Eric Hansen on mon 7 jun 10
What about using fine sand as grog? What kinds of things do I need to be
aware of? If my clay is fairly plastic, let's say I am using a mixture of
Fireclay, and OM4, a dash of spar, etc., and I want to use sand instead of
grog, what kinds of problems does this create/ Do I have to limit how much =
I
use or what the mesh size is?
h a n s e n
Ron Roy on tue 8 jun 10
Hi Eric,
Cristobalite forms theoretically from 1100C on and goes through its
inversion in a more gradual way around 200C.
I have measured hundreds of cone 6 bodies and only found cristobalite
in a few from one manufacturer - I do believe it was coming in with a
raw material. You can add cristobalite to low fire clay to help
counteract crazing for instance.
I have measured gold art and it was one of the few clays that did not
produce any cristobalite at cone 10R. Most raw clays will have some -
some will have a lot!
I think you will get cristobalite at cone 8 and 9 - I'll be willing to
run a few samples for you if you are interested - read the excellent
article -
Peter Sohngen in Studio Potter Volume 28 #1.
In essence his research (I did the dilatometery) shows you need at
least 10% spar and go easy on micro fine silica (what we usually get
in the bag.)
Graded silica - like 200M with few fines is not a problem.
The more iron involved the more complicated the problem becomes.
The time you stay above 1100C is when cristobalite forms - on the way
up and on the way down - so crash cooling down to 1100C from you top
temp will cut down the amount if it is a problem.
On the other hand staying at top temperature longer will reduce the
amount of free quartz in a body - especially if it's fine sand and you
have lots (at least 10%) spar - its the sodium and potassium that
likes melting silica.
RR
Quoting Eric Hansen :
> Ron: I know from past posts that you have researched the topic of quartz =
in
> clay bodies, I have another related question - at what cone,
> time/temperature does cristobalite formation occur? Can this be avoided b=
y
> staying in the cone 8 & 9 range? Earlier you mentioned that Gold Art and
> Foundry Hill Creme tend to form less cristobalite, but the Gold Art has a
> sulphur problem so I am told.
>
> One idea I have heard to control the difference between plate rim and foo=
t
> is to use wadding -
>
> My kiln is going to cool quickly unless I ramp it down; I can't imagine h=
ow
> to keep it from dropping fairly quickly through Q inversion, unless I jus=
t
> turn it back to "low" as in candling type of LOW
> h a n s e n
>
> On Tue, Jun 8, 2010 at 6:08 PM, wrote:
>
>> Hi Eric,
>>
>> The mesh size and amount will be a factor - just remember - at the quart=
z
>> inversion on the way down for both bisque and glaze firing - the more fr=
ee
>> quartz in the body the bigger the change in volume of the clay at 573C.
>> Plate rims go through the inversion at exactly 573C - not gradually but
>> suddenly. The foot - still in contact with a hot shelf goes through the
>> inversion later - this can result in the classic quartz dunting.
>>
>> If your kiln cools slow enough you don't have to worry but most kiln coo=
l
>> fast enough, especially at the bottom, for this to be a problem.
>>
>> There is a classic micro photo of a clay body with pieces of quartz in t=
he
>> clay - there are voids around the quartz because it contracted at 573C a=
nd
>> separated from the clay. Those voids are useful in that any micro cracks=
in
>> a clay will stop at those voids.
>>
>> So there can be advantages and disadvantages but if you understand what =
is
>> happening you can design around the disadvantages.
>>
>> RR
>>
>>
>>
>> Quoting Eric Hansen :
>>
>> What about using fine sand as grog? What kinds of things do I need to b=
e
>>> aware of? If my clay is fairly plastic, let's say I am using a mixture =
of
>>> Fireclay, and OM4, a dash of spar, etc., and I want to use sand instead=
of
>>> grog, what kinds of problems does this create/ Do I have to limit how m=
uch
>>> I
>>> use or what the mesh size is?
>>> h a n s e n
>>>
>>>
>>
>>
>>
>
Eric Hansen on tue 8 jun 10
Ron: I know from past posts that you have researched the topic of quartz in
clay bodies, I have another related question - at what cone,
time/temperature does cristobalite formation occur? Can this be avoided by
staying in the cone 8 & 9 range? Earlier you mentioned that Gold Art and
Foundry Hill Creme tend to form less cristobalite, but the Gold Art has a
sulphur problem so I am told.
