Eleanora Eden on sun 2 jul 06
These are quotes from Nelson and Rhodes. I am not arguing these
positions, only
sharing what I have read. I would love to see comments from
clayarters on their experiences
that may underscore or deviate from these comments. I have found
this very educating.
I am not adding any of my own comments or questions.
Nelson (p144 Ceramics, A Potter's Handbook)
"In preparing a clay body several things can be done to control the
effects of quartz inversion.
A combination of pyrophyllite and talc substituted for part of the
feldspar will reduce thermal
expansion, as will the addition of petalite. Spodumene in a clay
body creates an irreversible
expansion, while the inclusion of wollastonite also reduces firing
shrinkage and aids in
developing a tougher body that is more resistant to heat shock.
Rhodes (highlights of chapter 3, Stoneware and Porcelain, freely excerpted)
1.natural stoneware clays
Stoneware clay is closely related to fireclay and might be thought of
as a low-grade, plastic fireclay.
Ideal properties of stoneware:
Physical:
-excellent plasticity yet with enough coarse material to give a
slight tooth and to
permit the making of larger shapes without slumping.
-drying shrinkage no more than 5% and no tendency to warp or crack on drying.
-freedom from alkalies which would cause scumming or from excessive
organic matter.
Fired properties:
-firing shrinkage of not more than 6%.
-color in oxidized firing medium buff, in reduction light orange-brown.
2. blending stoneware bodies
Whenever possible, stoneware bodies whould be compounded using a high
percentage
of some natural stoneware clay.
Commonly used ingredients for stoneware bodies with usual maximum amounts:
Stoneware clay to 100%
Ball clay to 50%
Kaolin to 30%
Fireclay to 75%
Sagger clay to 75%
Red earthenware clay to 25% (fires to ^4)
Red earthenware clay to 10% (fires to ^04)
Grog to 30%
Flint to 25% (not less than 10%)
Feldspar or neph sy to 25%
Talc to 10%
Pyrophyllite to 20%
3. materials for stoneware
BALL CLAY
Ball clay is unusable by itself because of its great drying and
firing shrinkage, which
may be as high as 20%. It is used primarily as an addition to clay
bodies to bring
about an increase of plasticity, dry strength, and fired density.
Ky ball clay is relatively dark in the raw state., It is very
plastic and quite light when fired.
Tenn ball clay is much lighter in raw state but not quite as plastic.
It burns to a very light color.
No matter what ball clay is used, it is well to use the air-floated form.
Ball clay does have a tendency to inhibit the development of a burnt-orange
color in reduced ware.
FIRECLAY
Fireclays as mined and sold are quite various, but they are all refractory
clays with a softening point of cone 15 or higher.
In selecting a fireclay to use it is well to try them out first
without any additions.
Amt of fireclay added to plastic stoneware is 10-30%, but if the
fireclay is plastic then up to 50%.
RED EARTHENWARE CLAY
Red clay is used in stoneware bodies to darken the color and to serve
as a flux.
Common red clay which matures by itself at around ^04 will darken a stoneware
body if used in quantities above 5%, and usually more than 10% will
darken it too
much and may cause warping and excessive vitrification or bloating.
Red clay which matures at about ^4 may be introduced in larger
amounts, as much as 25%.
GROG
For throwing the usual amount is 10-20%. In a throwing body better results are
obtained if the grog is screened to eliminate all dust smaller than
80 mesh. Particles
smaller than 100 mesh may have an adverse effect on plasticity. Range between
30 and 80 mesh together with the mass of very fine-grained clay gives a dense
packing and maximum strength.
FLINT
Flint adds the valuable property of hardness to the fired clay, and
since it is a
nonplastic substance, it also serves to "open" the body for better drying, and
it lessens the drying and firing shrinkage.
In throwing bodies the flint should be limited to 15%. In casting
bodies, usually 20% or more.
The addition of flint to a body greatly simplifies the problem of
getting glazes
to fit without crazing. Clays which have less than 10% of flint may give
trouble by causing most glazes to craze.
FELDSPAR
Feldspar melts at stoneware temperature and its presence in a body
has the effect
of tightening it, fusing it together, and reducing its absorbency.
Feldspar, like flint, has an adverse effect on the plasticity of a
clay, and in formulating
throwing bodies, care must be taken not to let the nonplastic part of the body,
including the flint, the feldspar, and the grog, exceed about 25%.
Nepheline syenite is a kind of feldspar, but it has a relatively low
fusion point.
Using it in a body instead of feldspar may bring down the maturing
temperature 2
or more cones.
The use of some feldspar in a body tends to improve the fit of glazes.
TALC may be used in stoneware in small amounts to increase the
density of the fired body.
PYROPHYLLITE is especially useful in casting bodies. Its behavior in
the fire is
similar to that of kaolin, but in the raw state it functions as a non-plastic.
