Ivor and Olive Lewis on sun 23 jan 05
Dear Wayne Seidl,
You have given us a great authoritative answer but there is one thing
which comes to mind that puzzles me. Could there be an additional
source or cause of salts that will effloresce.
Is there some additional chemistry going on here? Most clays are
classed as "Ion Exchange Media". As such they are able to adsorb both
Group 1 and Group 2 Ions.
But if the electronic forces that govern these things allow Calcium to
swap for Sodium from the clay then things would happen as described.
Now Gypsum Plaster is Calcium Sulphate and mostly insoluble. But if
the Calcium Ions from the wet plaster migrate to the clay and their
place is taken by Sodium Ions you would get a build up of Sodium
Sulphate, a soluble salt, in the bed of plaster. During drying water
will migrate to the surface, especially at the base where it is drawn
b y gravity and as the concentration of salt builds up, long before
dryness is achieved you will get this crystal fur.
Just a speculative thought.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
wjskw@BELLSOUTH.NET on sun 23 jan 05
Ivor and all...
in between...
... there is one thing
which comes to mind that puzzles me. Could there be an additional
source or cause of salts that will effloresce.
Is there some additional chemistry going on here?=20
Yes, of course, Ivor. Most potable water in this part of
the country is softened with calcium salts prior to being
distributed. Those salts can indeed be part of the problem.
Most clays are
classed as "Ion Exchange Media". As such they are able to adsorb
both
Group 1 and Group 2 Ions.
But if the electronic forces that govern these things allow Calcium
to
swap for Sodium from the clay then things would happen as described.
Now Gypsum Plaster is Calcium Sulphate and mostly insoluble. But if
the Calcium Ions from the wet plaster migrate to the clay and their
place is taken by Sodium Ions you would get a build up of Sodium
Sulphate, a soluble salt, in the bed of plaster. During drying water
will migrate to the surface, especially at the base where it is
drawn
b y gravity and as the concentration of salt builds up, long before
dryness is achieved you will get this crystal fur.
Just a speculative thought.
The buildup and re-crystallization of the calcium and salts
in the plaster can also be responsible for the spalling
Ingeborg notes in her post, especially where the plaster is
allowed to remain damp, and therefore weakened over time
(against the plywood substrate).
This has also been noted as a cause of concrete spalling in
the building industry. I have seen a prime example of it on my
own house, where the moist ground, in contact with the concrete,
has resulted in a surface spalling, bubbling and
lifting the paint. Closer examination revealed a buildup of
those very same salts directly beneath the paint. It requires
constant maintenance, and does not stop, even when the concrete is
allowed to dry thoroughly for months.
=09
I wonder if those salts could be put to use, perhaps induced
to occur in a clay body and then used as a surface treatment or a
decoration during subsequent firing, much as one "salts" a
kiln. That is an interesting thought indeed...creating
self-glazing clay bodies. I've spent most of this career
combating just such an occurrence. Quite a switch to start
inducing it in my next one .
Best,
Wayne
=09
Snail Scott on sun 23 jan 05
At 08:34 AM 1/23/2005 -0500, Wayne S. wrote:
I wonder if those salts could be put to use, perhaps induced
>to occur in a clay body and then used as a surface treatment...creating
>self-glazing clay bodies.
I have often seen this phenomenon. The 'scumming'
which causes a whitish cast to earthenware will
(if severe enough) cause a slight darkening and
glossing of stoneware. I have observed it mainly
when a piece was partially covered to control
drying, and the solubles had migrated to the
exposed surface area. If the surface acquires the
classic look of scummed clay, those areas will
look slightly more vitrified after the stoneware
firing, in exactly the pattern that the scumming
had beforehand.
If everyone worked with unglazed stoneware, as I
do, it would probably be common knowledge, but
glaze eliminates the effect.
I suspect that the effect is only obvious when a
sharp contrast between the scummed and non-scummed
areas exists, as when a sheet of plastic prevents
evaporation across part of an otherwise smooth
surface. The scumming also has to be quite strong,
so clay made with more mineral-laden water (or
clay with more solubles) would make the effect more
pronounced. I've never done this on purpose - it's
mainly been a minor pain in the butt - but perhaps
a cut-plastic stencil laid across the drying clay
would result in a subtle deliberate pattern. It
IS pretty subtle, even compared to the effect of
a very thin iron-oxide wash, but a very minimalist
form and surface might show it to advantage. I would
assume that this also works best with thick-walled
forms in an 'open' clay body (like mine), and that
most thin-thrown pottery just wouldn't have enough
'stuff' in it to result in a visible efflorescence.
Slow drying would also be important, to give the
solubles time to migrate out to the surface. Adding
extra solubles would probably help, too, sort of
like making high-fire Egyptian paste. ;)
-Snail
wjskw@BELLSOUTH.NET on sun 23 jan 05
Snail!
What a thought! Perhaps "salting" a wet mug, then covering it with
plastic wrap to induce scumming? Partially covering it and
woodfiring when ready? Salting a slip and decorating with it?