One idea I have heard to control the difference between plate rim and foot
is to use wadding -
My kiln is going to cool quickly unless I ramp it down; I can't imagine how
to keep it from dropping fairly quickly through Q inversion, unless I just
turn it back to "low" as in candling type of LOW
h a n s e n
On Tue, Jun 8, 2010 at 6:08 PM, wrote:
> Hi Eric,
>
> The mesh size and amount will be a factor - just remember - at the quartz
> inversion on the way down for both bisque and glaze firing - the more fre=
e
> quartz in the body the bigger the change in volume of the clay at 573C.
> Plate rims go through the inversion at exactly 573C - not gradually but
> suddenly. The foot - still in contact with a hot shelf goes through the
> inversion later - this can result in the classic quartz dunting.
>
> If your kiln cools slow enough you don't have to worry but most kiln cool
> fast enough, especially at the bottom, for this to be a problem.
>
> There is a classic micro photo of a clay body with pieces of quartz in th=
e
> clay - there are voids around the quartz because it contracted at 573C an=
d
> separated from the clay. Those voids are useful in that any micro cracks =
in
> a clay will stop at those voids.
>
> So there can be advantages and disadvantages but if you understand what i=
s
> happening you can design around the disadvantages.
>
> RR
>
>
>
> Quoting Eric Hansen :
>
> What about using fine sand as grog? What kinds of things do I need to be
>> aware of? If my clay is fairly plastic, let's say I am using a mixture o=
f
>> Fireclay, and OM4, a dash of spar, etc., and I want to use sand instead =
of
>> grog, what kinds of problems does this create/ Do I have to limit how mu=
ch
>> I
>> use or what the mesh size is?
>> h a n s e n
>>
>>
>
>
>
Eric Hansen on tue 8 jun 10
Mike: Thanks for your input. I'm wondering if the geological sedimentary
process resulting in sandstone is what isolates so much material with fine
particle size. I have noticed the difference between it and beach sand,
where the smaller sands are removed by water and wind. Once we camped out o=
n
Mustang Island down by Corpus and the wind blew all night and in the mornin=
g
the tent had sand the size of flour which worked its way through the weave
of the tent canvas.
As mentioned in another post, we have sandy soil here which is mica,
feldspar, clay, and sand. Depending on where you dig of course, everything
here is cross-bedded (the sedimentary layers are pitched on a diagonal) wit=
h
the present earth surface cutting through it. The problem with digging this
clay is that it has all this other fun stuff but not quite enough of the
clay fraction, so unless you're making bricks, it cracks in drying. The fix
here is probably "just add ball clay"
At the same time I am testing the other way: 3 single-component clay bodies=
,
Gold Art, Roseville, and Kentucky Stone. So I have tests going of 100% clay
+ grog and was wondering how 100% clay + sand might differ. At the same tim=
e
I am mixing reclaim to which I am adding increased amounts of the local
clay, which has too much sand alone by itself, and some of that sand is
feldspar as well as mica and quartz. One of the local commercial clay
manufactures is doing basically the same thing, they have a color palate
which looks like local clay and you can see the mica in the clay too. There
are all kinds of local clay pits still operating in the region.
h a n s e n
On Tue, Jun 8, 2010 at 1:53 PM, Mike wrote:
> Hello Eric,
>
> I routinely use sandstone which I've pounded to powder in a mortar in my
> clay bodies. I find that it gives them more strength in the firing
> (sometimes the clay gets a little wiggly at high temps, and rims warp a b=
it)
> while at the same time helping to prevent leaking and sweating.
>
> The mesh size varies, probably from 100 mesh or finer, up to particles
> about 0.5 mm in size. By far, most of the particles seem at the finer end=
of
> the range. I'm just at the beginning of using this material (only a year =
or
> two now), but I think that the varied particle size is important for gett=
ing
> the two benefits listed above.
>
> The thing I suspect, though, is that there are many types of sand out
> there, some may melt/flux too much, some may not melt at all.
