Vince Pitelka on sun 21 jan 07
Dave Finkelnburg wrote:
> I have never seen the carefully researched science
> that would reassure me a porous clay can withstand
> serious freezing cycles. If you have some references
> to published research that I could review, I'd be
> grateful for the opportunity to look for it. Thanks!
Dave -
I never said "a porous clay." I don't know the specific references, and I'm
not sure when I could find the opportunity to look for them, but you could
do that, through your connections at Alfred. If you consider the magnitude
of the architectural terracotta industry at the turn of the century (19th to
20th), and you know that ceramic engineers in academia were focusing serious
research on this subject. Somewhere I have a copy of the outdoor claybody
info from Val Cushing's handbook, but I am not sure how easily I could even
put my hands on that. As I remember the story, his information was based
into his own research into earlier architectural terracotta literature.
Several years ago we had an exhibtion of the work of Robert Wood, a ceramics
professor from SUNY Buffalo, who makes large-scale terracotta sculpture,
much of which is installed outdoors. He had done thorough research into
architectural terracotta and outdoor claybodies, and he corroborated what I
had always understood - that a terracotta claybody fired to cone 1 or 2 will
develop great mechanical strength, but will still have enough porosity to
allow the pressure of freezing water to escape. As you know, when water is
freezing, it will follow the path of least resistance, but if it has no easy
path of release, it will build tremendous forces and simply shatter or warp
whatever is surrounding it.
I can claim no expertise in this specific subject, but when I think of it,
it does seem logical. If a porous low-fire piece is soaked with water and
then put into a hard freeze, it will disintegrate even though there are
plenty of avenues for the pressure to escape, because there is so little
mechanical strength in the claybody, and it is easier for the freezing water
to shatter the claybody than to "bother to" escape through the porosity.
But in a claybody fired high enough to develop great mechanical strength but
still retain some porosity, the pressure of freezing water will seek the
easier way, escaping through the remaining porosity.
I'll look forward to hearing what you find through your research.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Dave Finkelnburg on sun 21 jan 07
Vince,
I appreciate your thoughtful post on this
important subject. I have been thinking about this
too. I know there's a single piece of architectural
terra cotta that has to weigh several hundred pounds
that is on top of a church in downtown Boston. It is
at least a hundred years old. Perhaps there is
something to thickness, but I would like to see the
result of an objective test that shows this.
I have never seen the carefully researched science
that would reassure me a porous clay can withstand
serious freezing cycles. If you have some references
to published research that I could review, I'd be
grateful for the opportunity to look for it. Thanks!
Dave Finkelnburg, watching the snow on the
birdbath reflect the brilliant sunshine on a winter
day in Idaho
Quoting Vince Pitelka :
> It's hard to argue with your experience, but the
information that
> I've seen
> on architectural terracotta clearly says that 1.5%
absorption is
> way too
> tight for a long-lasting outdoor claybody. It could
be the fact
> that your
> planters are not inches thick that makes them
survive, in which
> case it
> might well be true that a tightly-vitrified claybody
will hold up
> fine as an
> outdoor planter in a hard-freezing climate.
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Dave Finkelnburg on mon 22 jan 07
Eva, Stephani,
Thanks for taking time to post information on this
subject. I know it's been on the list before, but
it's helpful to have it here in front of us.
I believe the formula for Saturation Coefficient
(SC) is accurate. However, it does say that whether a
fired clay body has 1.5% absorption or 5% absorption
or 15% or even 50%, the body will still withstand
damage from freezing as long as the C/B ratio is
correct. It just says that for every gram of water
absorbed into open pores, 0.275 grams (27.5% of the
open pore absorption) must be absorbed into closed
pores. At least, Val Cushing doesn't mention any
absolute value for absorption.
