Earl Brunner on mon 9 oct 00
I think you are getting a couple of processes mixed up and
inferring some things that are not accurate.
"Curtis G. Nelson" wrote:
>
> Hello All,
> In 1979, in his "Contemporary Ceramic Techniques," John Conrad, on page 87, wrote:
>
> "With the same techniques used for salt glazing, [a] salt substitute [sodium
> carbonate, also known as soda ash] is vaporized in the kiln's flames, causing
> decomposition of the chemicals. The steam and sodium vapors combine with the
> silica of the clay to form a thin coating of glaze. Water and carbon dioxide,
> the by-products from the use of [sodium carbonate], are no more damaging than
> the by-products of regular stoneware or porcelain gas firing."
What Conrad is talking about here is the exhaust vapors
coming out of the kiln. The sodium in the soda ash will
still interact with silica and the other fluxable materials
in the kiln, (you are vapor coating the pots, the vapor will
come in contact with and interact with all surfaces that it
is able to interact with) this includes the kiln furniture
and the walls or bricks of the kiln.
>
> My questions are these:
If you go to the archives and look up simulated or
artificial salt glaze or firing (check here:
http://www.potters.org/category086.htm) You will find the
procedures and techniques that you are talking about.
In essence, yes you can dip or or paint soda ash onto the
surface of bisque and get flashing that can resemble salt or
wood fired, fluxed surface. Some of the posts in the
archives will describe this technique.
> Curious
>
> Curtis Nelson in Chicago
>
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--
Earl Brunner
http://coyote.accessnv.com/bruec
mailto:bruec@anv.net
Curtis G. Nelson on mon 9 oct 00
Hello All,
May I have some confirmation and reassurance on my conclusions reached after
reading parts of Conrad's book as quoted below . . .
In 1979, in his "Contemporary Ceramic Techniques," John Conrad, on page 87, wrote:
"With the same techniques used for salt glazing, [a] salt substitute [sodium
carbonate, also known as soda ash] is vaporized in the kiln's flames, causing
decomposition of the chemicals. The steam and sodium vapors combine with the
silica of the clay to form a thin coating of glaze. Water and carbon dioxide,
the by-products from the use of [sodium carbonate], are no more damaging than
the by-products of regular stoneware or porcelain gas firing."
Further, he wrote:
"To obtain the air movement necessary to force the damp mixture into the kiln,
many ceramists use innovative techniques such as sand blaster, heavy-duty
blower, or drip feed in front of the burners."
My questions are these:
1. Is it generally agreed that this kind of glaze firing, with it's only
by-products being water and carbon dioxide, is benign and harmless to the
interiors of fuel-burning kilns, including gas?
2. To have a successful soda ash glaze firing, does the soda ash have to be
introduced into the kiln only after volatilizing temperatures have been reached?
3. In other words, as an alternate to this procedure, would proper chemical
decomposition still occur during the firing if the soda ash solution were
first sprayed on the bisqueware, dried, and loaded in the kiln at room temperature?
If the answer to the first question is "yes," and the second is "no," and the
third is "yes," then:
Can an individual piece of bisqueware be sprayed with soda ash solution and
subsequently fired as part of a load of regular-glazed stoneware and porcelain
without danger of sodium vapor migration affecting the regular pots' surfaces?
That is to say, does soda ash glaze firing depend on creating a steam/sodium
whole-kiln atmosphere, or is it possible that the thin coating of glaze can be
developed locally, i.e., just on the pre-sprayed pot's surface, therefore not
affecting the other pots in the firing?
Curious
Curtis Nelson in Chicago
vince pitelka on mon 9 oct 00
Curtis -
Conrad's books usually have lots of good information, and some
misinformation.
> 1. Is it generally agreed that this kind of glaze firing, with it's only
> by-products being water and carbon dioxide, is benign and harmless to the
> interiors of fuel-burning kilns, including gas?
No that is not generally agreed upon. Just as is true with a salt kiln, a
kiln which is soda fired is considered a soda kiln, and the residual soda
vapors will affect subsequent firings. With either salt firing or soda
firings, you are creating sodium vapor which glazes both the wares and the
inside of the kiln. This sodium accumulation subsequently re-volatizes,
affecting subsequent firings even if additional salt or soda is not charged.
This is what we call a "residual" salt or soda firing.
The by-products of soda firing are by no means completely benign. In the
external atmosphere, neither salt-firing nor soda-firing by-products are
particularly toxic, but both produce highly corrosive fumes which will
attack metal (especially iron) surfaces in the area.
> 2. To have a successful soda ash glaze firing, does the soda ash have to
be
> introduced into the kiln only after volatilizing temperatures have been
reached?
> In other words, as an alternate to this procedure, would proper chemical
> decomposition still occur during the firing if the soda ash solution were
> first sprayed on the bisqueware, dried, and loaded in the kiln at room
temperature?