>
> As little as 10% is enough to seal the body for me in most cases, though =
I
> add up to 50% sometimes because it really makes the clay fun and interest=
ing
> to throw and trim.
>
> Mike
>
>
> Mike
> in Taku, Japan
> http://karatsupots.comhttp://karatsupots.blogspot.com
>
> Kiln Building Workshop, Oct. 15 - 22
> http://karatsupots.com/workshop2010/workshop2010.htmlhttp://karatsupots-w=
orkshop.blogspot.com/
>
>
> (2010/06/08 7:21), Eric Hansen wrote:
>
> What about using fine sand as grog? What kinds of things do I need to be
> aware of? If my clay is fairly plastic, let's say I am using a mixture of
> Fireclay, and OM4, a dash of spar, etc., and I want to use sand instead o=
f
> grog, what kinds of problems does this create/ Do I have to limit how muc=
h I
> use or what the mesh size is?
> h a n s e n
>
>
>
>
Ron Roy on tue 8 jun 10
Hi Eric,
The mesh size and amount will be a factor - just remember - at the
quartz inversion on the way down for both bisque and glaze firing -
the more free quartz in the body the bigger the change in volume of
the clay at 573C. Plate rims go through the inversion at exactly 573C
- not gradually but suddenly. The foot - still in contact with a hot
shelf goes through the inversion later - this can result in the
classic quartz dunting.
If your kiln cools slow enough you don't have to worry but most kiln
cool fast enough, especially at the bottom, for this to be a problem.
There is a classic micro photo of a clay body with pieces of quartz in
the clay - there are voids around the quartz because it contracted at
573C and separated from the clay. Those voids are useful in that any
micro cracks in a clay will stop at those voids.
So there can be advantages and disadvantages but if you understand
what is happening you can design around the disadvantages.
RR
Quoting Eric Hansen :
> What about using fine sand as grog? What kinds of things do I need to be
> aware of? If my clay is fairly plastic, let's say I am using a mixture of
> Fireclay, and OM4, a dash of spar, etc., and I want to use sand instead o=
f
> grog, what kinds of problems does this create/ Do I have to limit how muc=
h I
> use or what the mesh size is?
> h a n s e n
>
Eric Hansen on wed 9 jun 10
Ron: Cedar Heights recommends both Gold Art and Roseville as
single-component clay bodies, I think they state up to cone 8 for Gold Art
and cone 6 for Roseville. Their thinking is that Roseville has nicer
fire-color; I haven't fired my tests yet, we'll see. Gold Art (Cedar Height=
s
Bond Clay, not Gold Art 200 mesh air-floated) has a slightly unpleasant
color and texture when fired to cone 6-8 in an electric kiln environment. I
don't have a UPA for Roseville unfortunately.
My thinking is that the KNaO in Gold Art has something to do with the low
cristobalite formation; your earlier post indicated fired Roseville to have
higher cristobalite forming. I'm wondering what would be wrong with just
adding some of Red Art to the Gold Art to get a better color, and with Red
Art having over twice the KNaO as Gold Art. The high iron 7-8% shouldn't be
a problem unless you fire it in reduction. So I can simply drill some red
art into the slurry and add that to the dry gold art and mix.
Another problem with Gold Art is the over 13% shrinkage much of which
shockingly occurs during the leather hard stages. I'm debating exactly how
much medium grog I want, if I have to add a lot of grog maybe finer grog
might be nicer.
The other clay I'm testing is Kentucky Stone, but my suspicions about it ar=
e
that it isn't going to be open and toothy enough. I think it is 200 mesh
air-floated. It probably is a good match for local clay I'm digging however=
.
I just mixed about 50 lbs. of it into a batch of reclaim slurry, it sure is
slow on stiffening up. What I have read about it is that it is a kind of
half-way point between an illite-type stoneware clay like gold art and a
standard ball clay.
The local clay has to be crushed, dried, blunged, made into a slip, and
poured through a 10 mesh screen, allowed to settle, excess water poured off=
.
It has feldspar, mica, and quartz sands in it and not quite enough clay
fraction for pottery, hence the idea to add ball clay, so then it opens up
the ball clay more. This process makes it hard to weigh the dry local clay,
however. If I weigh it before hand, then whatever I remove can be dried I
can weigh that and subtract I suppose, and that will give me a number to us=
e
in future clay mixing.
