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Salt v. Soda


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#1 timbo_heff

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Posted 06 June 2012 - 08:59 AM

Hi All.
Sort of a follow up to the types of salt question I posited in another thread:

Wondering if you can help enlighten me on "salt versus soda".

I thought that soda broke apart into the same sodium vapor that salt breaks into minus the the chlorine gas emissions.

If they are the same, why do they look so different?

Is it that folks doing soda are just doing less of it in order to achieve a more subtle look?
If you do tons of soda can you get a traditional thick and runny salt glaze appearance ?
or are they actually different animals with somewhat similar chemistry ?

Thanks for any help on this !

#2 Mark C.

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Posted 06 June 2012 - 10:01 AM

For me soda is a drier less shiny surface on pots-never had it get runny orange peel. It is more caustic to handle in hot liquid form and seems to clog the sprayers more easy than salt.
I like the orange colors of soda.
I consider them completely different animals.
We choose salt over soda -soda does less damage to kiln.
Mark
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#3 neilestrick

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Posted 06 June 2012 - 10:28 AM

For me soda is a drier less shiny surface on pots-never had it get runny orange peel. It is more caustic to handle in hot liquid form and seems to clog the sprayers more easy than salt.
I like the orange colors of soda.
I consider them completely different animals.
We choose salt over soda -soda does less damage to kiln.
Mark


This is a good example of how different clay bodies, kilns and firings can affect the results. I have a lot of soda fired pots in my collection that have plenty of orange peel and 'snot'. It's a different look than salt, though. Kind of looks wetter, with more green/yellow hues.

For drier surface with flashing color, use clay or slips that are higher in alumina, like kaolin slips. For more orange peel and glaze use clay bodies and slips that are higher in silica.
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#4 TJR

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Posted 07 June 2012 - 12:56 AM

Just my opinion but I think people choose soda for the warm tones of the clay body-usually on porcelain. If you heavily apply soda, you get a grey thick snot[not my term.]
People choose soda over salt because it is less polluting. The by product from salt is hydrochloric acid and those grey fumes coming out of the kiln are from chlorine gas which is poisonous.The by-product from soda is water vapour. Can't remember what the chemical make up for soda is... it's 1:00 am. The chemical make-up for salt is sodium chloride.
Would soda be sodium nitrate? Help me out here. Salt is a lot more caustic on your kiln as well.
TJR.

#5 JBaymore

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Posted 09 June 2012 - 12:35 PM

There have been some formal studies on the "salt" and "chlorine gas" production issue relatively recently......... and the amount of chlorine gas that is actually prduced in salt firings is far less than what was always assumed or expected. WAY less. The majority by far of that dense cloud of stuff coming out of a salt kiln that you see is water which has nucleated around small particles of sodium chloride ejected from the kiln........ a salt water mist.

Also contained in this effluent is some hydrochloric acid mist. Chlorine gas is readily disolved in liquid water, and at elevated water temperatures this reaction is enhanced. So most of the chlorine gas which was produced by the cracking of the NaCl molecule, instead of remaining as chlorine gas coming out of the kiln's stack, disolves into the water vapor present from combustion by-products and atmospheric moisture as the gases exit the kiln, cool a bit, and then become an acid mist.

That pesky salt water and hydrochloric acid mist is what causes all the corrosion of metal stuff around a salt kiln (just like metal stuff takes a beating at the ocean beach area). Most people assumed it was the chlorine gas doing all the damage ... for some reason.

I believe that CM had an article on this salt kiln research in the past 10 years or so.....can't remember the author or the issue. Or it might have been Ceramics Technical magazine. Or both.

Soda firing usually involves the use of soda ash as the sodium source. Soda ash is Na2CO3...... sodium carbonate. It is usually introduced into the kiln via a wet, water based solution sprayed into the hot kiln, often prepared as a super saturated solution by using very hot water for the liquid mixture. Solid soda ash, unlike solid salt (sodium chloride - NaCl) does not tend to dissipate in the hot kiln envoronment very well, hence the use of the water spray method to aid dispersal.

Soda ash when placed into in a hot "soda kiln" along with the combustion by-products produces significant sodium hydroxide (NaOH), which itself is readily hygroscopic and disolves readily in any available water if the temperature allows any liquid water to form. Solid dry sodium hydroxide has a melting point of something just over 600 F....well below soda ash's 1560-ish point. I believe that sodium hydroxide is the main "functional agent" in the glazing process called "soda firing".

