Joan Slack on fri 21 may 10
Hi,
I have been having an occasional problem with bloating, the clay kind! I =
=3D
use the same clay body in an electric kiln, firing to cone 7, and also =3D
an olympic updraft kiln, firing to cone 10. In the gas kiln, only =3D
sometimes, the ware is bloated especially near the top half of the kiln, =
=3D
and pinholed. Never happens in the electric. Some gas firings are fine, =3D
others do this, yet the firing log is similar. Does anyone have any =3D
suggestions? I've read the books and resources and can't see how to =3D
identify and fix the problem...I wonder if I am reducing too much, too =3D
fast? I know there is a correlation with bisque firing and bloating, but =
=3D
I have an old-fashioned kiln with cones as my guide, no programming to =3D
go by...any suggestions appreciated!
Thanks,
Joanie
Joan Molloy Slack
Riverrun Center for the Arts
PO Box 95 6938 Bridge Road
McNaughton, Wi 54543
www.riverrunarts.com
715-277-4224
Ron Roy on fri 21 may 10
Hi Joan,
What clay is it - what cone does the manufacturer recommend - what
colour is it?
I would not expect a clay that is vitrified enough to not leak at cone
7 to be useable at cone 10 - especially if it's a brown clay.
Sounds like the clay is being over fired at cone 10.
RR
Quoting Joan Slack :
> Hi,
>
> I have been having an occasional problem with bloating, the clay
> kind! I use the same clay body in an electric kiln, firing to cone
> 7, and also an olympic updraft kiln, firing to cone 10. In the gas
> kiln, only sometimes, the ware is bloated especially near the top
> half of the kiln, and pinholed. Never happens in the electric. Some
> gas firings are fine, others do this, yet the firing log is similar.
> Does anyone have any suggestions? I've read the books and resources
> and can't see how to identify and fix the problem...I wonder if I am
> reducing too much, too fast? I know there is a correlation with
> bisque firing and bloating, but I have an old-fashioned kiln with
> cones as my guide, no programming to go by...any suggestions
> appreciated!
> Thanks,
> Joanie
>
> Joan Molloy Slack
> Riverrun Center for the Arts
> PO Box 95 6938 Bridge Road
> McNaughton, Wi 54543
> www.riverrunarts.com
> 715-277-4224
>
William & Susan Schran User on fri 21 may 10
On 5/21/10 11:48 AM, "Joan Slack" wrote:
> I have been having an occasional problem with bloating, the clay kind! I =
use
> the same clay body in an electric kiln, firing to cone 7, and also an oly=
mpic
> updraft kiln, firing to cone 10. In the gas kiln, only sometimes, the war=
e is
> bloated especially near the top half of the kiln, and pinholed. Never hap=
pens
> in the electric. Some gas firings are fine, others do this, yet the firin=
g log
> is similar. Does anyone have any suggestions? I've read the books and
> resources and can't see how to identify and fix the problem...I wonder if=
I am
> reducing too much, too fast? I know there is a correlation with bisque fi=
ring
> and bloating, but I have an old-fashioned kiln with cones as my guide, no
> programming to go by...any suggestions appreciated!
From what you write: "I use the same clay body in an electric kiln, firing
to cone 7, and also an olympic updraft kiln, firing to cone 10. In the gas
kiln, only sometimes, the ware is bloated especially near the top half of
the kiln, and pinholed" - I would suggest your clay is being overfired at
^10 in the gas kiln and might be fired higher than ^10, causing gas
formation in the clay, resulting in bloating and pinholing - especially if
there is sufficient quantity of iron in the clay. You may also find the cla=
y
fired to ^7 in the electric kiln may not be fully mature. I have not seen
many or any clays that are fully matured and functional at ^7 oxidation and
^10 reduction.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Snail Scott on sun 23 may 10
On May 21, 2010, at 10:48 AM, Joan Slack wrote:
> In the gas kiln, only sometimes, the ware is bloated especially
> near the top half of the kiln, and pinholed. Never happens in the
> electric. Some gas firings are fine, others do this, yet the firing
> log is similar. Does anyone have any suggestions?
