Leslie St. Clair on mon 18 oct 04
Hi Ron,
I think I understand what you're saying about gasses
causing pinholes if the carbon materials aren't burned
out in the bisque. Is the iron becoming a flux a
separate problem? Or does this occur only if the
carbonaceous materials aren't burned out?
If it does occur separately, how can the problem be
fixed?
thanks for explaining this.
Leslie St. Clair in Cincinnati, Ohio
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Ron Roy on wed 20 oct 04
Good question Leslie - I did not make that very clear.
The gases are coming from the clay - which is being overfired - because of
the extra fluxing from the reduced iron.
When clay starts to melt too much the clay becomes covered with small
blisters - you can see em easy if there is no glaze over the clay - it's
called blebbing.
As the clay is fired more the blisters become bigger - from the gas being
liberated as the melting progresses - at some point it is described as
bloating - the bubbles get together and become big bubbles.
It happen to glazes as well on the way to being properly melted.
If the firing of the clay continues - eventually the clay melts into a
glaze - as will Albany slip at cone 6.
If you want to see this happening - make a small sample of just RedArt clay
and fire it to cone 6 - many earthenware clays will be blebbed or bloated
at cone 6.
When any burning goes on in any clay - oxygen is used up - if there is not
a fresh supply of air - to replace the oxygen that is used in the burning -
then the carbon or carbon monoxide - will take it where ever it can find
it. Red Iron (Fe2O3) is right there and will give up some of it's oxygen
easily - so the iron gets reduced and become a flux.
Much depends on the way the clay body is formulated - if it is close to
vitrification at glaze temperature then it does not take much of that to
make the clay be overfired - think if finely divided iron throughout a clay
body - from ball clay for instance - or chunks of iron from fire clay.
This is one of the reasons every potter should know the amount of
absorbency in their clay bodies - it tells you how close you are to
problems and you can take the right steps to avoid problems before they
happen.
RR
>Hi Ron,
> I think I understand what you're saying about gasses
>causing pinholes if the carbon materials aren't burned
>out in the bisque. Is the iron becoming a flux a
>separate problem? Or does this occur only if the
>carbonaceous materials aren't burned out?
>If it does occur separately, how can the problem be
>fixed?
> thanks for explaining this.
> Leslie St. Clair in Cincinnati, Ohio
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on thu 21 oct 04
And interesting thread.
I have back tracked through the posts to Mel's original question.
It would seem appropriate to suggest that there are a lot of separate
issues to sort out.
First among these are the assumptions that have been made. Second is
to separate fact from conjecture. Third is to clearly distinguish
between the various processes that are involved. Fourth is to
distinguish between fact and fiction, or knowledge arising from myth,
folklore and legend in the resources that have been cited. Fifth is to
provide Physical and Chemical arguments or evidence that support
reasons for the phenomena under discussion. Finally, is it wise to
allow a reader to interpret sense or context when a generalised terms
such as Flux or Fluxing are being used, especially when what is said
is in conflict with published data.
As I said in my reply to Mel to his question "does clay contain gas?"
"You are correct, I do not think clay has gas either. But then, it all
depends on what you mean...........
I hope other people read your question carefully!!"
There are complex issues about material behaviour concealed in Mel's
simple question that are unresolved. Do not assume ready made answers
are to be found in current popular literature.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
From: "Ron Roy"
To:
Sent: Wednesday, 20 October 2004 3:54
Subject: Re: Ron Roy/bisque/gasses
> Good question Leslie - I did not make that very clear.
>
> The gases are coming from the clay - which is being overfired -
because of
> the extra fluxing from the reduced iron.
>
> When clay starts to melt too much the clay becomes covered with
small
> blisters - you can see em easy if there is no glaze over the clay -
it's
> called blebbing.
>
> As the clay is fired more the blisters become bigger - from the gas
being
> liberated as the melting progresses - at some point it is described
as
> bloating - the bubbles get together and become big bubbles.
>
> It happen to glazes as well on the way to being properly melted.
>
> If the firing of the clay continues - eventually the clay melts into
a
> glaze - as will Albany slip at cone 6.
>
> If you want to see this happening - make a small sample of just
RedArt clay
> and fire it to cone 6 - many earthenware clays will be blebbed or
bloated
> at cone 6.
