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salt kiln fumes

updated wed 5 aug 98


Nan Rothwell on wed 26 mar 97

Ric Swenson wrote:

"Come on careful with ALL dusty especially
watchful of vapors from Salt kilns...sodium chloride...soda ash or sodium
bicarb.......stand away a few feet...use a L O N G handled scoop to put in
the the kiln so you can add the salt in an area of the
firebox that makes it easy to "dump" into...without endangering your mindful that a long term exposure as well as a short term
exposure can hurt you."

Thanks Ric -- I agree -- but I take your warning a step further.

I started firing salt in the early 1970's and after a couple of hideous
post-firing hangovers bought myself an old military surplus gas mask. I used
to frighten the dogs wearing the thing, but it helped... When I finally wore
out, I acquired a high quality respirator, with the filters the supplier
recommended. (Yeah, of course I should have started out with one, but I was
in my early twenties at the time and thought I was invulnerable...)

Anyway, the high-quality mask worked a treat and I have used it (or ones like
it) ever since. Now I wouldn't consider working anywhere near my kiln once
it reaches red heat without wearing a respirator. In truth, when I
occasionally forget or am too lazy to put the thing on early in the firing, I
always have a next-day headache.

I learned to fire salt while at the Harrow School of Art in England. When I
moved back to the US, I wanted to set up a studio in a small community in
Virginia. My next-door neighbor-to-be was an environmental scientist at UVA.
I described my firing cycle to him, and he did some research about my
process before deciding whether or not to oppose my initial request for a
zoning variance, when I wanted to set up my studio. When he came to the
zoning hearing, he spoke up on my behalf, saying that the amount of dilute
hydrochloric acid my kiln would produce would have no adverse effect on
people unless someone was standing right next to the kiln, breathing deeply.
As for the larger issue of damaging plants or air quality, he predicted that
in that sparsely populated community the emissions would do no harm and might
even promote nitrogen-fixing in the soil.

Since 1978, I've been fortunate enough to live way out in the country (our
county doesn't even have a traffic light or fast-food restaurant) so I don't
have to worry about those headache-causing fumes worrying any neighbors. But
I don't let my friends or family stand near the kiln shed area or downwind
when I am salting. And after about cone 06, I discourage anyone from going
into the shed without a respirator.

Nan Rothwell

Gavin Stairs on thu 27 mar 97

Nan Rothwell wrote:
>when I am salting. And after about cone 06, I discourage anyone from going
>into the shed without a respirator.

Hi Nan,

I hope you don't begin salting below about cone 10. Below cone 10 is when I
expect the most HCl to be produced, and this does nothing for your glaze.
It is merely liquid or solid NaCl being converted to NaOH in the pile or
pool on your kiln floor. That maximizes the HCl production, rather than
minimizing it. Minimal HCl is produced when the NaCl evaporates rapidly in
a hot kiln.

And yes, by all means use a long ladle to add the salt. And by all means
wear a respirator. Best not to breath any of the effluent of an active
kiln. When dining with the devil, use a long spoon. Hamada used to wrap
the salt in paper packets, and then throw them in the stoke hole, but that
probably won't work with a gas kiln.

And as for the conclusions of your erstwhile neighbor, I agree.

Do you experience your headaches and distress with other kiln firings, or
with salt firings alone? The reported symptoms, like headache particularly,
are consistent with CO poisoning. Perhaps in salt firings you remain closer
to the kiln than otherwise? I'm curious about the wide variability of
peoples' experiences in this regard. When something gives you illness or
pain, my response is "don't do it", but the question is what "it" is.


Richard Burkett on sat 18 jul 98

Perhaps the following will help to put some perspective on fumes from a
salt/soda kiln operation.
Not too long ago I read a report that declared the off-gases from
a high-fire kiln dosed with sodium chloride (NaCl, common salt) contained
up to 1% HCl. The report was in two parts, one a letter, that appeared in
UK's Ceramic Review within the past two/3 years. There was a hint that the
way the kiln was operated had a bearing on the amount of HCl generated.
In a kiln at C10+ (1300 C+) sodium chloride will become liquid
with a high vapour pressure (ie, much will go to vapour or single
unassociated molecules). These molecules of NaCl will exit the kiln in the
gaseous state along with the CO2/CO/H2O and HCl, all also as gases.
And, yes, uprotected carbon steel nearby will over time be
attacked/corroded by the action of the HCl and the condensed, solubilized
NaCl coming from each kiln firing. When kiln gases hit cold metal, the
water will condense first and then dissolve other molecules, chiefly NaCl
and HCl. It is the chloride solution (Na+, H+, Cl- ions) that
attacks/corrodes the steel.
Soda ash (Na2CO3) (or sodium carbonate decahydrate, aka washing
machine soda) will change to Na2O + CO2 at 850 C and the Na2O will go
directly to vapour at 1275 C. So when a C10 gas-fired kiln at 1300C
receives its dose of soda ash, most of the Na2O formed exits with the
combustion gases and soon forms NaOH (lye) which is hell on aluminum but
not on steel. But the NaOH will likely have greater impact on the local
environment than will an equivalent amount of NaCl coming out of the kiln.
Incidentally, the "salty water vapour" phrase is clear in its
meaning but not properly presented. Better to say the corrosion arises
from steam being condensed and the water then trapping/dissolving the
molecules of NaCl from the kiln's off-gases.
And the corrosion of auto metal arises from droplets of water
containing dissolved NaCl ... slush in the case of road salt, and seaspray
in the case of potters living near the ocean. While aqueous salt-corrosion
is straight-forward chemistry, it is somewhat too involved for this
Til later.

Tom Buck ) tel: 905-389-2339 & snailmail: 373 East
43rd St. Hamilton ON L8T 3E1 Canada (westend Lake Ontario, province of
Ontario, Canada).

Richard Burkett on tue 4 aug 98

Alistair Young writes about a Ceramic Review article:
>Special note was made of two points.
>No chlorine has ever been detected in any of their tests.
>Hydrogen Chloride levels in these tests were lower than previous tests.

I comes to mind that other volatile chlorides may well be forming during
the firing, iron chloride in particular comes to mind, as iron is present
both in the bricks and the clay in most salt kilns. The real answer to this
question might be to do a more extensive analysis of the flue gasses from a
soda kiln. Other testing that would probably be useful and perhaps
illuminating (so far no one has mentioned this factor to my knowledge)
would be to monitor emissions with changing firing conditions during
salting - whether the kiln is in oxidation or reduction mode.

Thanks for posting this message, Alistair.

Richard Burkett

Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
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