William Lucius on tue 27 jan 04
While doing fieldwork in central Utah a few years ago my wife and I =
sampled a number of clays and brought back a batch of gray clay to play =
with. It turns out that the clay is very plastic, quite forgiving (does =
not warp like a lot of native clays) and fires at bisque (cone 010) to a =
nice light orange color. However, the aromatheraphy house that I made =
for my wife for Christmas has had enough time to begin pitting. Sure =
enough, at the base of each pit is a small offending piece of gypsum. =
And given the number of pits that have developed so far, there is a lot =
of gypsum in this clay, although each piece is quite small given my clay =
processing procedures. Traditional Acoma pottery (a Pueblo Pueblo =
located west of Albuquerque) has the same problem, which can severly =
impact the painted decorations. =20
I remember reading somewhere that a higher firing temperature will =
sinter the gypsum and render it inactive, but I cannot remember where to =
even begin looking for that little scrap of data. I performed a cursory =
search of the archives but found no mention of that temperature. If =
someone happens to have this little bit of information and would share =
it, it would be appreciated. Otherwise I will have either begin =
reviewing the literature or start experimenting, which at present are =
highly unlikely behaviors.
William A. Lucius
iacr@msn.com
Ron Roy on sat 31 jan 04
Hi William,
Firing is not going to solve the problem - some of the lime is going to be
close to the surface and will rehydrate and pop.
The only way I know of is to pass the clay through a 30 mesh screen (35
would be better for low fire work) - do it dry or wet but get the bigger
chunks out.
If potters were looking for clay to mine they ignored any clay with Calcium
in it as more trouble that it's worth.
If high fired clay has more than a trace of calcium it can be a problem as
well - KNaO is the flux of choice for stoneware and porcelain because it
eats cristobalite as it forms - CaO and MgO do not. Not a problem at cone 6
but it sure is at cone 10.
RR
>While doing fieldwork in central Utah a few years ago my wife and I
>sampled a number of clays and brought back a batch of gray clay to play
>with. It turns out that the clay is very plastic, quite forgiving (does
>not warp like a lot of native clays) and fires at bisque (cone 010) to a
>nice light orange color. However, the aromatheraphy house that I made for
>my wife for Christmas has had enough time to begin pitting. Sure enough,
>at the base of each pit is a small offending piece of gypsum. And given
>the number of pits that have developed so far, there is a lot of gypsum in
>this clay, although each piece is quite small given my clay processing
>procedures. Traditional Acoma pottery (a Pueblo Pueblo located west of
>Albuquerque) has the same problem, which can severly impact the painted
>decorations.
>
>I remember reading somewhere that a higher firing temperature will sinter
>the gypsum and render it inactive, but I cannot remember where to even
>begin looking for that little scrap of data. I performed a cursory search
>of the archives but found no mention of that temperature. If someone
>happens to have this little bit of information and would share it, it
>would be appreciated. Otherwise I will have either begin reviewing the
>literature or start experimenting, which at present are highly unlikely
>behaviors.
>
>William A. Lucius
>iacr@msn.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Snail Scott on sun 1 feb 04
At 01:07 PM 1/31/04 -0500, you wrote:
>Firing is not going to solve the problem - some of the lime is going to be
>close to the surface and will rehydrate and pop.
I don't know how low you can fire and still
melt the gypsum into a pop-proof form, but it
is definitely possible at the higher stoneware
temperatures. Your local clay may not fire high
enough to make that feasable, though, and it
certainly wouldn't be earthenware anymore if it
did.
-Snail
Craig Edwards on sun 1 feb 04
Hello Snail: In my limited experience I have never been able to melt
gypsum into a " pop proof form". I have burned some pots pretty hot and
soaked them for a long time without melting lime. I use seashells (
calcium, lime ) to put pots on at high temps so they won't stick to the
shelves. Are we talking about the same thing?
The Best
Craig Edwards
New London MN
Snail Scott wrote:
>
>
>I don't know how low you can fire and still
>melt the gypsum into a pop-proof form, but it
>is definitely possible at the higher stoneware
>temperatures. Your local clay may not fire high
>enough to make that feasable, though, and it
>certainly wouldn't be earthenware anymore if it
>did.