Is your bisque firing also in gas? and is your clay
body iron-bearing?
A longer bisque can often help with bloating.
Iron bearing clays are more prone to bloating, and
the fluxing effect of the iron exacerbates this.
-Snail
Lee Love on sun 23 may 10
On Sun, May 23, 2010 at 8:55 AM, Snail Scott wr=
=3D
ote:
> On May 21, 2010, at 10:48 AM, Joan Slack wrote:
> Iron bearing clays are more prone to bloating, and
> the fluxing effect of the iron exacerbates this.
Iron clay with an iron saturated glaze are the worst.
--=3D20
--
Lee, a Mashiko potter in Minneapolis
http://mingeisota.blogspot.com/
=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi
douglas fur on sat 29 may 10
For Joan S
Are the gas fires reduction? If so decreasing the reduction (reducinging
the reduction) or scheduling it later in the fire can help by decreasing th=
e
ammount of Fe2O3 changed to FeO.
There's also the concept of "carbon trapping" (on the chemistry of which I'=
m
vague) that in too heavy a reduction you produce carbon which gets trapped
in the body which turns to CO or CO2 which forms the bubbles. If you reduce
the reduction you mainly produce CO (Avoid thready orange flames and go for
a purpley to yellow flame depending on the gas used)
DRB
Burien
On Sun, May 23, 2010 at 6:55 AM, Snail Scott wro=
te:
> On May 21, 2010, at 10:48 AM, Joan Slack wrote:
>
>> In the gas kiln, only sometimes, the ware is bloated especially
>> near the top half of the kiln, and pinholed. Never happens in the
>> electric. Some gas firings are fine, others do this, yet the firing
>> log is similar. Does anyone have any suggestions?
>>
>
>
> Is your bisque firing also in gas? and is your clay
> body iron-bearing?
>
> A longer bisque can often help with bloating.
>
> Iron bearing clays are more prone to bloating, and
> the fluxing effect of the iron exacerbates this.
>
> -Snail
>
Louis Katz on sun 30 may 10
Bloating.
If your body, or part of it is glassy and material then goes into solutio=
=3D
n
inside the glass, and that material is a carbonate or some other "gas
containing" compound like sulphates , then when the material goes into
solution it is likely to produce bubbles in the glass. As the temperature=
=3D
increases so does the size or at least pressure of these bubbles. As the
gasses or bubbles are already larger than the solid that contained them t=
=3D
his
increase due to temperature may not be important.
Strategies:
1. do not allow your clay to become glass like.
a. do not reduce your clay early, make sure your bisque is oxidized. This=
=3D
keeps some of the iron inside the clay from reducing and becoming a stron=
=3D
g
flux. When bloating is due to iron bearing clay this technique seems to w=
=3D
ork.
b. do not over fire.
2. reduce gas producing materials in clay.
a. If you use whiting in your clay body make sure it is pure calcium
carbonate. Magnesium carbonate has a higher dissociation temperature and =
=3D
may
cause problems(I don't think I have ever seen these but...). The term
whiting is very generic and as I understand it industrially is defined as=
=3D
a
material of at least 50% calcium carbonate, where as much as 50% can be
magnesium carbonate.=3D20
b. If your ware is very hard perhaps this is enough calcium suphate to ca=
=3D
use
a problem? This seems unlikely but possible.
c. keep the barium carbonate in your body to a minimum. The carbonate has=
=3D
to
go somewhere, it dissociates late. It takes very little to eliminate
scumming if well mixed.It could be a source of bloats.
3. If your bloats are in a white clay, try reducing at 1500F and then
reoxidizing. This has worked for others, and is based on advice from Rich=
=3D
ard
Burkett in consultation with his father. I have also read about this in a=
=3D
n
old book, perhaps by Binns. Should be more about this in the archives way=
=3D
back.
4. If all else fails try opening your clay body with grog or sand. This
seems like a long shot. It may cause more harm than good.
5. reformulate with less carbonates, sulphates, and iron., or decrease fl=
=3D
uxes.
Hope this helps.=3D20
Louis Katz
Clayer
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