search  current discussion  categories  kilns & firing - salt & soda 

gil's position on salt and soda

updated mon 31 mar 97


Gil Stengel on fri 14 mar 97

Clayart has been down the salt road more than a couple of times but
as long as I'm going to be quoted and all....

First off:
>Shauna Mulvihill wrote:

> We have cone 10 stoneware salt
>firings at our school once per semester. I usually help fire and
>always throw in the salt. At what temperature does the salt give
>off chlorine gas?
Salt kilns don't give off chlorine gas Shauna. Salt kilns do give
off alot of the same things that all fuel fired kilns give off. The
sodium chloride you put in your kiln probably comes out as sodium
chloride gas. Depending on how much sodium and chlorine stay in the
kiln you get different emission possibilities. The big problem is of
course the chlorine atom. One thing chlorine is not going to combine
with at kiln temperatures is another chlorine atom. It will combine
with a sodium or a hydrogen atom. With hydrogen it combines to form
gaseous HCl, definetely not something good for you. Peter Meanley,
in Ceramic Review magazine, published a gas analysis of several salt
firings that measured the HCl content in stack and spy emissions at
around 50 parts per million. Wanna play it safe? Mix your salt with
two thirds by volume sodium carbonate. You'll get a nice glaze coat
and an emission composed of NaCl and NaC03.

>Monona Rossol wrote:
>What bothers me is that your question clearly indicates that your
>teacher needs to go back to school. He/she has no business teaching a
>hazardous process like this without educating all the students
>involved about the hazards.

I don't know about your teachers education level but firing a salt kiln
is at most only slightly more hazardous than firing any other kiln.

>Monona Rossol wrote:
>I wonder why people keep reinventing the wheel. There were tons of
>salt glaze emission studies in the Brick Clay Record in the 1930's,
>1940's, and all the way up through the 60's if memory serves. There
>probably are even more sophisticated studies in the Ceramic
>Engineering literature--especially because in most developed
>countries, large ceramic operations must submit environmental data.

>This is an old hat ceramic engineering problem with no mysteries left
>for anyone who wants to look at the data.

I'd love to see these emissions studies. I'm unfamiliar with the
Brick Clay Record. Maybe you could tell us all
what they say?

>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.

The original post which I replied to was concerning mixing oxides
with NaCl and painting them on ware that was then pit fired. I will
repeat, the small amounts of NaCl involved in this instance are not a
health hazard.

I do not know how much HCl is safe to "snort". I normally refrain
from snorting when I'm salting a kiln. As for your dead tree limbs,
how do you know your kiln killed them?
I have seen many trees around many salt kilns that flourished but I
am not going to say that the kiln did this. Salt is certainly
explosive when inserted into a kiln. Eye protection is very
important, depending on one's method for introducing the material.

Gil Stengel

Carolyn Broadwell on sat 15 mar 97

As for your dead tree limbs, how do you know your kiln killed them? I
have seen many trees around many salt kilns that flourished but I am not
going to say that the kiln did this.

When Sandra Johnstone, well know for her salt glazing for over 25 years,
showed her slides, she showed a long line of trees on the back of her
property, most of which were obviously pretty sick. She had a slide
showing the fumes from her kiln hitting one group of those trees, and
those were green and flourishing. Over the years she felt the kiln was
the reason those trees were healthy. It was hard to argue with her, as
she had slides from many years, all the same general conclusion, as a
defense for her salting.


Richard Burkett on mon 17 mar 97

I'll chime in here, too, on the salt kiln vs. trees issue. In all the 10+
years that I fired salt kilns under trees the only damage that I saw -
ever - was related to heat from the stack - immediately above the flue a
few leaves were turned brown in one instance during a hot spell of hot,
dry weather. This would happen with any gas/fuel kiln. Salt kiln fumes
went through many a tree with no sign of damage. Excessive heat, on the
other hand, can quickly kiln plants.

Obviously this is all annecdotal evidence, but then that's all Monona was
offering in this instance. Got any hard science on salt kiln fumes and
trees, Monona? I, too, would like the references from the Brick and Clay
Record if you can provide them. I'm all for reducing the load of chlorine
compounds that is pumped into the atmosphere each year, but let's be
rational about this.
BTW, I spent a lot of time in the Wendt Engineering library when I taught
at the University of Wisconsin.

Richard, who now fires his soda kiln using sodium bicarbonate.

Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
E-mail: <-> Voice mail: (619) 594-6201
Home Page: