D20
effect, especially if you don't wait for each brushed=3D20
coating to set up between applications. You can find=3D20
yourself unwittingly wiping off the previous coat as you=3D20
work.
Conversely, a few commercial underglazes are actually=3D20
less vitrified than the clay, and when applied thickly will=3D20
act as a sponge during firing, soaking up some of the=3D20
glaze and giving a thin, patchy look to even a normally=3D20
sufficient glaze coating. This is less common than the=3D20
condition described above, but it can happen.=3D20
Without seeing the work, I can't really be certain of the=3D20
correct diagnosis.
It's easy to forget that commercial products can be quite=3D20
varied. Just as any new recipe requires testing under=3D20
conditions of use similar to what's intended, it's a good=3D20
idea to do the same with any new commercial product,=3D20
or a familiar one used in combo with a new clay, overglaze,=3D20
etc. Colorants are not inert additives like paint pigments,=3D20
either, and different colors in the same product line can=3D20
have radically different responses, especially when low-fire=3D20
products are pushed to their nominal limits. =3D20
Commercial ceramic surfaces are often formulated to be=3D20
consistent and predictable even across a wide range of=3D20
application styles, (a.k.a. 'boring'), but some are less so,=3D20
and will respond to variations in application with variations=3D20
in results (a.k.a. either 'flexible and responsive', alternatively=3D20
'squirrely and unreliable'). No guarantees. Testing, even of=3D20
commercial stuff, avoids much heartache and waste.
-Snail
Deborah Thuman on sat 26 nov 11
I made bowls. I used commercial underglazes on the inside of the
bowls. Bisque fired to ^04. Glazed outside of bowls with funky glazes.
Glazed inside of bowls with clear glossy glaze. All commercial glazes.
Glaze fired to ^05 with a 5 minute hold at the top. Slow firing -
scutt kiln with computer controller. Kiln has worked well for other
firings. The outsides of the bowls are fantastic. The insides of some
of the bowls are fantastic. The insides of other bowls are spotty and
look as if there is no glaze on them. Um.... what the heck happened
here? Clear glaze worked well on the mica clay. Clear glaze worked
well on Jim's pieces (with the same commercial underglazes).
Do I reglaze and refire to ^05?
Any clues to what happened appreciated. TIA.
Deb Thuman
http://debthumansblog.blogspot.com/
http://www.etsy.com/shop.php?user_id=3D5888059
http://www.facebook.com/pages/Deb-Thumans-Art-Page/167529715986
William & Susan Schran User on sun 27 nov 11
On 11/26/11 8:14 PM, "Deborah Thuman" wrote:
> I made bowls. I used commercial underglazes on the inside of the
> bowls. Bisque fired to ^04. Glazed outside of bowls with funky glazes.
> Glazed inside of bowls with clear glossy glaze. All commercial glazes.
> Glaze fired to ^05 with a 5 minute hold at the top. Slow firing -
> scutt kiln with computer controller. Kiln has worked well for other
> firings. The outsides of the bowls are fantastic. The insides of some
> of the bowls are fantastic. The insides of other bowls are spotty and
> look as if there is no glaze on them. Um.... what the heck happened
> here? Clear glaze worked well on the mica clay. Clear glaze worked
> well on Jim's pieces (with the same commercial underglazes).
> Do I reglaze and refire to ^05?
> Any clues to what happened appreciated. TIA.
Did you brush on the clear glaze for the interiors of the bowls?
Did you brush on all in one direction, like all horizontally?
How many coats did you apply?
I would expect the underglaze on the interiors would interact with the glaz=
e
and if the glaze is not thick enough, might cause the dry areas.
If you apply multiple coats of glaze all in one direction, this can lead to
uneven application, even pulling up some glaze as you apply more. Always
brush each layer of glaze in a different direction to resolve this.
Yes, you can apply more glaze and refire.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Lee on sun 27 nov 11
On Sat, Nov 26, 2011 at 7:14 PM, Deborah Thuman wrot=
=3D
e:
> Do I reglaze and refire to ^05?
