Eileen de Rosas on mon 4 apr 11
Hi,
I work at a low fire studio. One of our workhorse base glazes is
Worthington Clear--we use it to make a shiny green with copper, a shiny blu=
e
with cobalt, and an pretty amber with rutile. It is probably the most
popular glaze in the studio, and was generally quite reliable----
Until, about six months ago, it began to crawl. Thinking it had been a ba=
d
batch (even though it had tested fine initially), I added a small amount of
Gare lowfire commercial clear to help smooth over any problems. This worke=
d
for that bucket, but this glaze has been made many times since then, by
myself and others, and it has continued to crawl. I've been over everythin=
g
with the students about cleaning pieces before glazing, not burnishing the
pieces by overworking, not having oily-from-Greek-salad hands while glazing=
,
not glazing too thickly, not glazing 2 layers of glaze with a day of drying
in between, etc, etc.
It reached a point where something had to be done because everyone's work
was crawling miserably. It was flaking off the pots before they were fired=
,
or just in the kiln. I tested various combos while trying to keep the glaze
as close as possible to the original, which is:
Gerstley Borate 55
EPK 30
flint 15
In my tests, I subbed other clays for the EPK, added Frit 3134 and lowered
the G.B, subbed calcined kaolin for half the EPK, etc. The tests were done
with the Transparent Copper recipe with 4% copper carbonate added.
Most of the tests flaked off the test pieces before firing (automatic dq),
others crawled in the kiln, and some had an anemic color response. I picke=
d
the one that looked closest to the original and had no crawling, mixed up a
small batch for testing, and crossed my fingers.
Gerstly Borate 50
EPK 16
Calcined kaolin 17
flint 17
It worked fine.
Mixed up a large batch, tested, was happy. But now it is crawling too. I
have been trying to keep track of the thickness of the glaze, but in a larg=
e
studio with many students--alot of them teens and kids--it can thicken up
quickly or occasionally a student will thin it down too much.
Has anyone experienced this problem with the Worthington Clear base,
especially in the last year? My suspicion is that one of the materials has
changed, and since there are so few ingredients in the glaze, it affects it
more strongly than the other glazes. Even if that is true, we are in need
of a solution! Students pots are being ruined consistently, and we need a
reliable glaze that anyone can use for a decent result.
If you have any insight, I'd love some help solving this problem.
Eileen
Eric Hansen on tue 5 apr 11
Worthington Clear is a glaze that is very prone to crawling off the
pot. I would replace it with a reliable commercial glaze sold in bulk
- the cost will be about the same and your problems solved.
- h a n s e n -
On Mon, Apr 4, 2011 at 11:38 PM, Eileen de Rosas wrot=
=3D
e:
> Hi,
>
> I work at a low fire studio. =3DA0One of our workhorse base glazes is
> Worthington Clear--we use it to make a shiny green with copper, a shiny b=
=3D
lue
> with cobalt, and an pretty amber with rutile. =3DA0It is probably the mos=
t
> popular glaze in the studio, and was generally quite reliable----
>
> Until, about six months ago, it began to crawl. =3DA0 Thinking it had bee=
n =3D
a bad
> batch (even though it had tested fine initially), I added a small amount =
=3D
of
> Gare lowfire commercial clear to help smooth over any problems. =3DA0This=
w=3D
orked
> for that bucket, but this glaze has been made many times since then, by
> myself and others, and it has continued to crawl. =3DA0I've been over eve=
ry=3D
thing
> with the students about cleaning pieces before glazing, not burnishing th=
=3D
e
> pieces by overworking, not having oily-from-Greek-salad hands while glazi=
=3D
ng,
> not glazing too thickly, not glazing 2 layers of glaze with a day of dryi=
=3D
ng
> in between, etc, etc.
>
> It reached a point where something had to be done because everyone's work
> was crawling miserably. =3DA0It was flaking off the pots before they were=
f=3D
ired,
> or just in the kiln. I tested various combos while trying to keep the gla=
=3D
ze
> as close as possible to the original, which is:
>
> Gerstley Borate 55
> EPK 30
> flint 15
>
> In my tests, I subbed other clays for the EPK, added Frit 3134 and lowere=
=3D
d
> the G.B, subbed calcined kaolin for half the EPK, etc. =3DA0The tests wer=
e =3D
done
> with the Transparent Copper recipe with 4% copper carbonate added.
>
> Most of the tests flaked off the test pieces before firing (automatic dq)=
=3D
,
> others crawled in the kiln, and some had an anemic color response. =3DA0I=
p=3D
icked
> the one that looked closest to the original and had no crawling, mixed up=
=3D
a
> small batch for testing, and crossed my fingers.
>
> Gerstly Borate 50
> EPK 16
> Calcined kaolin 17
> flint 17
>
> It worked fine.
>
> Mixed up a large batch, tested, was happy. =3DA0But now it is crawling to=
o.=3D
=3DA0I
> have been trying to keep track of the thickness of the glaze, but in a la=
=3D
rge
> studio with many students--alot of them teens and kids--it can thicken up
> quickly or occasionally a student will thin it down too much.
>
> Has anyone experienced this problem with the Worthington Clear base,
> especially in the last year? =3DA0My suspicion is that one of the materia=
ls=3D
has
> changed, and since there are so few ingredients in the glaze, it affects =
=3D
it
> more strongly than the other glazes. =3DA0Even if that is true, we are in=
n=3D
eed
> of a solution! =3DA0Students pots are being ruined consistently, and we n=
ee=3D
d a
> reliable glaze that anyone can use for a decent result.
>
> If you have any insight, I'd love some help solving this problem.
>
> Eileen
>
--=3D20
Eric Alan Hansen
Stonehouse Studio Pottery
Alexandria, Virginia
americanpotter.blogspot.com
thesuddenschool.blogspot.com
hansencookbook.blogspot.com
"Simplify, simplify, simplify" - Thoreau
David Martin Hershey on wed 6 apr 11
Hi Eileen,
I use Vee Gum CER (ceramic)
to stop glazes from crawling.
(Hat-tip to Jonathan Kaplan)
It has always worked so far.
You only need a tiny bit- .3 to
1.5% max.
That's 3 tenths of one percent
of the dry weight of the glaze.
I typically use .5%
If you use more, the glaze
can become quite hard on the piece
and is difficult to "finger sand"
if necessary before firing.
Test various amounts
in a 200 gram test batch.
I mix it dry into the dry materials.
But you can also add it to a mixed
glaze
if you know the approx weight of
the dry materials.
Mix it as well as possible
into a little glaze or water before
adding.
Mix very well & screen twice after
adding to wet.
Vee Gum Ceramic is available from
Laguna,
or you can mix your own, which is
what I do.
I use Tony Hansen's formula:
50% vee gum T
50% CMC powder
1 pound goes quite a long way...
Best, DMH
David Martin Hershey
DMH Studio + Design
2629 Manhattan Ave #137
Hermosa Beach CA USA
90254-2447 424.241.3809
http://www.dmhstudio.com/
http://www.obamacardgold.com
http://www.winetastingtrolley.com/
On 4/4/2011 8:38 PM, Eileen de
Rosas wrote:
> Until, about six months ago, it began to crawl.
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