4. Plasticity and shrinkage of stoneware bodies
Should be at least 75% clay and no more than 35% nonplastic ingredients
such as flint, feldspar, or grog.
Aging does not seem to begin until the clay is in the plastic
condition, and clay
when it is in slip form does not seem to gain much in workability
from age. Aging
seems to be promoted by proper wedging before the clay is stored.
In any clay plastic enough for throwing a total shrinkage of 10% is
not excessive.
Bentonite can be added to increase plasticity up to 2-3%. Bentonite
makes the clay
slower to stiffen or dry, increases the shrinkage somewhat, and may
seriously aggravate
any tendency toward warping or cracking.
8. basic stoneware body compositions
plastic stoneware
stoneware clay 60% (he proposes Jordan which RR says is
discontinued)
ball clay 20%
flint 10%
feldspar 10%
Grog can be added to these ingredients in amounts depending on how rough a
body is desired. Five percent will give a sligiht roughness, 20%
will make quite a
rough body and effect the plasticity. If the available stoneware
clay is very plastic,
less ball clay night be needed. If the stoneware clay is very
nonplastic, more ball
clay would be needed.
Fired color of this clay is light grey-tan in oxidation and light
brown in reduction.
Its absorption at cone 9 is 1%. Its shrinkage at cone 9 is 11.5%.
Stoneware based on fireclay
Fireclay 40%
Ball clay 30%
Red clay 10%
Flint 10%
Feldspar 10%
The kind of fireclay used is critical. If a plastic one is
available, a body of this sort
may be very good to work with. At cone 9 the shrinkage is 10% and the fired
absorption is 15%.
--
Bellows Falls Vermont
www.eleanoraeden.com
Ron Roy on thu 6 jul 06
I have problems with both of them - too many to answer here and now.
If you are firing at cone 10 - be sure to read Peter Sohngen's atrticle in
Studio Potter - vol. 28#1 - forget the silica and make sure there is at
least 10% spar.
Most of the problems are because there is no cone mentioned - it is quite
different at cone 6 than at cone 10.
I would never advise the use of more than 30% of one kind of clay - and I
would make sure I knew that clay was one of the more stable clays.
The testing of raw materials is central to designing well behaved clays -
not eassy I know - most clay makers do not test their raw materials -
however - if they do they should be glad to share their data with you.
Using a low fire iron rich clay to get colour in a high fire body is a very
bad mistake. Been there - done that.
RR
>These are quotes from Nelson and Rhodes. I am not arguing these
>positions, only
>sharing what I have read. I would love to see comments from
>clayarters on their experiences
>that may underscore or deviate from these comments. I have found
>this very educating.
>I am not adding any of my own comments or questions.
>
>Nelson (p144 Ceramics, A Potter's Handbook)
>
>"In preparing a clay body several things can be done to control the
>effects of quartz inversion.
> A combination of pyrophyllite and talc substituted for part of the
>feldspar will reduce thermal
>expansion, as will the addition of petalite. Spodumene in a clay
>body creates an irreversible
> expansion, while the inclusion of wollastonite also reduces firing
>shrinkage and aids in
>developing a tougher body that is more resistant to heat shock.
>
>Rhodes (highlights of chapter 3, Stoneware and Porcelain, freely excerpted)
>
> 1.natural stoneware clays
>
>Stoneware clay is closely related to fireclay and might be thought of
>as a low-grade, plastic fireclay.
>
>Ideal properties of stoneware:
>
> Physical:
>
>-excellent plasticity yet with enough coarse material to give a
>slight tooth and to
>permit the making of larger shapes without slumping.
>
>-drying shrinkage no more than 5% and no tendency to warp or crack on drying.
>
>-freedom from alkalies which would cause scumming or from excessive
>organic matter.
>
> Fired properties:
>
>-firing shrinkage of not more than 6%.
>-color in oxidized firing medium buff, in reduction light orange-brown.
>
>2. blending stoneware bodies
>
>Whenever possible, stoneware bodies whould be compounded using a high
>percentage
>of some natural stoneware clay.
>
>Commonly used ingredients for stoneware bodies with usual maximum amounts:
>
>Stoneware clay to 100%
>Ball clay to 50%
>Kaolin to 30%
>Fireclay to 75%
>Sagger clay to 75%
>Red earthenware clay to 25% (fires to ^4)
>Red earthenware clay to 10% (fires to ^04)
>Grog to 30%
>Flint to 25% (not less than 10%)
>Feldspar or neph sy to 25%
>Talc to 10%
>Pyrophyllite to 20%
>
>3. materials for stoneware
>
>BALL CLAY
>Ball clay is unusable by itself because of its great drying and
>firing shrinkage, which
>may be as high as 20%. It is used primarily as an addition to clay
>bodies to bring
>about an increase of plasticity, dry strength, and fired density.