I've fired the local clay to cone 8, about to the color of dark brick red.
It starts to self glaze. At cone 10 reduction wood fire it becomes black
with a semi-gloss self glaze.
I was going to test Foundry Hill Creme from Spinks, but it is out of stock
right now.
h a n s e n
On Tue, Jun 8, 2010 at 11:42 PM, wrote:
> Hi Eric,
>
> Cristobalite forms theoretically from 1100C on and goes through its
> inversion in a more gradual way around 200C.
>
> I have measured hundreds of cone 6 bodies and only found cristobalite in =
a
> few from one manufacturer - I do believe it was coming in with a raw
> material. You can add cristobalite to low fire clay to help counteract
> crazing for instance.
>
> I have measured gold art and it was one of the few clays that did not
> produce any cristobalite at cone 10R. Most raw clays will have some - som=
e
> will have a lot!
>
> I think you will get cristobalite at cone 8 and 9 - I'll be willing to ru=
n
> a few samples for you if you are interested - read the excellent article =
-
>
> Peter Sohngen in Studio Potter Volume 28 #1.
>
> In essence his research (I did the dilatometery) shows you need at least
> 10% spar and go easy on micro fine silica (what we usually get in the bag=
.)
>
> Graded silica - like 200M with few fines is not a problem.
>
> The more iron involved the more complicated the problem becomes.
>
> The time you stay above 1100C is when cristobalite forms - on the way up
> and on the way down - so crash cooling down to 1100C from you top temp wi=
ll
> cut down the amount if it is a problem.
>
> On the other hand staying at top temperature longer will reduce the amoun=
t
> of free quartz in a body - especially if it's fine sand and you have lots
> (at least 10%) spar - its the sodium and potassium that likes melting
> silica.
>
> RR
>
>
>
> Quoting Eric Hansen :
>
> Ron: I know from past posts that you have researched the topic of quartz
>> in
>> clay bodies, I have another related question - at what cone,
>> time/temperature does cristobalite formation occur? Can this be avoided =
by
>> staying in the cone 8 & 9 range? Earlier you mentioned that Gold Art and
>> Foundry Hill Creme tend to form less cristobalite, but the Gold Art has =
a
>> sulphur problem so I am told.
>>
>> One idea I have heard to control the difference between plate rim and fo=
ot
>> is to use wadding -
>>
>> My kiln is going to cool quickly unless I ramp it down; I can't imagine
>> how
>> to keep it from dropping fairly quickly through Q inversion, unless I ju=
st
>> turn it back to "low" as in candling type of LOW
>> h a n s e n
>>
>> On Tue, Jun 8, 2010 at 6:08 PM, wrote:
>>
>> Hi Eric,
>>>
>>> The mesh size and amount will be a factor - just remember - at the quar=
tz
>>> inversion on the way down for both bisque and glaze firing - the more
>>> free
>>> quartz in the body the bigger the change in volume of the clay at 573C.
>>> Plate rims go through the inversion at exactly 573C - not gradually but
>>> suddenly. The foot - still in contact with a hot shelf goes through the
>>> inversion later - this can result in the classic quartz dunting.
>>>
>>> If your kiln cools slow enough you don't have to worry but most kiln co=
ol
>>> fast enough, especially at the bottom, for this to be a problem.
>>>
>>> There is a classic micro photo of a clay body with pieces of quartz in
>>> the
>>> clay - there are voids around the quartz because it contracted at 573C
>>> and
>>> separated from the clay. Those voids are useful in that any micro crack=
s
>>> in
>>> a clay will stop at those voids.
>>>
>>> So there can be advantages and disadvantages but if you understand what
>>> is
>>> happening you can design around the disadvantages.
>>>
>>> RR
>>>
>>>
>>>
>>> Quoting Eric Hansen :
>>>
>>> What about using fine sand as grog? What kinds of things do I need to =
be
>>>
>>>> aware of? If my clay is fairly plastic, let's say I am using a mixture
>>>> of
>>>> Fireclay, and OM4, a dash of spar, etc., and I want to use sand instea=
d
>>>> of
>>>> grog, what kinds of problems does this create/ Do I have to limit how
>>>> much
>>>> I
>>>> use or what the mesh size is?
