Jobst Frohberg on fri 25 may 07
This is my first posting to the list. I have recently returned to working
with clay after a 20 year absence. Used to fire cone 10 reduction, now
cone 6 electric. When trying to develop new glazes, we often mix an old
one and an interesting new one that is being tested anyway, varying the
proportions by volume; then do more specific tests later.
In one recent attempt I mixed the Light Stormy Blue (p.89 in MCG6) with an
interesting simple glaze from CM February: Colemanite 59, Talc 41, Rutile
18. The result was an interesting light blue background, breaking gold/tan
over edges and ridges. The recalculated formula is:
Nepheline Syenite 22.5
Wollastonite 8.2
Flint 19.3
EP Kaolin 6.29139
Talc 15.3
Colemanite 20
Zinc Oxide 6.3
Cobalt Carbonate 0.7
Rutile 9.8
We experimented with changing the colemanite (less gives a mat, flat
yellow glaze); or increasing the flint (more flint gives a glossy glaze,
with more even distribution of yellow). On researching this, I believe
this to be a 2 phase glaze (I think they are low in Alumina), leading to
the breaking over the edges.
The difficulty is that it looks great on tests, a glossy, smooth surface
with no defects. On larger pots, we get blisters, and at times a small
collection of blisters looking like solidified froth. We fire according to
the schedule in MCG6, a 20 minute soak, cooling at 70deg C/hour; cone 6
tip touching.
Any ideas of how to get rid of the blisters? I think it=92s worthwhile
pursuing this glaze, but I=92m nor sure what direction to go.
Thanks
Jobst Frohberg
Ron Roy on sun 27 may 07
Hi Jobst,
Are you really using Colemanite or Gers Borate? I calculated with both and
the alumina and silica are way low. I'm not surprised there are problems.
Let me know which and also send a total with the recipe - it's not adding
up to 100.
What kind of clay are you using?
This might be a similar problem to the rutile Blues problems that some have
cured by calcining the rutile and soaking longer.
RR
>This is my first posting to the list. I have recently returned to working
>with clay after a 20 year absence. Used to fire cone 10 reduction, now
>cone 6 electric. When trying to develop new glazes, we often mix an old
>one and an interesting new one that is being tested anyway, varying the
>proportions by volume; then do more specific tests later.
>In one recent attempt I mixed the Light Stormy Blue (p.89 in MCG6) with an
>interesting simple glaze from CM February: Colemanite 59, Talc 41, Rutile
>18. The result was an interesting light blue background, breaking gold/tan
>over edges and ridges. The recalculated formula is:
>
>Nepheline Syenite 22.5
>Wollastonite 8.2
>Flint 19.3
>EP Kaolin 6.29139
>Talc 15.3
>Colemanite 20
>Zinc Oxide 6.3
>Cobalt Carbonate 0.7
>Rutile 9.8
>
>We experimented with changing the colemanite (less gives a mat, flat
>yellow glaze); or increasing the flint (more flint gives a glossy glaze,
>with more even distribution of yellow). On researching this, I believe
>this to be a 2 phase glaze (I think they are low in Alumina), leading to
>the breaking over the edges.
>The difficulty is that it looks great on tests, a glossy, smooth surface
>with no defects. On larger pots, we get blisters, and at times a small
>collection of blisters looking like solidified froth. We fire according to
>the schedule in MCG6, a 20 minute soak, cooling at 70deg C/hour; cone 6
>tip touching.
>Any ideas of how to get rid of the blisters? I think it=92s worthwhile
>pursuing this glaze, but I=92m nor sure what direction to go.
>
>Thanks
>
>Jobst Frohberg
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Fred Parker on sun 27 may 07
Hello Jobst:
I am certainly no expert here, but I have learned from hard experience
that zinc can become very unfriendly in an electric firing -- especially
if your kiln is unvented. The result often is small clusters of bubbles
that freeze into small blisters and/or craters/pinholes. Also, I believe
rutile can have the same effect in larger amounts. I believe 9+% of
rutile might be a little too much.
Fred Parker
On Fri, 25 May 2007 18:36:50 -0500, Jobst Frohberg
wrote:
>This is my first posting to the list. I have recently returned to working
>with clay after a 20 year absence. Used to fire cone 10 reduction, now
>cone 6 electric. When trying to develop new glazes, we often mix an old
>one and an interesting new one that is being tested anyway, varying the
>proportions by volume; then do more specific tests later.
