shane mickey on mon 11 dec 06
hello clayarters
i would like to add some common sense to this discussion. Lubas kiln and the kiln at Auburn will both be used in an institutional setting. That said, you must factor in the number of firings these kilns will potentialy see. I am a firm believer in soft brick soda kilns, having seen one last over a decade and the replacement is holding up fine (thermal ceramics high CaO bricks were used). This kiln was also sprayed with ITC proir to being fired. Again remember this kiln is at a full time studio potters place, fired between 6-8 times a year. I would never reccomend a soft brick kiln for a school or clay center, they take way to much abuse and are fired way more than 6 times a year. I built the new salt kiln at penland in march of this year, do not know the number of firings on it but the bricks are well glazed up. WE used some High fire super duty brick in the fireboxs and lined the boxes with mizzour plus castable, the bricks and castable look great, but these bricks were like
$3 a piece! the rest of the kiln was built out of super duty brick, not high alumina per say, just regular ol clippers. i agree with ivor and hank, high alumina brick are generally more porous and absorb the salt/soda and can potentially show little effect for the first several firings, but after awhile they can spall and slag. as to the high silica brick, boy they should glass up in one firing and the whole idea of glazing up the surface to protect the brick is a good one. just be careful as to amount of silica, those bricks tend to spall quickly as they are designed for a slow temp rise(daysnot hours) and are designed to be at temperature for extended periods of weeks or months. The quick rise and fall of a pottery kiln can cause high silica bricks to fail. In conclusion the benefits of gas savings are not worth the shortend life of the kiln. you are talking about 100-200 dollars in gas difference! a kiln that could potentially hold $3-5K worth of pottery, or a firing that
a paying member of a clay center is going to have to pay for, charge them for the gas, and have a kiln that will last a long time. put more thought into the design and quality of construction so the kiln doesnt "walk" all over the place or fall apart. I have seen poorly designed soft brick soda kilns that take 20+ hours to fire and are showing alot of destruction, so where is they savings. build with hard brick and be safe.
shane mickey
shane mickey pottery and kiln design services
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Ivor and Olive Lewis on wed 13 dec 06
Dear Shane Mickey,
I have never attempted firing with Sodium Carbonate.
You comment << I agree with Ivor and Hank, high alumina brick are =
generally more porous and absorb the salt/soda and can potentially show =
little effect for the first several firings>>.
I do not recall saying that High alumina bricks are more porous. I was =
suggesting that Insulating Refractory Bricks might pose a problem when =
using Sodium Chloride due to their porosity. But now that would be =
debatable and depend on their contents and primary construction.
By the way, do you know the production temperature at which those High =
Calcium Oxide Bricks are manufactured.
Donovan Palmquist on wed 13 dec 06
Hi all.
I've been reading this thread and thought it was time to add my 2 cents
worth=85. Sorry, this is going to be long.
I have built over 50 soda kilns. Some of them have been soft brick, but
most are hardbrick with an IFB backup brick. I have found that over time
hardbrick kilns outperform softbrick ones. Shane Mickey makes a good point
that if you do not fire often, softbrick kilns are the most economical
choice. However, and he will agree, they are VERY high maintenance. You can=
expect 80 to 100 firings out of an IFB kiln depending on how much soda you
use and how diligent you are at keeping up the coatings. If you use a lot
of soda you can expect a much shorter kiln life. In my experience the
weakest link of soda kilns is the IFB/ coating interface. Your kiln is only=
as good as as the brick you coat. Used IFB are a disaster since they may
already be prone to spalling. If you use a small amount of soda you can
minimize the degredation of the kiln. I too have found that as you fire an
IFB soda kiln, the sodium will "soak" into the brick. After a time the
brick/coating interface begins to weaken and the coating peels off. Sodium
carbonate by nature does not like to be a vapor. It prefers to be a solid.
Every time you add soda it penetrates the porous brick and at some point
it reaches a eutectic with the brick and will melt or spall or fall apart.
Sodium carbonate is also hygroscopic, which means it has an affinity for
atmospheric moisture. Sodium carbonate will recrystallize and expand in
humid or moist conditions, much like freezing water, and will destroy some
refractories.
A long time ago I came across an article in an archeology magazine that
talked about this kind of destruction of Egyptian Faince hippos, basically
egyptian paste. Egyptian paste is high in soluable sodium. These hippos
were found in the dry desert and were covered in dirt. The conservators
soaked and washed these pieces in de-ionized water then set them out to
dry. Much to their amazement, the pieces self destructed because they
re-hydrated the sodium then dried it, causing the sodium to re-crystallize
as a solid. The crystallized sodium expanded in the matrix of the clay
and broke apart the hippos. This is a good lesson in what happens to
refractories when exposed to sodium vapor and atmospheric moisture. I
have seen it happen to kiln shelves, IFB and some low fire soda glazed
pots.
Another example of sodium vapor seeping its way into hard brick and
causing a mess is taking a used brick from a soda/salt kiln and using it
for kiln posts. I have seen these bricks literally fall apart in the kiln
at c10. They have so much sodium in them that at high temps they actually
flux out the brick and crumble it into a pile.
This can also happen to high alumina brick. While alumina resists the
buildup of sodium glass it acts like a sodium sponge, soaking up sodium
vapor until it reaches a eutectic with the materials in the brick and you
then have a catastrophic failure. The brick needs to be sealed (see
below).