>
> -Snail
>
>______________________________________________________________________________
>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.
>
>
>
Ivor and Olive Lewis on mon 2 feb 04
Dear Snail Scott,
I believe Gypsum, Calcium Sulphate will first Dehydrate at about 180=BAC
then Decompose above red heat, discharging Sulphur Trioxide gas
(irritant, corrosive), leaving behind a residue of Calcium Oxide. If
the original gypsum was finely divided the Calcium Oxide will be well
dispersed and will react with free silica if that is present. If the
Gypsum was granular and some CaO remains any water or water vapour
which penetrates into the ceramic will create Ca(OH)2. The increase in
volume may cause enough stress to cause "Plaster Pops"
Best regards,
Ivor Lewis. Redhill, South Australia
Snail Scott on mon 2 feb 04
At 01:37 PM 2/1/04 -0600, you wrote:
>Hello Snail: In my limited experience I have never been able to melt
>gypsum into a " pop proof form". I have burned some pots pretty hot and
>soaked them for a long time without melting lime.
I recently did a commission piece in which
I made a life-mold of the client's torso then
used it as a press-mold to make a 'portrait'.
I used an unglazed red stoneware fired to ^6.
The mold was a bit rough, and left plaster
residue in some areas. To keep the roughness
of the surface and keep it from looking too
'worked' I didn't try to remove all of the
plaster before firing, and afterward the
largest plaster chunks had become glaze-like
shiny spots.
I don't try to explain the chemical process -
I'm just reporting my observation. Perhaps
Ivor or someone else could provide some
theories?
-Snail
Dave Finkelnburg on mon 2 feb 04
Dear Snail,
I have researched this a little. One reference I found says that the
thermal decomposition of gypsum into calcium oxide and sulfur oxides
proceeds faster, and at a lower temperature, in a reducing atmosphere. The
presence of oxygen apparently slows the decomposition. So I would think
reduction would tend to produce more glaze from gypsum than oxidation
firing.
I am now wondering, and this is PURELY speculation, whether we observe
gypsum pop-outs rather than lime pop-outs, in biscuit ware???
Good potting,
Dave Finkelnburg
----- Original Message -----
From: "Snail Scott"
Sent: Monday, February 02, 2004 9:11 AM
> I recently did a commission piece in which
> I made a life-mold of the client's torso then
> used it as a press-mold to make a 'portrait'.
> I used an unglazed red stoneware fired to ^6.
> The mold was a bit rough, and left plaster
> residue in some areas. To keep the roughness
> of the surface and keep it from looking too
> 'worked' I didn't try to remove all of the
> plaster before firing, and afterward the
> largest plaster chunks had become glaze-like
> shiny spots.
Craig Edwards on mon 2 feb 04
Hello Snail: Yeah it would be good to hear some theories. It's got me
stumped. Lime has a melting point of 2900 C well beyond cone six. I have
used whiting CaCo2 for waddings in salt firings and had them be inert.
What's going on with it melting shiny at cone 6?
Cheers
Craig Edwards
New London MN
http://www.livejournal.com/~smilingpotter/
Snail Scott wrote:
>At 01:37 PM 2/1/04 -0600, you wrote:
>
>
>>Hello Snail: In my limited experience I have never been able to melt
>>gypsum into a " pop proof form". I have burned some pots pretty hot and
>>soaked them for a long time without melting lime.
>>
>>
>
>
>I recently did a commission piece in which
>I made a life-mold of the client's torso then
>used it as a press-mold to make a 'portrait'.
>I used an unglazed red stoneware fired to ^6.
>The mold was a bit rough, and left plaster
>residue in some areas. To keep the roughness
>of the surface and keep it from looking too
>'worked' I didn't try to remove all of the
>plaster before firing, and afterward the
>largest plaster chunks had become glaze-like
>shiny spots.
>
>I don't try to explain the chemical process -
>I'm just reporting my observation. Perhaps
>Ivor or someone else could provide some
>theories?