On the other hand, closed pores are only found in
the glass phase of a fired clay body. The glass melt
is what closes them in. So, it may not be possible to
get more than 27.5% of the pore volume of the open
pores to be present as closed pores without having a
pretty significant amount of vitrification, hence the
strength of the fired ware and also probably a
practical limit for the absorption that will meet the
SC ratio.
I don't know why, but for some reason I have had
the impression that the formula value cited applied to
fired clay with a water absorption of around 15 to
18%.
Stephani, do either the ASTM or CSA specifications
mention any value for absorption in relation to the
SC? Is there any information in these standards about
dimensions or weight of a sample to be tested?
Thanks!
Dave Finkelnburg, in Idaho where it's definitely
been cold enough for the freeze part of freeze-thaw
tests!
From: Eva Gallagher
Here is the calculation from Val's handbook - he says
that it is the
standard test used in the ceramics industry for
structural clay products.
D= dry weight of test sample
C= wet weight of sample after soaking at room temp for
24 hours (wipe off
surface water before weighing)
B = boiled weight after 2 hours in boiling water
(again wipe before
weighing)
1. C - D / D = "C" value
2. B - D / D = "B" value
3. "C" value / "B" value = C/B ratio. The C/B ratio
must be less than .78
From: stephani stephenson
the cold water absorption divided by the boiling water
absorption gives you the "Saturation" coefficient
The saturation coefficient must be 0.78 in order to
pass CSA and ASTM specs for outdoor use.
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Snail Scott on mon 22 jan 07
At 01:12 PM 1/21/2007 -0800, Dave wrote:
>...I have never seen the carefully researched science
>that would reassure me a porous clay can withstand
>serious freezing cycles...
Empirically, I have seen it with my own
eyes: hundred-year-old (and even older)
architectural terra cotta exposed to all
weather and definitely absorbent, with
no signs of damage at all. As an exchange
student in Britain, I used to walk past
the Russell Hotel in London on my way to
school and admire its multi-story ceramic
facade, all quite porous unglazed red
terra-cotta cladding, and some structural
terra cotta as well, I suspect.
The real research on this was done in the
20's and 30's, when architectural ceramics
were the 'wave of the future'. I can't
recall specific articles, but they are out
there, and the ones I've read confirm my
observations: it's the strength of the
bonds between particles that count, not
the porosity of the matrix. It appears that
a very porous fired clay copes with ice-
crystal formation by allowing the ice to
creep into the channels between particles,
and if all these channels interlink, there
is no containment of the pressure. It's
very tight bodies that are vulnerable, as
the tiny pockets between particles are
likely to be restricted or blind alleys.
Naturally, not every kind of porous clay
is frost-proof - far from it. But some
which are very well bonded in firing do
better than a really tight clay.
-Snail
Vince Pitelka on mon 22 jan 07
Stephanie -
Sephani Stephenson wrote:
> Vince you make a good point when you comment on the
> thickness of architectural pieces and the fact that
> bodies suitable for such work are usually quite 'open'
> i have observed thicker pieces of architectural
> ceramics which do have high % of grog. i have seen low
> fire tile bodies which are vitreous.
> so in my own mind i haven't ruled out a nice
> combination of vitreous and 'open'...i don't want to
> unnecessarily rule out something..
> I think,like Dave, i want to know more and am not
> entirely satisfied with relying on the S
> coefficient..at the same time until i learn otherwise,
> it is just about all i have to go on, in the written
> info anyway.
Stephani -
Your solution is simple. Take the Val Cushing statistics from Eva
Galagher's post today, prepare and test some claybodies, make a mess 'o
tiles, and then pull up stakes and move to Minneapolis so that you can test
them over time. What could be simpler?
I don't think there is any such thing as a combination of vitreous and open,
because "vitreous" pretty much precludes the possibility of "open." As I
have mentioned, it seems that the ideal body is fired high enough to develop
great mechanical strength, but the firing temperature is well below the
clay's vitrification point, in order to leave enough porosity to make the
clay work. You're right - I don't know this from personal experience.