>
>Ky ball clay is relatively dark in the raw state., It is very
>plastic and quite light when fired.
>Tenn ball clay is much lighter in raw state but not quite as plastic.
>It burns to a very light color.
>
>No matter what ball clay is used, it is well to use the air-floated form.
>
>Ball clay does have a tendency to inhibit the development of a burnt-orange
>color in reduced ware.
>
>FIRECLAY
>Fireclays as mined and sold are quite various, but they are all refractory
>clays with a softening point of cone 15 or higher.
>
>In selecting a fireclay to use it is well to try them out first
>without any additions.
>
>Amt of fireclay added to plastic stoneware is 10-30%, but if the
>fireclay is plastic then up to 50%.
>
>RED EARTHENWARE CLAY
>Red clay is used in stoneware bodies to darken the color and to serve
>as a flux.
>
>Common red clay which matures by itself at around ^04 will darken a stoneware
>body if used in quantities above 5%, and usually more than 10% will
>darken it too
>much and may cause warping and excessive vitrification or bloating.
>
>Red clay which matures at about ^4 may be introduced in larger
>amounts, as much as 25%.
>
>GROG
>For throwing the usual amount is 10-20%. In a throwing body better results are
>obtained if the grog is screened to eliminate all dust smaller than
>80 mesh. Particles
>smaller than 100 mesh may have an adverse effect on plasticity. Range between
>30 and 80 mesh together with the mass of very fine-grained clay gives a dense
>packing and maximum strength.
>
>FLINT
>Flint adds the valuable property of hardness to the fired clay, and
>since it is a
>nonplastic substance, it also serves to "open" the body for better drying, and
> it lessens the drying and firing shrinkage.
>
>In throwing bodies the flint should be limited to 15%. In casting
>bodies, usually 20% or more.
>
>The addition of flint to a body greatly simplifies the problem of
>getting glazes
>to fit without crazing. Clays which have less than 10% of flint may give
>trouble by causing most glazes to craze.
>
>FELDSPAR
>Feldspar melts at stoneware temperature and its presence in a body
>has the effect
>of tightening it, fusing it together, and reducing its absorbency.
>
>Feldspar, like flint, has an adverse effect on the plasticity of a
>clay, and in formulating
>throwing bodies, care must be taken not to let the nonplastic part of the body,
>including the flint, the feldspar, and the grog, exceed about 25%.
>
>Nepheline syenite is a kind of feldspar, but it has a relatively low
>fusion point.
>Using it in a body instead of feldspar may bring down the maturing
>temperature 2
>or more cones.
>
>The use of some feldspar in a body tends to improve the fit of glazes.
>
>TALC may be used in stoneware in small amounts to increase the
>density of the fired body.
>
>PYROPHYLLITE is especially useful in casting bodies. Its behavior in
>the fire is
>similar to that of kaolin, but in the raw state it functions as a non-plastic.
>
>4. Plasticity and shrinkage of stoneware bodies
>
>Should be at least 75% clay and no more than 35% nonplastic ingredients
>such as flint, feldspar, or grog.
>
>Aging does not seem to begin until the clay is in the plastic
>condition, and clay
>when it is in slip form does not seem to gain much in workability
>from age. Aging
>seems to be promoted by proper wedging before the clay is stored.
>
>In any clay plastic enough for throwing a total shrinkage of 10% is
>not excessive.
>
>Bentonite can be added to increase plasticity up to 2-3%. Bentonite
>makes the clay
>slower to stiffen or dry, increases the shrinkage somewhat, and may
>seriously aggravate
>any tendency toward warping or cracking.
>
>8. basic stoneware body compositions
>
> plastic stoneware
>stoneware clay 60% (he proposes Jordan which RR says is
>discontinued)
>ball clay 20%
>flint 10%
>feldspar 10%
>
>Grog can be added to these ingredients in amounts depending on how rough a
>body is desired. Five percent will give a sligiht roughness, 20%
>will make quite a
>rough body and effect the plasticity. If the available stoneware
>clay is very plastic,
>less ball clay night be needed. If the stoneware clay is very
>nonplastic, more ball
>clay would be needed.
>
>Fired color of this clay is light grey-tan in oxidation and light
>brown in reduction.
> Its absorption at cone 9 is 1%. Its shrinkage at cone 9 is 11.5%.
>
> Stoneware based on fireclay
>Fireclay 40%
>Ball clay 30%
>Red clay 10%
>Flint 10%
>Feldspar 10%
>
>The kind of fireclay used is critical. If a plastic one is
>available, a body of this sort
>may be very good to work with. At cone 9 the shrinkage is 10% and the fired
>absorption is 15%.
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>--
>Bellows Falls Vermont
>www.eleanoraeden.com
>
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>
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>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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