>>>> h a n s e n
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>
>
>
Mike on wed 9 jun 10
Hello Eric,
I routinely use sandstone which I've pounded to powder in a mortar in my
clay bodies. I find that it gives them more strength in the firing
(sometimes the clay gets a little wiggly at high temps, and rims warp a
bit) while at the same time helping to prevent leaking and sweating.
The mesh size varies, probably from 100 mesh or finer, up to particles
about 0.5 mm in size. By far, most of the particles seem at the finer
end of the range. I'm just at the beginning of using this material (only
a year or two now), but I think that the varied particle size is
important for getting the two benefits listed above.
The thing I suspect, though, is that there are many types of sand out
there, some may melt/flux too much, some may not melt at all.
As little as 10% is enough to seal the body for me in most cases, though
I add up to 50% sometimes because it really makes the clay fun and
interesting to throw and trim.
Mike
Mike
in Taku, Japan
http://karatsupots.com
http://karatsupots.blogspot.com
Kiln Building Workshop, Oct. 15 - 22
http://karatsupots.com/workshop2010/workshop2010.html
http://karatsupots-workshop.blogspot.com/
(2010/06/08 7:21), Eric Hansen wrote:
> What about using fine sand as grog? What kinds of things do I need to be
> aware of? If my clay is fairly plastic, let's say I am using a mixture of
> Fireclay, and OM4, a dash of spar, etc., and I want to use sand instead o=
f
> grog, what kinds of problems does this create/ Do I have to limit how muc=
h I
> use or what the mesh size is?
> h a n s e n
>
>
Ron Roy on thu 10 jun 10
Hi Eric,
I have some experience with Gold Art - will not use it anymore for a
couple of reasons.
The measured absorbency is up and down too much from batch to batch -
if you use a lot in a body you will need to test each batch of raw
material and adjust your recipe - that is if you want your clay to not
leak.
The Gold Art Bonding can vary in granular iron a lot as well - they
both have sulphur associated.
Kentucky stone is also variable so you have to keep an eye on it as well.
Foundry Hill Cream is a very stable clay.
My general rule for bodies is to not have more than 20% of any one
clay - if more than 20% then only with clays that do not vary more
than 1 or 2%.
If you send me a list of clays you have available I can rate them for
you in terms of variability based on 20 years of testing.
If you are firing to cone 10 you don't want to use Red Art - that
would be the quickest way to build cristobalite into your finished
product. You are going to need some highly refractory clay and at
least 10% spar. If you can get it - a good source of iron is Newman
clay - it still has an absorbency of about 8% at cone 10 reduction and
7% iron and is a fairly stable clay.
Let me know if you need more - RR
RR
Quoting Eric Hansen :
> Ron: Cedar Heights recommends both Gold Art and Roseville as
> single-component clay bodies, I think they state up to cone 8 for Gold Ar=
t
> and cone 6 for Roseville. Their thinking is that Roseville has nicer
> fire-color; I haven't fired my tests yet, we'll see. Gold Art (Cedar Heig=
hts
> Bond Clay, not Gold Art 200 mesh air-floated) has a slightly unpleasant
> color and texture when fired to cone 6-8 in an electric kiln environment.=
I
> don't have a UPA for Roseville unfortunately.
>
> My thinking is that the KNaO in Gold Art has something to do with the low
> cristobalite formation; your earlier post indicated fired Roseville to ha=
ve
> higher cristobalite forming. I'm wondering what would be wrong with just
> adding some of Red Art to the Gold Art to get a better color, and with Re=
d
> Art having over twice the KNaO as Gold Art. The high iron 7-8% shouldn't =
be
> a problem unless you fire it in reduction. So I can simply drill some red
> art into the slurry and add that to the dry gold art and mix.
>
> Another problem with Gold Art is the over 13% shrinkage much of which
> shockingly occurs during the leather hard stages. I'm debating exactly ho=
w
> much medium grog I want, if I have to add a lot of grog maybe finer grog
> might be nicer.
