Roeder on wed 13 aug 97
Hi folks,
For some time now, I've been meaning to plop a couple of my observations
regarding salt in the pitfire onto the clayart table....another
happening today reminded me to share this with you.
Salt has a rather nasty habit of being absorbed into the matrix of the
clay..into the clay body itself, during a pit fire. Then, some time
down the road, will swell with moisture gathered from the air, and cause
the body to break down, powdering off, flaking off...and no, I'm not
referring to a terra sig problem here.
Salt can cause immediate problems when in direct contact with the clay,
such as pitting, and cracking of the body. But this absorption problem
is much more subtle and problems can take years to become evident.
The first time I noticed it was a year after a pot came out of the
kiln. It was sitting on a shelf, and I noticed quite a bit of "stuff"
on the shelf around the base of the pot...white crumbly bits and dust.
I picked the pot up to clean the shelf, wondering where it had come
from, then I discovered the pot was eroding, where the salt coloration
had been. I could just rub away the surface of the pot with my thumb, in
the salt-flashed areas.
Now, today, as I was cleaning an area of my studio, I came across a
"test tile" that I had put salt on in a pit firing. This tile had been
fine all along, but just now, the surface has begun to flake away.
There was no sig on this piece. It had been fired to cone 05 before pit
firing.
I've noticed this phenomenon on raku clay, stoneware, earthenware, and
porcelain.
Those of you who are using salt, or salted materials (or salt water???)
in your firing may want to be more cautious in its use.
I wouldn't have known the salt was the cause of my pitting and eroding
had it not been for an article on primitive firing (a link on the
Ceramics Web).
So...there is a problem, are there any solutions out there? I've
stopped using it.
Candice Roeder
CRoeder@worldnet.att.net
Gavin Stairs on thu 14 aug 97
At 11:46 AM 13/08/97 EDT, Candice Roeder wrote:
....
>Salt has a rather nasty habit of being absorbed into the matrix of the
>clay..into the clay body itself, during a pit fire. Then, some time
>down the road, will swell with moisture gathered from the air, and cause
>the body to break down, powdering off, flaking off...
This problem is well known in the brick industry, as excess soluble salts
from either the brick or the mortar (or even from salt percolating into the
brick from groundwater, ice melting salts, etc.) can result in what is
known as efflorescence, or recrystallization of the salt in the evaporation
zone of the body. If this zone is beneath the surface, it will result in
exfoliation or crumbling of the surface. This can be a real problem with
brick walls and such.
In open ware bodies such a low fire earthenware, this can definitely be a
problem, especially if the salt has a chance to combine with calcium or
something else to produce a deliquescent salt. That is, CaCl2 takes water
out of the air and dissolves in it, making a brine which can easily migrate
to the surface and cause this problem.
When people speak of using salt in low fire, I can only think that it
should be used with extreme caution. In a high fire, the chloride is
driven off to some extent, and the remainder, if any, must be tightly
incorporated in the glass. This is not the case in the low fire. It may
just stay there as a fairly loosely bound constituent, capable of
decomposing the body as noted. If the soda is what is required, I would
try soda ash. If the salt gives something unique which can't be obtained
otherwise, which I doubt, then try to use it in very limited quantities,
and beware.
Gavin
Cindy on fri 15 aug 97
Gavin,
Is this just a problem with NaCl, or with any salt?
Cindy
> Hi folks,
>
> For some time now, I've been meaning to plop a couple of my observations
> regarding salt in the pitfire onto the clayart table....another
> happening today reminded me to share this with you.
>
> Salt has a rather nasty habit of being absorbed into the matrix of the
> clay..into the clay body itself, during a pit fire. Then, some time
> down the road, will swell with moisture gathered from the air, and cause
> the body to break down, powdering off, flaking off...and no, I'm not
> referring to a terra sig problem here.
>
> Salt can cause immediate problems when in direct contact with the clay,
> such as pitting, and cracking of the body. But this absorption problem
> is much more subtle and problems can take years to become evident.
>
> The first time I noticed it was a year after a pot came out of the
> kiln. It was sitting on a shelf, and I noticed quite a bit of "stuff"
> on the shelf around the base of the pot...white crumbly bits and dust.
> I picked the pot up to clean the shelf, wondering where it had come
> from, then I discovered the pot was eroding, where the salt coloration
> had been. I could just rub away the surface of the pot with my thumb, in
> the salt-flashed areas.
>
> Now, today, as I was cleaning an area of my studio, I came across a
> "test tile" that I had put salt on in a pit firing. This tile had been
> fine all along, but just now, the surface has begun to flake away.
> There was no sig on this piece. It had been fired to cone 05 before pit
> firing.
>
> I've noticed this phenomenon on raku clay, stoneware, earthenware, and
> porcelain.
>
> Those of you who are using salt, or salted materials (or salt water???)
> in your firing may want to be more cautious in its use.
>
> I wouldn't have known the salt was the cause of my pitting and eroding
> had it not been for an article on primitive firing (a link on the
> Ceramics Web).
>
> So...there is a problem, are there any solutions out there? I've
> stopped using it.
>
> Candice Roeder
> CRoeder@worldnet.att.net
Gavin Stairs on sat 16 aug 97
At 10:16 AM 15/08/97 EDT, Cindy wrote:
>----------------------------Original message----------------------------
....
>Is this just a problem with NaCl, or with any salt?
....
Hi Cindy,
It may be a problem with any salt that doesn't break down properly to oxide
at the firing temperature, and particularly those which are water soluable.
As it turns out, this means mostly the chlorides. The most important ones
are NaCl, KCl and CaCl2 (sodium chloride [table salt], potassium chloride
[not a common chemical, an important blood electrolyte] and calcium
chloride [road salt]), but I'd be a bit careful about any chlorides. NaCl
is the one most used in glazes on the supposition that it does something to
glaze character. CaCl2 is often cited as an amendment to floculate a
glaze. It acts by sequestering liquid water, which makes a slip less
runny. Normally used in very small quantities. Test.
Gavin
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