search  current discussion  categories  kilns & firing - salt & soda 

gil's position on salt and soda(long)

updated mon 31 mar 97


Gil Stengel on fri 21 mar 97

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.

Gavin Stairs wrote:

> I wonder if you can give me some citations. I'm not familiar with
> the Brick Clay Record. Is that a current journal? <

Monona Rossol wrote:

>I believe the Brick and Clay Record died in the 1960's, but all the
>old issues should be in any good Engineering library. I know the
>ceramics industry started another similar publication, but its been so
>many years that I can't come up with the name.

Monona Rossol wrote:

>Get over thinking that salt glazing is an "ancient art." Sewer tile,
>dinner ware, and tons of other salt glazed wares are being made as we
>speak. Its the same exact process. These industries will have a
>technical publication somewhere that will be very useful.

Sorry to laboriously repost what has been written previously on
clayart but these posts are germaine to my recent visit to the
American Ceramics Society research library in Westerville, Ohio. For
those of you unfamiliar with this place, it is housed in the American
Ceramics Society headquarters, complete with a fascinating collection
of industrial and "art" ceramics and the offices of Ceramics Monthly
magazine. The ACS is known for it's "Journal of the American
Ceramics Society" and the "Bulletin of the American Ceramics

The journal in particular contains abstracts of virtually all
published ceramic engineering and ceramic science articles, books,
patent applications etc. dating back some sixty years. These
abstract are indexed by subject and author and published yearly. The
library staff at ACS are very helpful. They can perform computer
searches of all material published in the field dating back to 1976.
The abstracts are also available on site for years previous to 1976.
The entire "Brick Clay Record" is also available onsite, though it is
not indexed. This journal ceased publication in the 80's. The
"Ceramics Abstracts" of the Journal of American Ceramics Society
contain abstracts for anything published in the "Brick Clay Record".

Of course I was there searching for any scientific or industrial
studies concerning salt firing and particularly the emissions from
such processes. The library staff performed a database search for
me, under several keywords designed to make the search as all
inclusive as possible. The staff were very clear in explaining to me
that this search covered all the available scientific literature in
the realm of ceramic science and engineering. The results of this
search were seven references, of which only one and possibly two
concern themselves with kiln emissions, albeit very peripherally. If
anyone has the "Proceedings of the Intenational Clay Conference" for
1985, aparently a journal published in english, on pages 391-5 there
is an article authored by Fabbri, B. and Fiori, C. that could contain
some kiln emission studies of firing clay high in NaCl. That's as
close as I could come to a pertinent study in the years 1976 -.

I searched all the ceramic abstracts from the years 1935 to 1975.
I came up with three or four studies of the properties of
salt glaze itself, one in particular in German that might have
quantitative data that could help understand emissions. Two which
are thermal analyses of NaOh and NaCl which could help a great deal
in extrapolating data (these were the best references I could find
really but the actual articles in this case were not onsite). I'll
try to run them down through interlibrary loan here at WIU. The
article in German I plan to have translated. I think Gavin Stairs
wil find these very useful.

As for the "Brick Clay Record", I was unable to find any studies in
the ceramic abstracts concerning kiln emissions. This puzzled me greatly and
I feared then and now that I was searching under the wrong headings
so I pulled down all the BCR's for the years 1950 -59 and went
through them one by one. I did not have time to go through the
forties and thirties but I reasoned that perhaps this truffle pig
approach might turn something up. I came up with several studies of
the characteristics of salt glaze and how to improve its thermal
expansion properties (which I had already noted in the abstracts)
with additions of boron etc. but no hard data on
the breakdown of NaCl in a kiln or the emissions from said kilns.
What I did find, in the articles which I read, (rather like going
through old newspapers only way more fun) was a strong indication
that the sewer tile industry in particular abandoned salt glazing in
favor of sprayed on high boron glazes beginning in the late forties.
The reason for this is simple really. Because of thermal expansion
problems with salt glaze and the lack of "fit" with clays in use in
industry for sewer tile, some very mechanically weak ware was
generally the end product from factories using sodium vapor
techniques. Competition by companies spraying on glazes with a
better fit, and the ensuing higher mechanical strengths of their
tile, caused a general market shift away from salt glazing.

I need to do further research on this topic but I think that my
inability to find data related to sodium vapor kiln emissions might
be simply that this technique does not enjoy widespread industrial
use. The resultant lack of economic incentive for such studies could
explain the relative dearth of material in the abstracts.

As for the "Brick Clay Record", the lack of emissions data there is no
surprise. This journal was entirely pro industry in the '30s, '40s
and '50s (the only years I had time to look at)
and you will find little mention there of anything which
could reflect negatively on ceramic industry. There are some
retrospectively amusing photos, for instance, of men working in
factories, naked from the waste up and literally covered with barium
carbonate. In addition, from the fifties on back, I would theorize
that environmental safety was not a big concern in industry in

One other explanation of my inability to find many references could
be that I was simply looking in the wrong place or in the wrong
fashion. I know Monona Rossol has some data which hopefully she will share
with us all.

If you have read this far then I thank you for your attention. Sorry
for the length of this post.

Gil Stengel

Gavin Stairs on fri 21 mar 97

I see that Gil has done my work for me. It remains to search the remaining
numbers of "Brick and Clay Record", and to search for monographs and texts
that may touch on this, although Gil's search seems to have covered at least
some of these.

It seems that salt glaze may not be quite as current as Monona suggested. I
have had a hard time understanding how a salt glaze can be as durable as it
is purported to be, since by my estimates, the fit must be extremely poor.
The industry apparently came to this conclusion independently. It would be
nice to know if there are any industrial potteries still using a salt
process. I rather imagine there may be one or two in Germany, producing the
traditional wares, like steins and jugs, for the traditional pubs and for
the tourist trade. But that is no more than conjecture on my part.

I remember such ware as being decidedly on the heavy side, and made of very
dense stoneware. Gil measured some of his glaze thicknesses for me, and
found them to be quite thin. One example was perhaps half a millimeter, and
another (a soda glaze) was less than he found it possible to measure. Gil
also says that thicker glazes tend to craze, and this is what I recall as
well. The abstract of the article, to which Gil referred in his note, on
NaCl and NaOH seems to indicate that NaCl remains coherent in at least one
oxide system, except while the glaze is partially melted, in a slurry form.
I intend to try to understand this better by experiment. It may be that
some of the mystery is explained by reason of the material being other than
what we have assumed.

So far, rather than finding this to be ground well understood, it seems to
me that we are in the position of discovering, or at least rediscovering,
what is no longer current for lack of interest in the industrial ceramics
community. It would seem that the process has been discarded for its
shortcomings in quality and economic viability. This says nothing about the
viability of the process in small scale, studio potteries, where the
variability and industrial shortcomings may be positive features.

I can still say that I have seen no evidence that studio salt glazing
releases significant quantities of HCl, certainly negligible Cl2, and the
dispersal if a few pounds of salt per firing in ordinary studio practice is
probably not significant either. That is, it would not normally be
measurable in soil even in the immediate vicinity of the kiln. On an
industrial scale, these statements might be contestable, but we are not here
discussing industrial scale production.

Quite apart from its merits or demerits as a studio glaze process, I am
interested to pursue this topic for its intrinsic interest. In particular,
I am interested to know if the conventional assumption of all glaze
consituents being oxides is supported. At least the halides (Fl, Cl, I,
Br...) are more electronegative than Oxygen, and may well persist.

So far, the literature does not seem to have anything strong to say about
any of this. I look forward to whatever new developments may appear.