Centa Uhalde on sun 1 aug 10
Can someone explain the logic behind a using a higher bisque than glaze fir=
e
for earthenware (01 bisque, 04 glaze).
I always thought you need a higher glaze fire than bisque for absorption.
Thanks,
Centa
Www.centatheresa.com
Snail Scott on mon 2 aug 10
On Aug 2, 2010, at 1:59 AM, Centa Uhalde wrote:
> Can someone explain the logic behind a using a higher bisque than
> glaze fire
> for earthenware (01 bisque, 04 glaze).
>
> I always thought you need a higher glaze fire than bisque for
> absorption.
The purposes of a bisque firing are several:
to harden the clay so it won't be accidentally
damaged during decorating, to sinter the clay
so it won't turn back into mud when wetted,
and to drive out various substances that might
affect the glaze badly if they were to outgas
after the glazes were present.
All of this has very little to do with the final firing
temperature of the clay. The burnout temperature
of carbon, sulfur, and such remains the same
regardless of the clay's vitrification point. A high-
temp clay might be less strong after a bisque to
a given temp than a low-temp clay, but that's
usually a minor factor compared to burnout.
The relevant point is that for earthenware, the
best bisque temp - the one that drives out the
most impurities - may be the same as the temp
of maturity for the clay body. In any case, you
certainly can't bisque fire higher than that or the
clay will bloat or warp! Because of this, burnout
may not be perfect, and some of those nasty
glaze-affecting materials may remain. If the
bisque temp is lower than the maturation temp,
some will definitely remain, and though not all
glazes are equally affected by such outgassing,
most people figure to bisque to a consistent safe
temp suitable for all their glazes.
Cadmium-based red glazes are the most
notoriously vulnerable, often turning clear or
grey if outgassing occurs, but others can be
affected as well.
So, you want to bisque hot enough to drive off the
maximum amount of 'nasties', which means for
earthenware, bisqueing to the maturation temp,
then glaze firing slightly lower so as to not stir up
any remaining nasties. Unlike stoneware,
earthenware is still plenty absorbent enough for
glazing even after being fired to maturity.
The two-cone disparity that the commercial glaze
makers have settles on as a standard (bisque to ^04,
and glaze to ^06) is a good enough margin of safety
to allow for inaccurate kilns and inconsistent firing
and still get reliable results. It's not mandatory,
though. As long as the underlying principle is
understood (that some low-fire glaze colorants are
sensitive to outgassing), you can make your own
choices about how to conduct your firings, and to
what temperatures.
-Snail
Robert Harris on mon 2 aug 10
I would add to Snail's excellent rationale that in industry the glost
firing is almost always at a lower temperature than the bisque firing
- even for stoneware items and temperatures. In this case the lack of
absorbency for stoneware items is countered by the use of gums and
adhesives in industrial glazes.
In these cases we may also note that industry want the advantages of
stoneware - full vitrification and thus strength non-porosity etc -
with some of the advantages of lower temperature glazes - i.e. colour
stability etc
R
John Britt on mon 2 aug 10
Centa,
The main reason is for majolica potters who bisque hotter during the bisq=
=3D
ue
so the all the impurities are driven out and then you dont' get pinholes
during the lower second firing (glaze firing) with the hours and hours sp=
=3D
ent
glazing and decorating. Since you are not approaching the heatwork of t=
=3D
he
bisque you should be less likely to get gases released or pinholing in th=
=3D
e
majolica.
Marcia Selsor on mon 2 aug 10
I agree with all three :Robert, Snail and John.
I am more familiar with this practise for majolica and as john says, =3D
higher bisque helps eliminate pinholing. Also if one bisques lower for =3D
majolica, some crazing may happen also.
Marcia
On Aug 2, 2010, at 10:16 AM, Robert Harris wrote:
> I would add to Snail's excellent rationale that in industry the glost
> firing is almost always at a lower temperature than the bisque firing
> - even for stoneware items and temperatures. In this case the lack of
> absorbency for stoneware items is countered by the use of gums and
> adhesives in industrial glazes.
>=3D20
> In these cases we may also note that industry want the advantages of
> stoneware - full vitrification and thus strength non-porosity etc -
> with some of the advantages of lower temperature glazes - i.e. colour
> stability etc
>=3D20
> R
>=3D20
Marcia Selsor
http://www.marciaselsor.com
Lee Love on mon 2 aug 10
I've been having good results with single fire jermolickers. I
started with terra sig but also tested both majolica and several clear
glazes. I glazed leather hard, so the unfired clay was not porous
and so it did not bubble.
--
=3DA0Lee, 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
Lee Love on mon 2 aug 10
On Mon, Aug 2, 2010 at 12:02 PM, Marcia Selsor wrote:
>:helps eliminate pinholing. Also if one bisques lower for majolica, some c=
=3D
razing may >happen also.
This only happens if you don't fit the glaze to the claybody.
My single fire majolica and terra sig is to cone 1. It
rings better than porcelain and works well in the microwave.
--
=3DA0Lee, 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
Frank Gaydos on tue 3 aug 10
I fire our schools terracotta to cone 05 bisque. Cone 04 for glaze. I never=
=3D
have a problem. It helps if you soak the clay for a few minutes at tempera=
=3D
ture. We have 20 glazes made including majolica and clear.=3D20
Frank=3D20
----- Original Message -----=3D20
From: "Lee Love" =3D20
To: Clayart@LSV.CERAMICS.ORG=3D20
Sent: Monday, August 2, 2010 4:49:52 PM=3D20
Subject: Re: Higher bisque than glaze fire?=3D20
On Mon, Aug 2, 2010 at 12:02 PM, Marcia Selsor wrote:=3D20
>:helps eliminate pinholing. Also if one bisques lower for majolica, some c=
=3D
razing may >happen also.=3D20
=3DC2=3DA0=3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 This only =
happens if you don't fit=3D
the glaze to the claybody.=3D20
=3DC2=3DA0=3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 My single =
fire majolica and terra =3D
sig is to cone 1. =3DC2=3DA0 =3DC2=3DA0It=3D20
rings better than porcelain and works well in the microwave.=3D20
--=3D20
=3DC2=3DA0Lee, a Mashiko potter in Minneapolis=3D20
http://mingeisota.blogspot.com/=3D20
=3DE2=3D80=3D9CObserve the wonders as they occur around you. Don't claim th=
em. Fe=3D
el=3D20
the artistry moving through and be silent.=3DE2=3D80=3D9D --Rumi=3D20
----- Original Message -----=3D20
From: "Lee Love" =3D20
To: Clayart@LSV.CERAMICS.ORG=3D20
Sent: Monday, August 2, 2010 4:49:52 PM=3D20
Subject: Re: Higher bisque than glaze fire?=3D20
On Mon, Aug 2, 2010 at 12:02 PM, Marcia Selsor wrote:=3D20
>:helps eliminate pinholing. Also if one bisques lower for majolica, some c=
=3D
razing may >happen also.=3D20
=3DC2=3DA0=3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 This only =
happens if you don't fit=3D
the glaze to the claybody.=3D20
=3DC2=3DA0=3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 =3DC2=3DA0 My single =
fire majolica and terra =3D
sig is to cone 1. =3DC2=3DA0 =3DC2=3DA0It=3D20
rings better than porcelain and works well in the microwave.=3D20
--=3D20
=3DC2=3DA0Lee, a Mashiko potter in Minneapolis=3D20
http://mingeisota.blogspot.com/=3D20
=3DE2=3D80=3D9CObserve the wonders as they occur around you. Don't claim th=
em. Fe=3D
el=3D20
the artistry moving through and be silent.=3DE2=3D80=3D9D --Rumi=3D20
Brant Palley on tue 3 aug 10
The reason is that you need to burn out any carbon in the clay so it does=
=3D
n=3D92t=3D20
bubble out through the glaze during the glaze fire (also other reactions =
=3D
are=3D20
happening). Each time you fire hotter the clay shrinks more squeezing the=
=3D
air=3D20
out of the body.=3D20
Earthenware is not vitrified and can have absorption from 7-17% and st=
=3D
ill=3D20
have good glaze fit. This is why it is good to glaze the bottoms of func=
=3D
tional=3D20
ware to prevent water absorbing into the body.=3D20
Also, we design the clay bodies to fit the glaze. The bisque for New M=
=3D
exico=3D20
Clay's earthenware is cone 04, this insures you will not get crazing or=3D2=
0=3D
shivering. Duncan and Mayco both recommend 2 cones difference between=3D20
glaze and bisque.
Brant Palley
New Mexico Clay www.nmclay.com
3300 Girard Blvd. NE
Albuquerque NM 87107
505 881-2350 800 781-2529
www.fiberclay.com www.micaclay.com
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