>
> When any burning goes on in any clay - oxygen is used up - if there
is not
> a fresh supply of air - to replace the oxygen that is used in the
burning -
> then the carbon or carbon monoxide - will take it where ever it can
find
> it. Red Iron (Fe2O3) is right there and will give up some of it's
oxygen
> easily - so the iron gets reduced and become a flux.
>
> Much depends on the way the clay body is formulated - if it is close
to
> vitrification at glaze temperature then it does not take much of
that to
> make the clay be overfired - think if finely divided iron throughout
a clay
> body - from ball clay for instance - or chunks of iron from fire
clay.
>
> This is one of the reasons every potter should know the amount of
> absorbency in their clay bodies - it tells you how close you are to
> problems and you can take the right steps to avoid problems before
they
> happen.
>
> RR
>
>
>
> >Hi Ron,
> > I think I understand what you're saying about gasses
> >causing pinholes if the carbon materials aren't burned
> >out in the bisque. Is the iron becoming a flux a
> >separate problem? Or does this occur only if the
> >carbonaceous materials aren't burned out?
> >If it does occur separately, how can the problem be
> >fixed?
> > thanks for explaining this.
> > Leslie St. Clair in Cincinnati, Ohio
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
>
______________________________________________________________________
________
> 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.
Louis Katz on thu 21 oct 04
Hi Ivor,
O.K. ( some of the material in this post is for others )
My assumption about bloating and pinholes, I don't have the means to =20
determine if it is fact, just my working theory my model is:
That as carbonates and sulphates go into solution the CO2 or S02 that =20=
would be given off if they were heated enough goes into solution until =20=
the glaze is saturated with the gas and then the gases come out of =20
solution as bubbles.
This process I believe can happen in a glaze, or in the glassy matrix =20=
of a clay body. Soubility of gases in glass is a big field. (
If gases come out of solution as glass cools the glass maker is never =20=
able to comletely fine out his glass, a big problem)
Also there is always gas coming out of an open pore porous object as it =20=
being heated as the gases expand under constant pressure as the =20
temperature increases. The volume of the gasses increases =20
proportionally to the absolute temperature (Kelvin) assuming the =20
pressure does not change.
Of these two the first reason is far more likely to evolve large =20
quantities of gases. It does not take much solid material to produce a =20=
vast volume of gas, one gram of CO2 fills about 1/2 liter I think at =20
Standard temperature and pressure (1). Because this gas is being =20
evolved at near 1274 degrees Kelvin it takes up roughly 4 times that =20
volume or in the realm of two liters.
This is about the same volume as a 2000 grams of clay, (just eyeballing =20=
here but I have given the density of clay below(2))
I don't know what the volume of pores are at as clay approaches =20
maturity but it is small by weight of water and therefore small by =20
volume. Since you have to double the absolute temperature to double the =20=
volume of gasses as small 50 or even 100 K degree climb in temp will =20
produce only a small amount of expansion in the pore space. I suspect =20=
that impuritie CO2 and SO2 that evolve as a material goes into solution =20=
are much more likely to cause problems than pore spaces.
I would assume that as Iron oxide looses its O in reduction it either =20=
gets attached to an H, HO becoming steam or to a CO becoming CO2. =20
Neither of these processes would seem to create bubbles, They look like =20=
there owuld be no net change in the volume of gas. Am I dreaming any of =20=
this or does it make sense from your sense of things.
Just by the way, it would not surprise me if some other mechanism =20
created pinholes.
Have to go stretch a University technology budget that has effectively =20=
gone down as the population of students and faculty have gone up.
Louis
(1) At O C and sealevel 22.4 liters of CO2 weights about 44 grams. So =20=
one gram of CO2 is about 1/2 liter, In the same ballpark by volume as a =20=
1000 gr. of clay.
http://www.all-science-fair-projects.com/=20
science_fair_projects_encyclopedia/=20
Standard_conditions_for_temperature_and_pressure
"In chemistry, the term standard temperature and pressure (abbreviated =20=
STP) denotes an exact reference temperature of 0=B0C (273.15 K) and =20
pressure of 1 atm (defined as 101.325 kPa). These values approximate =20
freezing temperature of water and atmospheric pressure at sea level."
2()Wet clay about 1602 Kg/cu meter
On Oct 20, 2004, at 11:36 PM, Ivor and Olive Lewis wrote:
> And interesting thread.
> I have back tracked through the posts to Mel's original question.