Whatever degree of certainty I have about this information is just based on
having read about this and talked to people about this now and then over the
last thirty years.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Dannon Rhudy on mon 22 jan 07
Dave, there are a great many buildings in Indiana and
Ohio that have decorative terra cotta work on the exterior. They've been
there since early 1900's, maybe
longer. Here the freeze-thaw cycles are every year, and
the climate is a wet one, too. I believe that Stephani
Stephenson has some information on some of these, too.
Maybe in the archives, but I am not sure of that.
It is NOT the same terra cotta that comprises the cheap
imported flower pots. Those don't last more than a season or two
outside/winter in this climate.
regards
Dannon Rhudy
stephani stephenson on mon 22 jan 07
to my mind there are a couple of layers to this
question
one is what kind of information you learn from..
1. science and results of scientific tests, whether
the tests are conducted by scientists, industry , or
individuals.
2. anecdotes and/ or here-say.... when someone says
"i did this or i have seen this, read this,heard this"
3. Direct experience "i have done this or used this
under these conditions and here is what the result
was.....
ALL of the above have value
though there is no better substitute than #3 for the
maker... you have tested it, and have direct
experience ,over time, and in first person. with
certain materials under certain conditions...and know
what works
#1 and #2 are also valuable, but especially valuable
when they back each other up.
now, there seem to be 2 approaches to this
'weather/water/frost resistant' question
One says make it vitreous! with porosity under 3%
or 2% or some even say 1%... but the general consensus
is
Keep the water OUT! keep the water out to begin with!
the other approach, which doesn't speak to whether
the body is vitreous or not, but does allow for the
the body to be open.....(.i.e there are either air
capillaries in there... now, whether those capillaries
or voids are caused by how particles fit together ,
i.e., inclusions of large un- melted grog particles
i don't know..... but this approach,
, also makes a differentiation between 'absorption '
and 'porosity' (Cushing)
or perhaps in updated terminology, between 'open
porosity ' and 'closed porosity'( Insight, referenced
later in this post)
The idea of a capillary system as i read it is
..that clay will be porous, enough so that water will
either drain or evaporate out of it OR
t even if water soaks into the clay , stays there ,
then freezes, there will be room for the water to
expand as it turns to ice.
that even ice 'heave' can be tolerated to a certain
degree .
that is how i read the second approach. I can
visualize this second approach . it makes sense to me.
yet i still don't trust it entirely, though i know
that porpous clay pieces have and do survive in
northern climates. but that information is way too
broad to be useful to me .
i need details. I need either 1. Science 2. more
data. or 3. direct experience over time.
Back to the difference between 'open porosity ' and
'closed porosity'
this refers to the difference between the amount of
room temperature liquid water a fired body will absorb
versus the amount of boiling water a fired body will
absorb ....
the cold water absorption divided by the boiling water
absorption gives you the "Saturation" coefficient
The saturation coefficient must be 0.78 in order to
pass CSA and ASTM specs for outdoor use.
i can tell you that i have tested bodies for use
outdoors which had coefficients less than .78 which
had higher absorption than many clay professionals
thought was prudent, but then too, most of the people
consulted are coming from a stoneware pottery
background versus an architectural ceramics
background.
this second approach has sounded dubious to those in
the field who feel that only a vitreous body will do.
I think they, like I, like anyone who wonders,
need to do more testing.
The alternative and the back up to the mathematical
approach is to simply run a claybody through actual
freeze thaw cycles and see what happens. For you the
home potter or tilemaker or sculptor, this is a good
way to learn about your clay , especially your clay
if it is glazed, in combination with your glaze.....
In my perfect world i would like to have a body that
was vitreous at the temp i fire to..I would love to
know that the water would just not get in!