You can get some slight soda effects by applying soda-ash solution to the
bisque-fired pots, especially over slip-decorated effects. It is kind of
like a very slight wetting of the surface. But you run the risk of serious
carbon-coring, bloating, and/or slumping. Soda ash is of course
water-soluble, and therefore it soaks into the claybody with the water when
you glaze the pots. To get enough soda ash onto the piece, you end up with
a great deal soaked into the body. It is a powerful flux, and can simply
overflux the claybody.
> Can an individual piece of bisqueware be sprayed with soda ash solution
and
> subsequently fired as part of a load of regular-glazed stoneware and
porcelain
> without danger of sodium vapor migration affecting the regular pots'
surfaces?
As mentioned, you can do this to a limited extent, but you cannot produce
anything like true soda-glazed effects.
> That is to say, does soda ash glaze firing depend on creating a
steam/sodium
> whole-kiln atmosphere, or is it possible that the thin coating of glaze
can be
> developed locally, i.e., just on the pre-sprayed pot's surface, therefore
not
> affecting the other pots in the firing?
No, I don't see how that would be possible. If you really had enough soda
on the pot to create a soda-like effect, some would most likely volatize and
could effect other pots, and the kiln itself, especially if this was done
with any regularity.
Best wishes -
- Vince
Vince Pitelka
Home - vpitelka@dekalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
http://www.craftcenter.tntech.edu/
Paul Taylor on wed 11 oct 00
Dear Curtis.
Salt glazing can be done with several intentions. Just to get a glaze on
a pot or to get an orange peel like surface.
I do not believe the orange peel effect can be "got" except by vapor
glazing. What seems to be happening is that the glaze gives a fine crawling
but does not show as crawling because the exposed clay or slip is constantly
receiving a glaze flux - sodium.
All other effects can be made with a glaze or spraying soda in a media on
biscuit.
I could be wrong and would like to be.
Regards Paul T
Hank Murrow on wed 11 oct 00
Paul wrote;
>I do not believe the orange peel effect can be "got" except by vapor
>glazing. What seems to be happening is that the glaze gives a fine crawling
>but does not show as crawling because the exposed clay or slip is constantly
>receiving a glaze flux - sodium. All other effects can be made with a
>glaze or spraying soda in a media on biscuit. I could be wrong and would
>like to be.
Dear Paul;
Would you consider that the constitution of a typical body is very high in
alumina; and therefore would make an Al-rich glaze at the surface when
fluxed by sodium vapor? As with shinos(which are very high in Al) such
glazes have a high coefficient of contraction. They seem to bead up easily;
and it has been my conjecture that the 'pigskin' effect is due to this high
Al content. Now with a silica-rich porcelain body there is much less
pigskin effect, due to the reduced Al and consequent higher Si. Easy to
test; put an alumina wash on one side of a pot and a silica wash on the
other side and salt the pot, reporting to this forum. Something to consider
anyway, Hank in Eugene
Paul Taylor on thu 12 oct 00
Dear Hank
I did not explain my self very clearly.
What I meant was, that even on an alumina and/or iron rich surface a
mottled effect can only be achieved by vapor glazing, because although the
high alumina/soda glaze beads up, it is the thin vapor glazed surface that
exists between the beading that is all important to the effect. This is the
effect that makes salt glazing unique. All other effects can be made with a
thin spray of glaze material.
A similar orange peel effect can be had from ash but the effect is very
streaky and not the same.
But as you state the color and texture depends on the alumina in the
body. I also suspect that the sort of alumna in the body makes a difference
to the pigs skin effect. Would a body that contains china clay give a more
mottled appearance than just a high alumina secondary clay ? It seems to
make a difference to the color. Which I think is to do with the titanium
impurities in the secondary clays. china clay has a larger particle size and
I suspect a reluctance to mix in properly with other ingredients this could
also add to the effect. But from your post I have lost faith in this theory.
Regards Paul T
----------
>From: Hank Murrow
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: Non-salt Salt Glazing
>Date: Wed, Oct 11, 2000, 10:09 pm
>
>Dear Paul;
>
>Would you consider that the constitution of a typical body is very high in
>alumina; and therefore would make an Al-rich glaze at the surface when
>fluxed by sodium vapor? As with shinos(which are very high in Al) such
>glazes have a high coefficient of contraction. They seem to bead up easily;
>and it has been my conjecture that the 'pigskin' effect is due to this high
>Al content. Now with a silica-rich porcelain body there is much less
>pigskin effect, due to the reduced Al and consequent higher Si. Easy to
>test; put an alumina wash on one side of a pot and a silica wash on the
>other side and salt the pot, reporting to this forum. Something to consider
>anyway, Hank in Eugene
>
>______________________________________________________________________________
>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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