vince pitelka on sun 6 may 01
Ivor -
I appreciate your interest and involvement in these discussions, and your
search for the truth, but I think that everyone would do well to avoid
alarmism about salt firing until you come up with the scientific evidence to
prove these things. I do not have the time or inclination to research this
now, but it bothers me when others spread information which seems
unnecessary inflamatory. There is no evidence at all of toxic reactions to
salt fumes in industry or in the studio, and I recall hearing of recent
tests which indicate that the predominant effluent from the stack and ports
of a salt kiln is hydrochloric acid fumes diluted with everything else
coming out of the kiln. Anyone who sticks their head in front of the stack
or ports and breathes deeply might experience harmful effects from both heat
or fumes, but we would hope that no one is that stupid. Anyone with a bit
of chemistry or a modicum of common sense is going to avoid breathing any
kind of fumes.
You said:
"By the way, when tested separately as pure materials Silica and alumina
seem
to resist the action of salt at cone 8."
I guess I do not understand what you are trying to say here. The sodium
vapors combine with silica in the clay and with other materials to form the
surface glaze. It is the silica that makes it all work. Bodies with a
little more silica do better in salt, and silica sand promotes orange peel
effect by creating blooms of glass.
"This is another technical aspect in ceramics arts where there is a lot of
conjecture, assumption and opinion but very little in the way of definitive
research to establish some facts."
Be more specific. Where is the conjecture and assumption? Salt glazing has
been studied very intensively in industry, and the chemistry is well
understood. Do some research in the ceramic chemistry journals and you will
find all the facts you want.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Stephen Grimmer on wed 9 may 01
Hi,
In no way do I wish to discount anyone's experience with respiratory
illness, as I've had my share of diagnosed extrinsic asthma aggravated by
(non ceramic) occupational conditions. However, we must be careful when
attributing acute health problems to incidental, short-term exposure to
anything.
When I was in the restaurant business, I took the Illinois Department of
Health's Food Service Hygiene class. An interesting observation from the
hygienist was that when people eat at home and later experience nausea,
vomiting, and diarrhea, it's referred to as "24 hour flu; the whole family
has it." The same symptoms observed after eating out in a restaurant are
certainly food poisoning and trigger a call to the health department. The
point? Sometimes people just get sick. Sometimes it's food poisoning. Either
way, anecdotal evidence is not always a good diagnostic tool. If 20 people
got sick after eating at a particular restaurant on a given night, food
poisoning is a good candidate; one person getting sick out of two hundred
customers is another story.
Similarly, if 10 people at a soda firing experience respiratory
distress, one might make a good case that the firing played a part. If only
one person in ten had problems, or one person out of 100 over a semester,
then the case is much harder to make.
Best
Steve
--
Stephen Grimmer
Southern Illinois University at Carbondale
iandol on wed 9 may 01
Vince,
I said: "By the way, when tested separately as pure materials Silica and =
alumina seem to resist the action of salt at cone 8."
Your query "I guess I do not understand what you are trying to say =
here."
My comment. When I was delving into the question of the chemistry of =
salt glaze, starting in 1983, I realised that I needed to isolate each =
component which might be included in a salt glazing clay. So I made test =
pieces from a stoneware clay then put on a slip coating of each of the =
major materials used in the process as individual samples. These were =
aluminium hydrate, #200 silica, kaolin, ball clay, mica, neph syenite, =
potash felspar and soda felspar. These smaples were fired to cone 8 in a =
regular kiln load. I repeated these tests for the 1988 Australian =
Ceramics Conference and presented them as slides to the Salt Glaze =
Seminar audience. I will send you GIF images by separate post. Free =
Aluminium oxide and free silica did not react to salt vapour. =20
You say "The sodium vapours combine with silica in the clay and with =
other materials to form the surface glaze."=20
My comment: In the light the above evidence and of the reading I have =
done, I find the information you convey difficult to comprehend. It =
seems to be a repetition of what was being written at the turn of the =
nineteenth century. Values for the rate of dissociation of Sodium =
chloride are available and they are insignificant until temperatures =
exceeds 2000 Celsius. Above its boiling point, Sodium chloride is =
reported to be dimeric, represented by a molecular formula of Na2Cl2. As =
temperature is increased it becomes increasingly monomeric, represented =
by a molecular formula of NaCl. No, I did not do that investigation. =
That was a lucky find when hunting for information in the University =
Library stacks.
You need to read information from; A Confidential Report: The Final =
Report of the Joint Salt Glazed Ware Air Pollution Working Party. The =
British Ceramic Research Association (1961). This is very clear about =
the compounds in particulate substances emitted from industrial kilns, =
confirming that Potassium chloride equal in amount to Sodium chloride is =
present in the effluent. This is difficult to explain using the popular =
chemistry of the process, that is, the hypothesis of author Daniel =
Rhodes
I agree with your assessment of the safety aspects, but temper my own =
actions with the experience of a severe asthma attack following two =
firings done over a period of about five days. After an episode of =
severe breathing difficulty, to be diagnosed as having asthma when this =
had not been detected in any previous medical examination was a sever =
shock. It was also accompanied by extremely high blood pressure, =
carrying a threat of permanent medication, which was a worry because =
that had always been on the low side of the acceptable range. I =
eliminated additions of salt to my diet, bought a suitable respirator to =
wear until I stopped using the process. The asthmatic episode has not =
been repeated and as I approach the biblical three score years and ten =
BP is text book according to my Physician. I do not claim a connection =
between my illness and salt firing. Purely circumstantial or merely =
coincidence. But given a similar record, would you fail to take =
precautions.
Sodium Carbonate, by the way, does react directly as a whole compound =
with pure silica to form sodium silicate with the evolution of Carbon =
dioxide. This was reported in 1952 by P. J. Durrant in his 2nd. ed. of =
"General and Inorganic Chemistry". I am sure you will appreciate that, =
as a fact.
Best regards, enjoying our exchanges. Now I must also get back to the =
main task.
Ivor
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