.If it also had good working properties.
with architectural clay this can mean thickness.One
difficulty with accessing archival architectural terra
cotta information is that the information was often
proprietary,
used in industry, locked up ,kept secret.
also native materials were often used and sometimes
surprisingly high temperatures and very long firing
cycles were used. so not all the info would be
applicable on a studio scale. at the same time, i
know that a lot of the founders of the tile and clay
companies in the late 1800s and early 1900s in the
U.S. and in other countries were trained and skilled
ceramic engineers ,etc. so that info lurks somewhere,
probably in some of the old articles stashed in
Scholes library or abroad or in dusty boxes
somewhere.... but a lot of it in this country went
down with the companies in the 1930s with the
depression and with the favoring of new architectural
materials.
Vince you make a good point when you comment on the
thickness of architectural pieces and the fact that
bodies suitable for such work are usually quite 'open'
i have observed thicker pieces of architectural
ceramics which do have high % of grog. i have seen low
fire tile bodies which are vitreous.
so in my own mind i haven't ruled out a nice
combination of vitreous and 'open'...i don't want to
unnecessarily rule out something..
I think,like Dave, i want to know more and am not
entirely satisfied with relying on the S
coefficient..at the same time until i learn otherwise,
it is just about all i have to go on, in the written
info anyway.
Stephani Stephenson
P.S.
I have read Cushing's study on freeze thaw and in fact
retyped it in a post to the list at one time and we
have discussed on list numerous times. that
information and Cushing's formula on how to figure out
whether a clay-body is OK for freeze/thaw is also
posted on the Insight , go to the following URL to
find it
the article is entitled "Outdoor Weather Resistant
Ceramics"
the URL is
http://ceramic-materials.com/cermat/education/105.html
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stephani stephenson on mon 22 jan 07
you know , in typing the last post i made a goof
BOTH Cushing and the Insight Article state that the
Saturation coefficient should be LESS than 0.78...i
left out the word "Less than" when i referred to the
Insight article
also i referred to a "porpous" clay body 0:
sorry about that, i don't do that very dolphin.....
also , one other observation i made about the VALUE of
a porous claybody.
I had some porous slightly underfired pavers
some of them i sealed thoroughly with a penetrating
sealant
the others i did not seal
they were about 1 to 1 1/4 " thick,heavily grogged
body.
the grog grains stood out slightly on the paver
surface, helping to make a nice slip proof surface
i put them outside during a rainy spell
rainwater immediately beaded up on the sealed tiles
even though the pavers had a slightly domed center the
water collected in pools...in an enclosed patio,it
would have pooled .
it made even the grogged pavers somewhat slippery.
meanwhile i noticed a beautiful phenomena
the unsealed pavers absorbed a good deal of rain...
with intermittent rains
the water NEVER pooled on the surface
i.e. the surface always remained slip proof
AND
in between the intermittent showers the water which
had absorbed into
the clay evaporated back out.
these pavers were like a wonderful breathing organism,
inhaling and exhaling the water, always providing sure
footing
in certain climates you couldn't get a more ideal
surface
so , the lesson is that one size does not fit all!
Stephani Stephenson
revivalsteph@yahoo.com
http://www.revivaltileworks.com
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Marek & Pauline Drzazga-Donaldson on tue 23 jan 07
Dear Stephani,
I know you speak with a great deal of experience as you work in the =
realm of Architectural Ceramics.=20
I have also had to go through rigorous testing for my goods (roof tiles =
etc) to be allowed into the States.
Several observations on your brilliant posting.
After the fire of London - 1666 - a decree was implemented that all =
houses in London had to be made from tiles and not thatch as previuosly =
was the case. So a standard size and general thickness was ordered. Now =
even after the mini Ice Age Europe went through in the mid 17Century, =
some of these tiles still exist on roofs in England and they are so =
porous it is unbelievable - I have seen and felt them myself. The tiles =
are regularly (every fourty years or so) taken off so thst the timber =
and felting can be repaired/maintained. These buggers keep on keeping =
on.
