Richard mahaffey on thu 13 mar 97
Gil Stengel wrote:"latest issue of Ceramic Review magazine,
indicating that no trace of Chlorine could be found in the gas
from a couple of salt firings. H20, NaCl and HCl are the much more
My dim memory of chemistry indicates that HCl is Hydrocloric acid. Great
stuff to breathe. HCl is almost surely present when one wets the salt to
increase the distribution and vaporization of the salt.
We Used to salt at night in town , I know not nice but it was the 70's.
The metal roof had more holes than metal after awihle...... makes one
Rick Mahaffey, Tacoma Community College, Tacoma WA 98466
Gavin Stairs on fri 14 mar 97
Rick Mahaffey wrote ...
>My dim memory of chemistry indicates that HCl is Hydrocloric acid. Great
>stuff to breathe. HCl is almost surely present when one wets the salt to
>increase the distribution and vaporization of the salt.
>We Used to salt at night in town , I know not nice but it was the 70's.
>The metal roof had more holes than metal after awihle...... makes one
Monona Rossol wrote ...
>I'm wondering what kind of salt glazing you are talking about that is so
>safe. And just how much HCl is it good for you to snort? I remember the
>almost explosive force with which the salt fumes and how careful I had to be
>not to get a lung full. I remember that cloud of pungent HCl-containing fog
>chasing me down the hill away from my outdoor gas kiln. I remember the dead
>tree limbs above the stack. This is no project near most school
>yards--reform school, maybe.
NaCl + H2O + kiln heat <<-> NaOH + HCl
NaCl is strongly bonded, and likes to stay that way. NaOH is a strong BASE,
which likes to combine with HCl, which is a moderately strong ACID (stomach
acid) (I'm not kidding!) You start out neutral, you end up neutral, with
most of the salt leaving the kiln AS SALT. Except what goes into the glaze
(and the kiln walls...)
I started out with the standard assumption, that NaCl goes into the glaze
and becomes Na2O in some form. If that were true, then something like
2 NaCl + H2O + kiln heat <<-> Na2O + 2 HCl
has to occur. Once again, Na2O is a strong BASE. This means that the HCl
has lots of chance to react with the Na2O (and other alkali oxides, like
Li2O, K2O and CaO, for example) to produce chlorides and water once again,
before some of it escapes to go up the chimney. The data Gil referred to
shows that SOME of it does just that. I haven't been able to do a
quantitative analysis of that data to see how much, but I am quite sure that
it is far from all of it.
Now the punch line: TO THE EXTENT that these reactions go forward, and NOT
BACK, AND NO MORE, there will be HCl coming out of the kiln. In the case of
the first reaction, there will also be the same amount of NaOH, a strong
base, which will have a strong tendency to react and neutralize the acid,
whether in the kiln or outside it.
As near as I can tell, something between 1/4 and 1/2 of the NaCl in a salt
firing actually makes glaze. The rest goes up the pipe. By far the
predominant fraction of this must come out as salt. So if you put 12lbs of
salt into the kiln, something between 6 and 9 lbs goes up the flue... and
onto your roof, maybe. To find out what that will do to a metal roof, ask
someone who lives by the sea. Not good.
Monona's story is evidence of the strong unpleasantness of the emission of a
salt kiln. However, I would like to point out that an HCl mist from a kiln
would mostly rise. The molecular weight of HCl is slightly less than that of
CO2. The heavy mist she saw was most likely salt vapor, particles and salt
fog. This would not be a nice mixture to breathe. It would certainly
poison local vegitation chronically exposed to it. So, I might add, would
ordinary kiln emissions, if the tree were close enough.
Monona also referred to plentious previous literature. I haven't seen it
yet, and I look forward to reading it. I'll ask her for references.
There is far, far more acid coming out of the kiln as CO2 and CO, than as
HCl. Add water to CO2 and you get H2CO3, a weak acid. This stuff is one of
the agents of urban decay, slowly dissolving our buildings, highways and
bridges, among other things. As well as both being greenhouse gasses and
contributors to smog.
So, HCl is not nice stuff to emit, no question. Locally, salt can do as
much damage or more. And in terms of global impact, the emission of HCl
pales in comparison to the emission of ordinary combustion products.
Running a kiln is not the most environmentally friendly thing you can do.
Running a salt kiln is worse, but probably not much worse.
Chlorides of organics are a big nasty, and HCl is one route to forming them.
Undoubtedly, some are formed in a salt kiln, but I have seen no evidence
that the quantities produced are even measurable.
If you wish to avoid emitting Cl in any form (e.g., as salt), consider soda
ash. Soda ash is slightly alkaline, which tends to fight the excess acid of
the rest of the kiln, but the chemistry is more convoluted, and I won't go
into it here. I know there are problems, but life is all compromises.
I've been working with Gil Stengel on trying to understand this stuff, and I
plan to continue it. We need more data on how much NaCl stays in the glaze,
and other questions, before we can say the definitive word. Perhaps some of
the literature Monona refers to can answer that. And as to why we have to
keep returning to these questions, well, that's how we learn, isn't it?
In the mean time, go to Gil's session on salt kilns at NCECA. I won't be
there, alas, but he will be. Tear him limb from limb, what do I care?
Karl David Knudson on sun 16 mar 97
On Thu, 13 Mar 1997, Richard mahaffey wrote:
> ----------------------------Original message----------------------------
> Gil Stengel wrote:"latest issue of Ceramic Review magazine,
> indicating that no trace of Chlorine could be found in the gas
> from a couple of salt firings. H20, NaCl and HCl are the much more
> probable components."
> My dim memory of chemistry indicates that HCl is Hydrocloric acid. Great
> stuff to breathe. HCl is almost surely present when one wets the salt to
> increase the distribution and vaporization of the salt.
I believe that they were testing with wetted salt. 7lbs salt to 1 pint
water? Testing the flue gas, gas 3" from a spyhole emitting flame/gas and
the cloud that forms. Really good graphs of the results.
Karl in Eugene