>
> -Snail
>
>
>
Ivor and Olive Lewis on tue 3 feb 04
Dear Snail Scott,
Minor amounts of Calcium salts or plaster residues on the surface of
the clay would no doubt enter into some form of chemical reaction by
the time your sculptural work reached cone 6. David suggests the
potential for decomposition above a temperature or 1100 so there is
the possibility for Calcium oxide to be available. If your clay had
free silica in its make up there is potential for a reaction given
Wollastonite which is a recognised flux in your firing range. As you
say, a theory ! Bet it was a lovely surface ! !
Best regards,
Ivor Lewis. Redhill, South Australia
Snail Scott on wed 4 feb 04
At 02:45 PM 2/2/04 -0700, Dave F wrote:
>...One reference I found says that the
>thermal decomposition of gypsum into calcium oxide and sulfur oxides
>proceeds faster, and at a lower temperature, in a reducing atmosphere...
>
>----- Original Message -----
>From: "Snail Scott"
>Sent: Monday, February 02, 2004 9:11 AM
>>...I didn't try to remove all of the
>> plaster before firing, and afterward the
>> largest plaster chunks had become glaze-like
>> shiny spots.
The piece I referred to was single-fired to ^6
in an electric kiln, so by normal standards it
was not reduction-fired. There was surely
some burn-out occurring, but could it have been
enough to make a difference? Seems unlikely.
Could the high iron content of the clay have
caused some fluxing effect?
-Snail
Ivor and Olive Lewis on fri 6 feb 04
Dear Snail Scott,
At Cone 6, 1200 =BA C, 2192=BA F there is a lot of chemistry going on
inside clay as well as at the surface that we know nothing about. In
addition we are not dealing with pure substances. It is possible that
any sample of Potter's Plaster will contain a fraction of Calcium
Carbonate. Even one percent could be enough to initiate or trigger
some sort of reaction and contribute to a melting process.
People seem willing to believe that highly refractory solids like
Chromium Sesquioxide are intensely volatile at the temperature you
mention. Why should they draw the line and disbar a material we know
will decompose only a hundred or so degrees higher than your firing
temperature.
As I said earlier, there may be a catalytic effect which assists the
decomposition of CaSO4. Iron oxide might be responsible. Try it out.
Make up a sample, fire it and test to see if the residue will dissolve
to give an alkaline solution.
Enjoy the weekend.
Best regards,
Ivor Lewis. Redhill, South Australia
----- Original Message -----
From: "Snail Scott"
To:
Sent: Thursday, 5 February 2004 4:13
Subject: Re: Gypsum Pitting
> At 02:45 PM 2/2/04 -0700, Dave F wrote:
> >...One reference I found says that the
> >thermal decomposition of gypsum into calcium oxide and sulfur
oxides
> >proceeds faster, and at a lower temperature, in a reducing
atmosphere...
> >
> >----- Original Message -----
> >From: "Snail Scott"
> >Sent: Monday, February 02, 2004 9:11 AM
> >>...I didn't try to remove all of the
> >> plaster before firing, and afterward the
> >> largest plaster chunks had become glaze-like
> >> shiny spots.
>
>
> The piece I referred to was single-fired to ^6
> in an electric kiln, so by normal standards it
> was not reduction-fired. There was surely
> some burn-out occurring, but could it have been
> enough to make a difference? Seems unlikely.
> Could the high iron content of the clay have
> caused some fluxing effect?
>
> -Snail
>
>
______________________________________________________________________
________
> 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.
William Lucius on fri 6 feb 04
I have been following the Gypsum Pitting thread with interest. I =
realize now that when I posed the question that it was a necessary step =
for me to tackle the long-term memory storage bins of my brain. In the =
meantime Dave Finkelburg and I struck up an interesting e-mail =
conversation off the list - we both share an interest in the technical =
side of ceramics. A few e-mails and an aha moment reminded me where to =
find an almost forgotten paper on the addition of rock salt to clay to =
prevent lime pops, which I forwarded to Dave. He agreed that we should =
post a summary of our findings to Clayart, and gave me the assignment.