Now I have made some tiles (hand made in exactly the same way as =
centuries ago), and was asked to make them as close to vitreous as =
possible. I argued against this, as I had heard, and had had a little =
experience, that fully vitrified tiles would easily crack under =
massively varying heat variations. Chicago, for instance, would be hot =
in the day and freezing by night. Luckily I had some tiles fired at two =
different states, fully vitrified and what I called frost proof ( a =
Staffordshire TerraCotta fired up to Cone 3 - 1170 C), which is very =
slightly porous but well fired. The vitrified tiles were the only ones =
that broke over a winter season - none of my frost proof tiles did, =
whereas 15% (nearly) of the vitrified tiles did - an unacceptable level. =
Go figure.
Happy Potting Marek
Hand made Architectural Ceramics from No9 Studio UK www.no9uk.com
Fully Residential Pottery Courses and more at Mole Cottage =
www.moleys.com
"Tips and Time Travel from a Vernacular Potter" reviews on =
www.keramix.com
an irreverent point of view after 35 years in the game Marek =
Drzazga-Donaldson =20
Assemble a dragon finial at www.dragonfinials.co.uk
Free Works and Mole Cottage DVD's and Video content on all the sites
Drzazga Video Promotions at www.drzazga.co.uk Submit address for DVD
Jeoung-Ah Kim on tue 23 jan 07
Dear Vince and all others,
Very interesting posts and thank you for the article. The article was usable but not enough.
How cold temperature are you guys talking about? And, what humidity?
I have had discussions here with colleagues some years ago about the outdoor ceramics since we are living in a really cold and dry climate freezing from September to next April (snow usually not melts during this period). And then, start raining almost everyday. How many are you interest in this matter? Perhaps I can conduct a research in this topic. Do you have any serious problem with outdoor ceramics in USA?
Yours,
Kim
Dr. Kim Jeoung-Ah
Speldosegatan 4
SE-42146, Västra Frölunda
Sweden
Tel: +46-739-849906
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Eva Gallagher on tue 23 jan 07
Vince, Dave,
Here is the calculation from Val's handbook - he says that it is the
standard test used in the ceramics industry for structural clay products.
Dave you may find that your clay falls right in the correct range. Also the
glaze must have an effect - terracotta is usually unglazed, fully allowing
the pores to function, unlike many stoneware planters that are glazed at
least on one side if not both.
D= dry weight of test sample
C= wet weight of sample after soaking at room temp for 24 hours (wipe off
surface water before weighing)
B = boiled weight after 2 hours in boiling water (again wipe before
weighing)
1. C - D / D = "C" value
2. B - D / D = "B" value
3. "C" value / "B" value = C/B ratio. The C/B ratio must be less than .78 -
represents room for expansion in the pore structure.
However I think that Dave is right in that planters even though they may
have the right porosity may still crack because of the expansion of the
contents. I think that perhaps these type of pots if large enough do OK in
Europe because it never stays cold long enough for the earth in the pots to
freeze completely, - it may just freeze on the outer edges. Here the outer
edges freeze first, then later as the insides freeze - they push out a hard
shell of earth against the clay - the frozen earth cannot move up and out -
Actually the planters that have worked best here in Ontario are very
v-shaped and had very thick walls. Others that I have seen work were very
thick large salt glazed planters and troughs in the Czech and Slovak
Republics - but again the shape is important - all had a nice smooth at
least 1" thick wall, wider at the top than the bottom.
Eva Gallagher
Still looking for the perfect frost resistant planter,
Deep River, Ontario
----- Original Message -----
From: "Vince Pitelka"
To:
Sent: Sunday, January 21, 2007 11:03 PM
Subject: Re: Freeze-resistant Ceramics--reply to Vince
> Dave Finkelnburg wrote:
>> I have never seen the carefully researched science
>> that would reassure me a porous clay can withstand
>> serious freezing cycles. If you have some references
>> to published research that I could review, I'd be
>> grateful for the opportunity to look for it. Thanks!