> It would seem appropriate to suggest that there are a lot of separate
> issues to sort out.
> First among these are the assumptions that have been made. Second is
> to separate fact from conjecture. Third is to clearly distinguish
> between the various processes that are involved. Fourth is to
> distinguish between fact and fiction, or knowledge arising from myth,
> folklore and legend in the resources that have been cited. Fifth is to
> provide Physical and Chemical arguments or evidence that support
> reasons for the phenomena under discussion. Finally, is it wise to
> allow a reader to interpret sense or context when a generalised terms
> such as Flux or Fluxing are being used, especially when what is said
> is in conflict with published data.
> As I said in my reply to Mel to his question "does clay contain gas?"
> "You are correct, I do not think clay has gas either. But then, it all
> depends on what you mean...........
> I hope other people read your question carefully!!"
> There are complex issues about material behaviour concealed in Mel's
> simple question that are unresolved. Do not assume ready made answers
> are to be found in current popular literature.
> Best regards,
> Ivor Lewis.
> Redhill,
> S. Australia.
>
>
>
> From: "Ron Roy"
> To:
> Sent: Wednesday, 20 October 2004 3:54
> Subject: Re: Ron Roy/bisque/gasses
>
>
>> Good question Leslie - I did not make that very clear.
>>
>> The gases are coming from the clay - which is being overfired -
> because of
>> the extra fluxing from the reduced iron.
>>
>> When clay starts to melt too much the clay becomes covered with
> small
>> blisters - you can see em easy if there is no glaze over the clay -
> it's
>> called blebbing.
>>
>> As the clay is fired more the blisters become bigger - from the gas
> being
>> liberated as the melting progresses - at some point it is described
> as
>> bloating - the bubbles get together and become big bubbles.
>>
>> It happen to glazes as well on the way to being properly melted.
>>
>> If the firing of the clay continues - eventually the clay melts into
> a
>> glaze - as will Albany slip at cone 6.
>>
>> If you want to see this happening - make a small sample of just
> RedArt clay
>> and fire it to cone 6 - many earthenware clays will be blebbed or
> bloated
>> at cone 6.
>>
>> When any burning goes on in any clay - oxygen is used up - if there
> is not
>> a fresh supply of air - to replace the oxygen that is used in the
> burning -
>> then the carbon or carbon monoxide - will take it where ever it can
> find
>> it. Red Iron (Fe2O3) is right there and will give up some of it's
> oxygen
>> easily - so the iron gets reduced and become a flux.
>>
>> Much depends on the way the clay body is formulated - if it is close
> to
>> vitrification at glaze temperature then it does not take much of
> that to
>> make the clay be overfired - think if finely divided iron throughout
> a clay
>> body - from ball clay for instance - or chunks of iron from fire
> clay.
>>
>> This is one of the reasons every potter should know the amount of
>> absorbency in their clay bodies - it tells you how close you are to
>> problems and you can take the right steps to avoid problems before
> they
>> happen.
>>
>> RR
>>
>>
>>
>>> Hi Ron,
>>> I think I understand what you're saying about gasses
>>> causing pinholes if the carbon materials aren't burned
>>> out in the bisque. Is the iron becoming a flux a
>>> separate problem? Or does this occur only if the
>>> carbonaceous materials aren't burned out?
>>> If it does occur separately, how can the problem be
>>> fixed?
>>> thanks for explaining this.
>>> Leslie St. Clair in Cincinnati, Ohio
>>
>> Ron Roy
>> RR#4
>> 15084 Little Lake Road
>> Brighton, Ontario
>> Canada
>> K0K 1H0
>> Phone: 613-475-9544
>> Fax: 613-475-3513
>>
>>
> ______________________________________________________________________
> ________
>> 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.
>
> =
_______________________________________________________________________=20=
> _______
> 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 =20
> melpots@pclink.com.
>
Ivor and Olive Lewis on fri 22 oct 04
Dear Louis Katz,
It is saddening to learn that you are having to cope with severe
financial constraints as teaching burdens escalated.
I became highly suspicious about this whole business, of bloating in
clay bodies and porosity or bubbles and pinholing in Clay/glaze
systems, when I had a series of glaze tests which gave high degrees of
gas inclusions on a tight porcelain clay which was both low in Iron
contamination and not known to contain compounds of Sulphur (as
sulphates) or Carbon (as carbonates).