Unlike the majority of you, I use only native clays, which have the =
propensity to contain significant amounts of gypsum. Even at cone 5-6 =
glaze temperatures the gypsum eventually leads to gypsum pitting. =
Rather than trying to melt the offending bits, which takes a temperature =
something over 1200C, one answer is to add salt to the clay. The =
aforementioned technical paper reveals that brickmakers add salt (NaCl) =
to inhibit lime (CaCO3) spalling. To summarize the paper, the addition =
of .5% NaCl acts as a flux, leading to combination of CaCO3 and iron to =
form stable compoiunds below 900 to 1000C, lowers the onset of =
vitrification and seals off the lime from water vapor, increases the =
pore size of the CaCO3 or some combination of all three. If you need =
the bibliographic reference of the paper, contact me off list. Although =
we do not know exactly how gypsum differs from lime or from Plaster of =
Paris and I have not tried to add salt to my already salty clay, it is =
possible that the addition of rock salt to your clay might minimize =
pitting from those stray bits of Plaster of Paris. =20
William A. Lucius
iacr@msn.com
Snail Scott on sat 7 feb 04
At 03:57 PM 2/6/04 +1030, Ivor L wrote:
>...It is possible that
>any sample of Potter's Plaster will contain a fraction of Calcium
>Carbonate...
This may not be relevant, but the plaster stuck
to the clay was almost certainly not #1 Pottery
Plaster, but plaster from the plaster-impregnated
medical-gauze wrappings (as used for broken leg
casts, etc) which comprised the inner layer of the
life-cast I used for the press-mold. I have no
idea how much it may diverge from other plaster
formulations, but it _may_ be another factor.
-Snail
Ivor and Olive Lewis on sat 7 feb 04
Dear William Lucius,
It is doubtful adding Sodium Chloride would have any effect, in a
chemical sense, on Calcium Sulphate.
If the behaviours of raw acids are anything to go by, Sulphuric acid
will discharge Hydrogen chloride from Sodium Chloride but Hydrochloric
acid will not discharge Sulphur Trioxide from Calcium Sulphate, though
it will discharge Hydrogen Sulphide from Metallic Sulphides and
Sulphur dioxide from Metallic Sulphites.
In the end, to get to answers we need to do the Calculations and the
chemical bench work and that should happen in our Universities where
they have the resources to undertake the tasks and people trained in
critical research.
Best regards,
Ivor Lewis. Redhill, South Australia
----- Original Message -----
From: "William Lucius"
To:
Sent: Saturday, 7 February 2004 3:25
Subject: Re: Gypsum Pitting
I have been following the Gypsum Pitting thread with interest. I
realize now that when I posed the question that it was a necessary
step for me to tackle the long-term memory storage bins of my brain.
In the meantime Dave Finkelburg and I struck up an interesting e-mail
conversation off the list - we both share an interest in the technical
side of ceramics. A few e-mails and an aha moment reminded me where
to find an almost forgotten paper on the addition of rock salt to clay
to prevent lime pops, which I forwarded to Dave. He agreed that we
should post a summary of our findings to Clayart, and gave me the
assignment.
Unlike the majority of you, I use only native clays, which have the
propensity to contain significant amounts of gypsum. Even at cone 5-6
glaze temperatures the gypsum eventually leads to gypsum pitting.
Rather than trying to melt the offending bits, which takes a
temperature something over 1200C, one answer is to add salt to the
clay. The aforementioned technical paper reveals that brickmakers add
salt (NaCl) to inhibit lime (CaCO3) spalling. To summarize the paper,
the addition of .5% NaCl acts as a flux, leading to combination of
CaCO3 and iron to form stable compoiunds below 900 to 1000C, lowers
the onset of vitrification and seals off the lime from water vapor,
increases the pore size of the CaCO3 or some combination of all three.
If you need the bibliographic reference of the paper, contact me off
list. Although we do not know exactly how gypsum differs from lime or
from Plaster of Paris and I have not tried to add salt to my already
salty clay, it is possible that the addition of rock salt to your clay
might minimize pitting from those stray bits of Plaster of Paris.
William A. Lucius
iacr@msn.com
______________________________________________________________________
________
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.
Ivor and Olive Lewis on sun 8 feb 04
One of the penalties of using materials of unknown provenance.
As little as one percent CaCO3 as a contaminant will convert to CaO
above 850=BAC. Sufficient to initiate some form of fluxing action which
could propagate sintering is initiated as temperature rises.
Best regards,
Ivor Lewis. Redhill, South Australia
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