I now recommend building soda kilns out of hard firebrick, either high or
super duty brick and a backup of IFB. These kilns require a little less
maintanence and certainly last longer. The cost of firing a hard brick
kiln is probably about 20% more than with soft brick....but they=92ll
certainly last more than 20% longer. Back in the 70's it was common
knowledge that a soda/salt kiln needed to be "seasoned" so that you got
good salting in the kiln. I blindly followed this advice for years,
waiting for my kiln to get properly aged so that I got good salting.
When we built our new soda kiln I didn't want to wait for a seasoned kiln.
After a few firings in the new kiln and lots of soda we still weren't
getting great surfaces. Where was all the soda going?... Aha! into the
walls and every porous part of the kiln. How can we stop the soda from going=
into the walls of the kiln? I looked into the kiln glaze idea and came
across a recipe in Emmanuel Coopers Clay and Glazes book from Jane Hamlyn
that addressed the very issue. So...we glazed both kilns with her recipe
and =85viola! We now use 1-2# of a soda mix in the kiln with fantasic result=
s.
The kiln walls are juicy and beautiful with glass and with no sodium
penetrating the walls of the kiln. We have done ~15 firings in each of our
kilns (using the Gail Nichols soda/soda ash/whiting mixture) and everything
is holding up great.
We still have an IFB door coated with a zircon coating and I know that
this will not last forever. A door is cheaper than an entire kiln to
replace. Pete Sherzer, the tech at the Northern Clay Center in Mpls, is
using a silica and clay coating on the IFB doors with very promising
results. It appears to have a good glass coating that remelts each time
they fire. I am looking forward to seeing how this holds up over time.
Jane Hamlyn's Kiln Coating: potash spar 65oz., whiting 8oz., kaolin 26oz.
Soda wants to start breaking down your kiln with the very first firing....no=
way around that. In my opinion, it's worth the fuss because we're getting
nice work out of our kiln. Isn't that, ultimately, what we're after?
Donovan
Lee Love on thu 14 dec 06
On 12/13/06, Ivor and Olive Lewis wrote:
>
> You comment << I agree with Ivor and Hank, high alumina brick are generally more >porous >and absorb the salt/soda and can potentially show little effect for the first several >firings>>.
We all judge things by personal experience. How the soda
effects the pots on the first firing depends on how you introduce the
soda. I have experiemented with the French method of salting where
they placethe soda in cups throughout the kiln, rather than blowing or
squirting it in. You get localized results this way and the soda
doesn't fill the kiln the way spraying does. Genvieve does every
firing this way in her kiln of the same design as mine.
Euan's method, and he gets heavy results on porcelainious
stoneware, is to mix soda ash with wet sawdust. He sandwiches this
mix between two boards and throws it
--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/
"Let the beauty we love be what we do." - Rumi
"When we all do better. We ALL do better." -Paul Wellstone
Lee Love on thu 14 dec 06
On 12/14/06, Lee Love wrote:
> On 12/13/06, Ivor and Olive Lewis wrote:
>
>
>
> Euan's method, and he gets heavy results on porcelainious
> stoneware, is to mix soda ash with wet sawdust. He sandwiches this
> mix between two boards and throws it
Oops! Didn't finish this: He throws the sandwich in with force to
scatter the paste. He gets orange peel with soda. He cleans
the surface of his softbrick every so many firings by scraping it with
a metal scraper. He fires once or twice a month. I don't think I
will ever go this way because of the wear on the softbrick. Right
now, I sometimes put in soda, placed on a seashell that is put on a
glazed cup on a prop. Small amounts work well but if you use too
much, it runs onto the shelf.
Genevieve's latest experiment is to use tall narrow test tube
shapes to hold the soda. She learned this from a visitor. The
idea is that the soda percolates and is popped into the air when it
vaporizes.
What makes more sense for me, is a small hardbrick soda chamber
between the main chamber and the chimney. In a woodkiln, because you
get brighter colors in oxidation in soda and salt, this chamber would
be a good place to put gas burners to burn up the carbon, rather than
blowing all that heat out the chimney.
--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/
"Let the beauty we love be what we do." - Rumi
"When we all do better. We ALL do better." -Paul Wellstone
Ivor and Olive Lewis on fri 15 dec 06
Dear Lee Love,=20
I would expect each potter to develop an idiosyncratic way of delivering =
either salt or soda to their ware in the kiln to ensure their own unique =
aesthetic achievement.
But to get back to the theme of the thread, what has been your =
experience of kiln degradation when using these vapour phase agents and =
how do you counteract their deleterious effects?
Best regards,
Ivor Lewis.
Redhill,
South Australia.
=20
Ivor and Olive Lewis on fri 15 dec 06
Dear Donovan,=20
I could find no reference to Jane Hamlyn, kiln wash or bat wash in the =
book by Cooper and Royle, nor was anything said about this in their =
notes on Salt Glaze.
It is an interesting proposition and I am certain that it will give a =
marvellous glossy surface to the interior of your kiln. Look forward to =
learning how this develops. I think Vince has given a test time target =
of three years which sound to me like a hundred and fifty firings from =
his "Kruzite" bricks.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Lee Love on fri 15 dec 06
On 12/15/06, Ivor and Olive Lewis wrote:
> But to get back to the theme of the thread, what has been your experience of kiln >degradation when using these vapour phase agents and how do you counteract their >deleterious effects?
With this method, the effects are localized (the ware and adjacent
shelves) and don't effect the interior soft brick, beyond the effects
of the wood firing. You can also place the cups or shells
strategically, closer to the flue, so fumes do not effect the rest of
the load.
I have also experimented with dipping bisque in old type
calgon and shino water. Calgon works on iron stoneware while the
shino water works better on porcelain.
--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/
"Let the beauty we love be what we do." - Rumi
"When we all do better. We ALL do better." -Paul Wellstone
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