>In one recent attempt I mixed the Light Stormy Blue (p.89 in MCG6) with an
>interesting simple glaze from CM February: Colemanite 59, Talc 41, Rutile
>18. The result was an interesting light blue background, breaking gold/tan
>over edges and ridges. The recalculated formula is:
>
>Nepheline Syenite 22.5
>Wollastonite 8.2
>Flint 19.3
>EP Kaolin 6.29139
>Talc 15.3
>Colemanite 20
>Zinc Oxide 6.3
>Cobalt Carbonate 0.7
>Rutile 9.8
>
>We experimented with changing the colemanite (less gives a mat, flat
>yellow glaze); or increasing the flint (more flint gives a glossy glaze,
>with more even distribution of yellow). On researching this, I believe
>this to be a 2 phase glaze (I think they are low in Alumina), leading to
>the breaking over the edges.
>The difficulty is that it looks great on tests, a glossy, smooth surface
>with no defects. On larger pots, we get blisters, and at times a small
>collection of blisters looking like solidified froth. We fire according to
>the schedule in MCG6, a 20 minute soak, cooling at 70deg C/hour; cone 6
>tip touching.
>Any ideas of how to get rid of the blisters? I think it=92s worthwhile
>pursuing this glaze, but I=92m nor sure what direction to go.
>
>Thanks
>
>Jobst Frohberg
>
>__________________________________________________________________________
____
>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.
Sal /Jobst Bodnar/Frohberg on mon 28 may 07
Thanks, Fred
It does look like froth; others have suggested that it may be overfired, an=
d I'll try reducing Zinc and Rutile next. Venting the kiln- well, it may be=
easier to work on the glaze!!
=20
Regards
=20
Jobst Frohberg
> Date: Sun, 27 May 2007 21:15:27 -0500> From: fhparker@PRODIGY.NET> Subjec=
t: Re: Introduction, glaze problems> To: CLAYART@LSV.CERAMICS.ORG> > Hello =
Jobst:> > I am certainly no expert here, but I have learned from hard exper=
ience> that zinc can become very unfriendly in an electric firing -- especi=
ally> if your kiln is unvented. The result often is small clusters of bubbl=
es> that freeze into small blisters and/or craters/pinholes. Also, I believ=
e> rutile can have the same effect in larger amounts. I believe 9+% of> rut=
ile might be a little too much.> > Fred Parker> > > On Fri, 25 May 2007 18:=
36:50 -0500, Jobst Frohberg > wrote:> > >This is my f=
irst posting to the list. I have recently returned to working> >with clay a=
fter a 20 year absence. Used to fire cone 10 reduction, now> >cone 6 electr=
ic. When trying to develop new glazes, we often mix an old> >one and an int=
eresting new one that is being tested anyway, varying the> >proportions by =
volume; then do more specific tests later.> >In one recent attempt I mixed =
the Light Stormy Blue (p.89 in MCG6) with an> >interesting simple glaze fro=
m CM February: Colemanite 59, Talc 41, Rutile> >18. The result was an inter=
esting light blue background, breaking gold/tan> >over edges and ridges. Th=
e recalculated formula is:> >> >Nepheline Syenite 22.5> >Wollastonite 8.2> =
>Flint 19.3> >EP Kaolin 6.29139> >Talc 15.3> >Colemanite 20> >Zinc Oxide 6.=
3> >Cobalt Carbonate 0.7> >Rutile 9.8> >> >We experimented with changing th=
e colemanite (less gives a mat, flat> >yellow glaze); or increasing the fli=
nt (more flint gives a glossy glaze,> >with more even distribution of yello=
w). On researching this, I believe> >this to be a 2 phase glaze (I think th=
ey are low in Alumina), leading to> >the breaking over the edges.> >The dif=
ficulty is that it looks great on tests, a glossy, smooth surface> >with no=
defects. On larger pots, we get blisters, and at times a small> >collectio=
n of blisters looking like solidified froth. We fire according to> >the sch=
edule in MCG6, a 20 minute soak, cooling at 70deg C/hour; cone 6> >tip touc=
hing.> >Any ideas of how to get rid of the blisters? I think it=92s worthwh=
ile> >pursuing this glaze, but I=92m nor sure what direction to go.> >> >Th=
anks> >> >Jobst Frohberg> >> >_____________________________________________=
_____________________________> ____> >Send postings to clayart@lsv.ceramics=
.org> >> >You may look at the archives for the list or change your subscrip=
tion> >settings from http://www.ceramics.org/clayart/> >> >Moderator of the=
list is Mel Jacobson who may be reached at> melpots@pclink.com.> > _______=
_______________________________________________________________________> Se=
nd postings to clayart@lsv.ceramics.org> > You may look at the archives for=
the list or change your subscription> settings from http://www.ceramics.or=
g/clayart/> > Moderator of the list is Mel Jacobson who may be reached at m=
elpots@pclink.com.
_________________________________________________________________
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