>
> The other clay I'm testing is Kentucky Stone, but my suspicions about it =
are
> that it isn't going to be open and toothy enough. I think it is 200 mesh
> air-floated. It probably is a good match for local clay I'm digging howev=
er.
> I just mixed about 50 lbs. of it into a batch of reclaim slurry, it sure =
is
> slow on stiffening up. What I have read about it is that it is a kind of
> half-way point between an illite-type stoneware clay like gold art and a
> standard ball clay.
>
> The local clay has to be crushed, dried, blunged, made into a slip, and
> poured through a 10 mesh screen, allowed to settle, excess water poured o=
ff.
> It has feldspar, mica, and quartz sands in it and not quite enough clay
> fraction for pottery, hence the idea to add ball clay, so then it opens u=
p
> the ball clay more. This process makes it hard to weigh the dry local cla=
y,
> however. If I weigh it before hand, then whatever I remove can be dried I
> can weigh that and subtract I suppose, and that will give me a number to =
use
> in future clay mixing.
>
> I've fired the local clay to cone 8, about to the color of dark brick red=
.
> It starts to self glaze. At cone 10 reduction wood fire it becomes black
> with a semi-gloss self glaze.
>
> I was going to test Foundry Hill Creme from Spinks, but it is out of stoc=
k
> right now.
> h a n s e n
>
>
>
> On Tue, Jun 8, 2010 at 11:42 PM, wrote:
>
>> Hi Eric,
>>
>> Cristobalite forms theoretically from 1100C on and goes through its
>> inversion in a more gradual way around 200C.
>>
>> I have measured hundreds of cone 6 bodies and only found cristobalite in=
a
>> few from one manufacturer - I do believe it was coming in with a raw
>> material. You can add cristobalite to low fire clay to help counteract
>> crazing for instance.
>>
>> I have measured gold art and it was one of the few clays that did not
>> produce any cristobalite at cone 10R. Most raw clays will have some - so=
me
>> will have a lot!
>>
>> I think you will get cristobalite at cone 8 and 9 - I'll be willing to r=
un
>> a few samples for you if you are interested - read the excellent article=
-
>>
>> Peter Sohngen in Studio Potter Volume 28 #1.
>>
>> In essence his research (I did the dilatometery) shows you need at least
>> 10% spar and go easy on micro fine silica (what we usually get in the ba=
g.)
>>
>> Graded silica - like 200M with few fines is not a problem.
>>
>> The more iron involved the more complicated the problem becomes.
>>
>> The time you stay above 1100C is when cristobalite forms - on the way up
>> and on the way down - so crash cooling down to 1100C from you top temp w=
ill
>> cut down the amount if it is a problem.
>>
>> On the other hand staying at top temperature longer will reduce the amou=
nt
>> of free quartz in a body - especially if it's fine sand and you have lot=
s
>> (at least 10%) spar - its the sodium and potassium that likes melting
>> silica.
>>
>> RR
>>
>>
>>
>> Quoting Eric Hansen :
>>
>> Ron: I know from past posts that you have researched the topic of quart=
z
>>> in
>>> clay bodies, I have another related question - at what cone,
>>> time/temperature does cristobalite formation occur? Can this be avoided=
by
>>> staying in the cone 8 & 9 range? Earlier you mentioned that Gold Art an=
d
>>> Foundry Hill Creme tend to form less cristobalite, but the Gold Art has=
a
>>> sulphur problem so I am told.
>>>
>>> One idea I have heard to control the difference between plate rim and f=
oot
>>> is to use wadding -
>>>
>>> My kiln is going to cool quickly unless I ramp it down; I can't imagine
>>> how
>>> to keep it from dropping fairly quickly through Q inversion, unless I j=
ust
>>> turn it back to "low" as in candling type of LOW
>>> h a n s e n
>>>
>>> On Tue, Jun 8, 2010 at 6:08 PM, wrote:
>>>
>>> Hi Eric,
>>>>
>>>> The mesh size and amount will be a factor - just remember - at the qua=
rtz
>>>> inversion on the way down for both bisque and glaze firing - the more
>>>> free
>>>> quartz in the body the bigger the change in volume of the clay at 573C=
.
