Snail Scott on fri 2 apr 04
At 01:09 PM 4/2/04 -0700, you wrote:
>Does adding salt to a gas kiln change the chemistry to effect at what
>point the cones bend in any way? Another student at my college says we
>are over firing out salt fires and it's because when we add the salt,
>the cones become unreliable, is this true?
The salt does seem to flux the cones, that's
true. As for 'unreliable', well, they seem
consistent to me, just not the same calibration
as non-salted cones. So forget what cone you
thought you were firing to, and judge by your
results. Does the ware seem overfired? Or
underfired? If not, it probably isn't,
regardless of what the cones say. Don't just
blindly say "this is ^10 clay" (or whatever) and
wait for that cone to drop. LOOK at what you get,
test it, and adjust your future firings
accordingly. Who cares if it's not the same temp
as a ^10 in a non-salt firing? It's not relevant.
Only the results count, not the number on the
cone. If the ware is properly vitrified when the
^8 drops or the ^12, or whatever, then that's
the right cone, period. You can always pull
draw rings as you fire, if you want 'real-time'
confirmation.
Everything above applies to non-salt firings as
well. Evaluate _results_, not correspondence to
some recommended number.
-Snail
BJ Clark on fri 2 apr 04
Does adding salt to a gas kiln change the chemistry to effect at what
point the cones bend in any way? Another student at my college says we
are over firing out salt fires and it's because when we add the salt,
the cones become unreliable, is this true?
Thanks In Advance,
BJ Clark
Patrice Murtha on fri 2 apr 04
Hi BJ--I have been involved in salt firings for the last 4-5 years
and I have never noticed the salt changing the timing on the cones
bending. If anyone else has noticed this please let us know, I'm
curious.
>Does adding salt to a gas kiln change the chemistry to effect at what
>point the cones bend in any way? Another student at my college says we
>are over firing out salt fires and it's because when we add the salt,
>the cones become unreliable, is this true?
>
>Thanks In Advance,
>BJ Clark
>
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Ivor and Olive Lewis on sat 3 apr 04
Dear B. J. Clark,
Perhaps what is being discussed are the effects of residual salt which
remains in a kiln after a firing. This is activated during subsequent
firings.
Oversalting and refraining from allowing the kiln to clear well
before closing down may lead to this problem.
Under normal circumstances it would be a technical error to commence
salting a kiln before your cones and draw rings indicate your clays
have fully matured, that is, become impervious and fully vitreous.
Given that the boiling point of Sodium Chloride is over 1300 =BA C and
that the vapour pressure is relatively low when it becomes a fluid at
805=BA there will be little free salt vapour to react with your cones
before they bend. The only cones which might react badly are those
which contain a proportion of Iron Oxide. Guessing, those would be
from ^010 to ^3. If there were an effect o higher cones I would
anticipate they would fall early rather than late so overfiring would
not be a problem. Best to rely on your draw ring tests.
Clay which is salted before it has reached maturity can absorb a lot
of molten salt which will cause persistent efflorescence and taint the
flavour of food or beverages.
Best regards,
Ivor Lewis
Susan Setley on sun 4 apr 04
"The salt does seem to flux the cones, that's
true. As for 'unreliable', well, they seem
consistent to me, just not the same calibration
as non-salted cones."
What about soda kilns? We have a soda kiln, and the current wisdom is that -- although we use cones -- by themselves they aren't reliable because the soda cause them to flux early.
Ivor and Olive Lewis on mon 5 apr 04
Dear Susan Setley,
We know so little about the Science of Soda Glazing, other than that
it will differ from Salt Glazing. So it may not be possible to make a
valid comment.
But I think the same technical principles hold true. Fire the clay to
maturity, then apply the Soda and follow the progress of build up of
the glaze coating using Draw Rings.
To be on the safe side use additional draw rings to confirm that your
cones are indicating that your clay had achieved its fired properties
of vitrification and impermeability.
One thing to note is that when firing with Soda, reactions which take
place at the surfaces of the Cones will be with the Silica fraction of
the Cone composition.
Best regards,
Ivor Lewis. Redhill, South Australia
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