Repeatedly dipping it in brine? Bisquing it and throwing it in the
sea for a year in a protective wire mesh box?
Oh, the possibilities. I can't wait to get dirty again.
I wonder, could/would it be "functional surface"? I suppose that
would be up to testing to determine.
Thanks for replying!
Best Regards,
Wayne
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Snail
Scott
Sent: Sunday, January 23, 2005 11:03 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Plaster Problems (long) and a speculative thought
At 08:34 AM 1/23/2005 -0500,Wayne S. wrote:
I wonder if those salts could be put to use, perhaps induced
>to occur in a clay body and then used as a surface
treatment...creating
>self-glazing clay bodies.
I have often seen this phenomenon. The 'scumming'
which causes a whitish cast to earthenware will
(if severe enough) cause a slight darkening and
glossing of stoneware. I have observed it mainly
when a piece was partially covered to control
drying, and the solubles had migrated to the
exposed surface area. If the surface acquires the
classic look of scummed clay, those areas will
look slightly more vitrified after the stoneware
firing, in exactly the pattern that the scumming
had beforehand.
If everyone worked with unglazed stoneware, as I
do, it would probably be common knowledge, but
glaze eliminates the effect.
I suspect that the effect is only obvious when a
sharp contrast between the scummed and non-scummed
areas exists, as when a sheet of plastic prevents
evaporation across part of an otherwise smooth
surface. The scumming also has to be quite strong,
so clay made with more mineral-laden water (or
clay with more solubles) would make the effect more
pronounced. I've never done this on purpose - it's
mainly been a minor pain in the butt - but perhaps
a cut-plastic stencil laid across the drying clay
would result in a subtle deliberate pattern. It
IS pretty subtle, even compared to the effect of
a very thin iron-oxide wash, but a very minimalist
form and surface might show it to advantage. I would
assume that this also works best with thick-walled
forms in an 'open' clay body (like mine), and that
most thin-thrown pottery just wouldn't have enough
'stuff' in it to result in a visible efflorescence.
Slow drying would also be important, to give the
solubles time to migrate out to the surface. Adding
extra solubles would probably help, too, sort of
like making high-fire Egyptian paste. ;)
-Snail
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Ivor and Olive Lewis on mon 24 jan 05
Self glazing clays were the "Buzz" in 1902.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ivor and Olive Lewis on mon 24 jan 05
Dear Wayne,
WE used Sodium Chloride to restore the Ion exchange medium.That was
because the water was rich in Calcium and the ion exchange medium
sequestered the Calcium ions, discharging the Sodium ions into the
water flow. Sodium has the ability to expel Calcium Ions.
There are several ways to identify of the salt in the efflorescence,
A flame test will show if it is a calcium salt or if it is a sodium
salt. Under the microscope These salts will have differing forms but
it would take a chemist or mineralogist to distinguish between them.
In a plaster slab my bet would be on Sodium Sulphate. In time the loss
of Calcium from the hydrated Calcium Sulphate will destroy the
mechanical integrity of the slab.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From:
To:
Sent: Monday, 24 January 2005 12:04
Subject: Re: Plaster Problems (long) and a speculative thought
Ivor and all...
in between...
... there is one thing
which comes to mind that puzzles me. Could there be an additional
source or cause of salts that will effloresce.
Is there some additional chemistry going on here?
Yes, of course, Ivor. Most potable water in this part of
the country is softened with calcium salts prior to being
distributed. Those salts can indeed be part of the problem.
Most clays are
classed as "Ion Exchange Media". As such they are able to adsorb
both
Group 1 and Group 2 Ions.
But if the electronic forces that govern these things allow Calcium
to
swap for Sodium from the clay then things would happen as described.
Now Gypsum Plaster is Calcium Sulphate and mostly insoluble. But if
the Calcium Ions from the wet plaster migrate to the clay and their
place is taken by Sodium Ions you would get a build up of Sodium
Sulphate, a soluble salt, in the bed of plaster. During drying water
will migrate to the surface, especially at the base where it is
drawn
b y gravity and as the concentration of salt builds up, long before
dryness is achieved you will get this crystal fur.
Just a speculative thought.
The buildup and re-crystallization of the calcium and salts
in the plaster can also be responsible for the spalling
Ingeborg notes in her post, especially where the plaster is
allowed to remain damp, and therefore weakened over time
(against the plywood substrate).
This has also been noted as a cause of concrete spalling in
the building industry. I have seen a prime example of it on my
own house, where the moist ground, in contact with the concrete,
has resulted in a surface spalling, bubbling and
lifting the paint. Closer examination revealed a buildup of
those very same salts directly beneath the paint. It requires
constant maintenance, and does not stop, even when the concrete is
allowed to dry thoroughly for months.
I wonder if those salts could be put to use, perhaps induced
to occur in a clay body and then used as a surface treatment or a
decoration during subsequent firing, much as one "salts" a
kiln. That is an interesting thought indeed...creating
self-glazing clay bodies. I've spent most of this career
combating just such an occurrence. Quite a switch to start
inducing it in my next one .
Best,
Wayne
______________________________________________________________________
________
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.
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