>>>> Plate rims go through the inversion at exactly 573C - not gradually bu=
t
>>>> suddenly. The foot - still in contact with a hot shelf goes through th=
e
>>>> inversion later - this can result in the classic quartz dunting.
>>>>
>>>> If your kiln cools slow enough you don't have to worry but most kiln c=
ool
>>>> fast enough, especially at the bottom, for this to be a problem.
>>>>
>>>> There is a classic micro photo of a clay body with pieces of quartz in
>>>> the
>>>> clay - there are voids around the quartz because it contracted at 573C
>>>> and
>>>> separated from the clay. Those voids are useful in that any micro crac=
ks
>>>> in
>>>> a clay will stop at those voids.
>>>>
>>>> So there can be advantages and disadvantages but if you understand wha=
t
>>>> is
>>>> happening you can design around the disadvantages.
>>>>
>>>> RR
>>>>
>>>>
>>>>
>>>> Quoting Eric Hansen :
>>>>
>>>> What about using fine sand as grog? What kinds of things do I need to=
be
>>>>
>>>>> aware of? If my clay is fairly plastic, let's say I am using a mixtur=
e
>>>>> of
>>>>> Fireclay, and OM4, a dash of spar, etc., and I want to use sand inste=
ad
>>>>> of
>>>>> grog, what kinds of problems does this create/ Do I have to limit how
>>>>> much
>>>>> I
>>>>> use or what the mesh size is?
>>>>> h a n s e n
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>
>>
>>
>
Edouard Bastarache on thu 10 jun 10
Eric,
The Fe2O3, in reduction firing, will become FeO, a very powerful flux and a=
t
high temp, will prevent the formation of cristobalite by
producing "Fayalite" in reacting with silica.
I have sent a few posts on this matter to Clayart, backed by the knowledge
of Smart.Conseil.
Do not forget to fire in reduction if KNaO is not present,,,
Gis,
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.flickr.com/photos/30058682@N00/
http://edouardbastarache.blogspot.com/
http://blogsalbertbastarache.blogspot.com/
http://cerampeintures.blogspot.com/
http://perso.wanadoo.fr/smart2000/index.htm
Ron Roy on thu 10 jun 10
Hi Ed,
I have lots of dilatometer charts showing all kinds of cristobalite in
high iron bodies - in fact I have read that iron even helps the
cristobalite to form.
What does Smart base his findings on - any diatometry or is it theory?
What temperature does he say this happens?
RR
Quoting Edouard Bastarache :
> Eric,
>
>
> The Fe2O3, in reduction firing, will become FeO, a very powerful flux and=
at
> high temp, will prevent the formation of cristobalite by
> producing "Fayalite" in reacting with silica.
> I have sent a few posts on this matter to Clayart, backed by the knowledg=
e
> of Smart.Conseil.
> Do not forget to fire in reduction if KNaO is not present,,,
>
> Gis,
>
> Edouard Bastarache
> Spertesperantisto
>
> Sorel-Tracy
> Quebec
>
> http://www.flickr.com/photos/30058682@N00/
> http://edouardbastarache.blogspot.com/
> http://blogsalbertbastarache.blogspot.com/
> http://cerampeintures.blogspot.com/
> http://perso.wanadoo.fr/smart2000/index.htm
>
ivor and olive lewis on fri 11 jun 10
I recall the discussion about four or five years ago which Edouard
Bastarache mentions <<...The Fe2O3, in reduction firing, will become FeO, a
very powerful flux and at high temp, will prevent the formation of
Cristobalite by producing "Fayalite" in reacting with silica...>>
In oxygen free situations Iron oxides can be reduced to metallic iron at
temperatures as low as 700 deg Celsius.
What happens in a system containing Iron oxides and Silicon dioxide will
very much depend on the ratio of Iron Oxide to Silica and the species of
both compounds.
There seems to be a window of opportunity for Fayalite to form when 30% to
40% Cristobalite reacts with 60% to 70% Iron oxide, in the absence of
Oxygen.
The Ancients achieved this by using massive quantities of Charcoal to creat=
e
Wrought Iron.
Best regards,
Ivor Lewis,
REDHILL,
South Australia
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