My knowledge of glass making is not extensive enough to comment about
gas solubility in that medium.
I respect opinions that Red iron oxide can be reduced to black iron
oxide by free carbon with the evolution of a gas. I respect opinions
that if this happens in a clay that has vitrified we may expect
bloating. I respect opinions that if this happens in a partially
porous but mature clay body there could be porosity and pinholing in
the covering glaze.
I accept your mathematical evaluation of the effects of heating an
evolving gas at high temperature.
I have yet to be convinced that, of itself, Black Iron oxide ("FeO")
melts preferentially four hundred degrees below its measured melting
point (CRC Handbook 1377=BA C. ACS Phase diagrams 1369=BA C) . Clues to
possible maturing processes for clay bodes which are prone to bloating
due to the presence of Iron compounds in their makeup may be deduced
by reading Ch !0 of "Introduction to Ceramics", (a text I refer to
frequently for enlightenment) and a collection of Phase Equilibrium
Diagrams.
Thank you for your response.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
.com.
Louis Katz on fri 22 oct 04
On Oct 22, 2004, at 1:04 AM, Ivor and Olive Lewis wrote:
> Dear Louis Katz,
> It is saddening to learn that you are having to cope with severe
> financial constraints as teaching burdens escalated.
Best committee I have ever been on. We spend about $800,000 a year. Two=20=
years ago we decided to pass printing costs onto students as about half=20=
the printed text on campus was never even picked up. Much more=20
economical. We will not have to raise computer use fees and the new=20
system is working like a charm. In the end it is costing students and=20
the University less.
Yesterday after a comment about some departments not buying 3 year=20
warranties on their computers and the computer services having to pick=20=
up the cost we decided to self insure. It looks like it may save=20
$30,000 a year to me. It does carry some risk. The committe is a great=20=
group of people representing their respective disciplines but alos=20
working for the good of all.
Nice, tightly run meetings. Little wasted time.
>
> My knowledge of glass making is not extensive enough to comment about
> gas solubility in that medium.
This is something I think we could learn more about. I think it may=20
have soemthing to do with pinholing in Rutile glazes (just a guess).
> I respect opinions that Red iron oxide can be reduced to black iron
> oxide by free carbon with the evolution of a gas. ]
Thanks for this concise statement. I kept assuming reduction by CO
>
> I accept your mathematical evaluation of the effects of heating an
> evolving gas at high temperature.
It was nice to quantify them and further confirm my suspicions
> I have yet to be convinced that, of itself, Black Iron oxide ("FeO")
> melts preferentially four hundred degrees below its measured melting
> point (CRC Handbook 1377=BA C. ACS Phase diagrams 1369=BA C) .
I may see if I can get a consortium of departments to go in on the big=20=
ACER's database of phase diagrams. Geology and the engineering=20
departments have expressed interest.
> Clues to
> possible maturing processes for clay bodes which are prone to bloating
> due to the presence of Iron compounds in their makeup may be deduced
> by reading Ch !0 of "Introduction to Ceramics", (a text I refer to
> frequently for enlightenment) and a collection of Phase Equilibrium
> Diagrams.
Thanks for your help.
Louis
KE5CVK
http://www.tamucc.edu/~lkatz/cs/files/
Ivor and Olive Lewis on sat 23 oct 04
Dear Louis,
Seems as though you have good control where you work and a good crew
to work with.
Access to the ACS database could be a good investment, though just
having the Levin et al Atlas of diagrams (4th reprint 1979) to hand
might be sufficient for your departmental needs. The current price is
reasonable. I bought my copy about fifteen years ago and it is
starting to show its age. The ACS wall charts are very good as well as
being a reasonable price. Ruling of the percentage lines ensures you
can read to about 0.10%. I have the CaO-Al2O3-Si02, Na2O-Al2O3-Si02,
K2O-Al2O3-Si02, MgO-Al2O3-Si02 and FeO-Fe2O3-SiO2 plates. I think they
should also do one for ZnO-SiO2-Al2O3.
The direct reaction between Red Iron Oxide and Carbon is interesting
since when carried to completion it gives Metallic Iron. Greenwood and
Earnshaw give an equation 2Fe2O3+3C=> 4Fe+ 3CO2 (p1072). However, I
think it to be more probable that at those lower temperatures we are
considering that reaction stops short and Carbon monoxide is involved.
Look forward to more chance discussions.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
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