Deb, If you can reheat the pots in your oven, it will help the
re-glaze hold, especially if you are dipping.
>
> Any clues to what happened appreciated. TIA.
If you poured glaze inside the bowls and then dipped the outside
second, the outside glaze might have less dry bisque to pick it up, so
the glaze is thinner. I try to submerge most forms and glaze both
interior and exterior at the same time, to get a uniformed
application.
What method are you using for glaze application?
--
=3DA0Lee Love in Minneapolis
http://mingeisota.blogspot.com/
=3DA0"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D9=
7that is, =3D
"The
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
Steve Mills on sun 27 nov 11
On occasion, when faced with this problem, I have found that re-firing is t=
h=3D
e answer, but crucially I needed to slow it down over the last 300oF (150oC=
)=3D
to allow the temperature INSIDE the pot to equalise with the outside, and =
t=3D
hen in future do the same with all firings.=3D20
If you fire fast, the inside can lag behind, especially with a more "closed=
"=3D
shape.=3D20
Steve M
Steve Mills
Bath
UK
www.mudslinger.me.uk
Sent from my Ipod touch
On 27 Nov 2011, at 01:14, Deborah Thuman wrote:
> I made bowls. I used commercial underglazes on the inside of the
> bowls. Bisque fired to ^04. Glazed outside of bowls with funky glazes.
> Glazed inside of bowls with clear glossy glaze. All commercial glazes.
> Glaze fired to ^05 with a 5 minute hold at the top. Slow firing -
> scutt kiln with computer controller. Kiln has worked well for other
> firings. The outsides of the bowls are fantastic. The insides of some
> of the bowls are fantastic. The insides of other bowls are spotty and
> look as if there is no glaze on them. Um.... what the heck happened
> here? Clear glaze worked well on the mica clay. Clear glaze worked
> well on Jim's pieces (with the same commercial underglazes).
>=3D20
> Do I reglaze and refire to ^05?
>=3D20
> Any clues to what happened appreciated. TIA.
>=3D20
> Deb Thuman
> http://debthumansblog.blogspot.com/
> http://www.etsy.com/shop.php?user_id=3D3D5888059
> http://www.facebook.com/pages/Deb-Thumans-Art-Page/167529715986
Gayle Bair on sun 27 nov 11
Deb,
I have experienced what Snail described:
"Conversely, a few commercial underglazes are actually
less vitrified than the clay, and when applied thickly will
act as a sponge during firing, soaking up some of the
glaze and giving a thin, patchy look to even a normally
sufficient glaze coating. This is less common than the
condition described above, but it can happen."
I solved that issue by adding clear glaze to the underglazes and keep them
in separate containers.
I also use this system with various stains.
Or you can look closely at the glazed pot and see the areas where the
underglaze has sucked up the glaze
and dab/brush those areas with the glaze but find my other solution easier
for me in the long run.
On Nov 26, 2011, at 7:14 PM, Deborah Thuman wrote:
> > I made bowls. I used commercial underglazes on the inside of the
> > bowls. Bisque fired to ^04....
> > Glazed inside of bowls with clear glossy glaze....The insides of...
> bowls are spotty and
> > look as if there is no glaze on them...
>
>
>
> On Sat, Nov 26, 2011 at 8:20 PM, Snail Scott w=
rote:
> Many commercial underglazes (a.k.a. engobes) are
> actually fairly vitrified after a high bisque like ^04 -
> enough to trap, or to exclude, moisture. If you are
> relying on the absorbency of the normal bisque-fired
> earthenware to get the glaze thickness correct, the
> semi-vitreous engobe layer can prevent that. When
> dipping, for which absorbency is everything, it is a
> real issue. Even when brushing, it can have a noticeable
> effect, especially if you don't wait for each brushed
> coating to set up between applications. You can find
> yourself unwittingly wiping off the previous coat as you
> work.
>
> >snip
>
--
Gayle Bair
Bainbridge Island WA
gayle@claybair.com
www.claybair.com
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