Interestingly, it is a known fact that solid sodium hydroxide is hightly reactive when in contact with with silica glass, forming a sodium silicate glass. Clay's surface (at a microscopic level) has much free silica (pure silica glass) present. Sodium silicate based glasses have very low melting points.......... and when the molecular equivalent of soda content is high enough relative to the silica content, it is even still a liquid at room tempeature (water glass). So one wonders if this fact is part of or all of the reason that soda kilns tend to produce a more smooth uniform "liquid" type of glaze surface when compared to salt's stiffer "orange peel" textures.

Picking up combustion by-product water as well as both atmospheric moisture and any water vaopr from the liquid solution of introduction, the gasseous emissions from a soda kiln include a mist of a corrosive sodium hydroxide solution, which is a strong base (like an acid but the other way around Posted Image ).

Sodium hydroxide solution, unlike a sodium chloride solution or hydrochloric acid, does not react with ferrous metal based materials much.... so a soda kiln's emissions will not attack the steelwork as aggressively as the emissions from a salt kiln will. This perception of "less rusting" may be one of the reasons that potters grew to feel that soda kiln emissions were not as "bad" as a salt kiln's.

The emissions from a soda kiln are not all that much better than those coming from a salt kiln. Strong acid versus strong base. Pick your poison. The myth that soda kilns are somehow "pollution free" when compared to salt kilns is just that... a myth.


best,

.................john
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#6 Mark C.

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Posted 09 June 2012 - 02:50 PM

Thanks for bringing this up John
-I read that article as well years back but did not recall enough of it to mention it here-I have read another piece recently on this as well (old brain).
I salt because I like the effects on pots- I like soda pots just not as much.
I have never thought ceramics as a green activity in any means anyway.
Mark
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#7 Marcia Selsor

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Posted 09 June 2012 - 05:07 PM

I think two good examples of soda colors and varying surfaces would be to contrast UK Ruthanne Tudball's work to Gail Nichols ,Ruathanne gets a wet orange peel andGails work is drier. That does not reflect on the soda but the way they use it or maybe the clays they are glazing.
marcia

#8 timbo_heff

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Posted 20 June 2012 - 02:00 PM

Thanks for the thoughtful answers ... it's all coming together now ... :D

#9 timbo_heff

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Posted 07 January 2013 - 11:16 PM

John B:
So you say "Soda ash when placed into in a hot "soda kiln" along with the combustion by-products produces significant sodium hydroxide (NaOH),"
Are you conversely saying that salt does not produce sodium hydroxide ?
( and that is why they look different)?

#10 JBaymore

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Posted 19 January 2013 - 01:21 PM

John B:
So you say "Soda ash when placed into in a hot "soda kiln" along with the combustion by-products produces significant sodium hydroxide (NaOH),"
Are you conversely saying that salt does not produce sodium hydroxide ?
( and that is why they look different)?


Tim,

Salt firing effluent is mostly fine particles of sodium chloride acting as a nucleating agent for water vapor. My understanding is that little sodium hydroxide is produced.

If you can get enough soda to actually circulate in the soda kiln (sodsa does not dissapate as easily as salt), you can get surfaces that are pretty reminicient of "salt" firings. Most people don't WANT that. The lighter treated soda surfaces give such nice blushes.

You can get similar results to soda with what is usually callesd "residual salt" in a salt kiln. Fire the kiln to cone 9-10 but don't put any new salt in......just use the fumes from the kiln's lining.

best,

..............john
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#11 neilestrick

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Posted 19 January 2013 - 02:27 PM

A friend of mine, Wil Shynkaruk, did a study on salt and soda vapors when he was a tech at Utah State University while I was in grad school there in 1997. He published his results in Ceramics Technical #5, titled Taking the Sin Out of Salt. If I remember correctly, he determined that almost 99% water vapor. He also determined that from a pollution standpoint, soda was worse than salt, and that driving your car was many, many times worse than both.
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#12 JBaymore

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Posted 19 January 2013 - 04:10 PM

Neil,

See eariler in the thread... that is likely the article I was referring to.

best,

............john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

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#13 Natania

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Posted 22 January 2013 - 09:20 PM

There is a lot of good info, in a book by Rosemary Cochrane as well. Can't think of the title T the moment but it has something to do with salt and soda!




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