>
> Dave -
> I never said "a porous clay." I don't know the specific references, and
> I'm
> not sure when I could find the opportunity to look for them, but you could
> do that, through your connections at Alfred. If you consider the
> magnitude
> of the architectural terracotta industry at the turn of the century (19th
> to
> 20th), and you know that ceramic engineers in academia were focusing
> serious
> research on this subject. Somewhere I have a copy of the outdoor claybody
> info from Val Cushing's handbook, but I am not sure how easily I could
> even
> put my hands on that. As I remember the story, his information was based
> into his own research into earlier architectural terracotta literature.
>
> Several years ago we had an exhibtion of the work of Robert Wood, a
> ceramics
> professor from SUNY Buffalo, who makes large-scale terracotta sculpture,
> much of which is installed outdoors. He had done thorough research into
> architectural terracotta and outdoor claybodies, and he corroborated what
> I
> had always understood - that a terracotta claybody fired to cone 1 or 2
> will
> develop great mechanical strength, but will still have enough porosity to
> allow the pressure of freezing water to escape. As you know, when water
> is
> freezing, it will follow the path of least resistance, but if it has no
> easy
> path of release, it will build tremendous forces and simply shatter or
> warp
> whatever is surrounding it.
>
> I can claim no expertise in this specific subject, but when I think of it,
> it does seem logical. If a porous low-fire piece is soaked with water and
> then put into a hard freeze, it will disintegrate even though there are
> plenty of avenues for the pressure to escape, because there is so little
> mechanical strength in the claybody, and it is easier for the freezing
> water
> to shatter the claybody than to "bother to" escape through the porosity.
> But in a claybody fired high enough to develop great mechanical strength
> but
> still retain some porosity, the pressure of freezing water will seek the
> easier way, escaping through the remaining porosity.
>
> I'll look forward to hearing what you find through your research.
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft, Tennessee Technological University
> Smithville TN 37166, 615/597-6801 x111
> vpitelka@dtccom.net, wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka/
> http://www.tntech.edu/craftcenter/
>
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>
Snail Scott on wed 24 jan 07
At 08:35 PM 1/22/2007 -0600, Vince wrote:
>I don't think there is any such thing as a combination of vitreous and open,
>because "vitreous" pretty much precludes the possibility of "open."
Seems to me that one can imagine a
material which is composed of fully
vitrified material, but is still full
of holes. The material itself is fused,
but does not fill every void. Materials
which burn out in firing often leave
unfilled voids, and the microcracks
caused by large grog seem to survive
being closed up in firing as well. My
own clay bodies which have done well
oputdoors have been both high in grog
and were made of quite coarse clays,
which I suspect may retain interstices
between particles even after firing to
maturity, grog or no grog. It seems
reasonable to extrapolate from this to
a well-vitrified but still 'porous'
(in the sense of literally having pores)
ceramic material.
-Snail
Vince Pitelka on wed 24 jan 07
Kim wrote -
> Very interesting posts and thank you for the article. The article was
> usable but not enough.
> How cold temperature are you guys talking about? And, what humidity?
Kim -
It's not that specific. Architectural terracotta was used all over the
country, but the best test is the stuff used in the Northeast and the
northern Midwest, where there were huge quanitites of architectural
terracotta on building exteriors. In those areas winter temperatures drop
to well below zero Fahrenheit. That could happen after a period of fairly
humid weather, so the porosity in the clay could still be well-saturated
with water. During the coldest part of the winter, the air is very dry, and
I doubt that that's when the problem occurs. It would more likely occur
earlier in the late fall or early winter when there is likely to be a lot of
residual moisture within the pores in the clay.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Vince Pitelka on wed 24 jan 07
> It seems
> reasonable to extrapolate from this to
> a well-vitrified but still 'porous'
> (in the sense of literally having pores)
> ceramic material.
Snail -
Maybe it's just a question of defining "porous." Certainly there are still
pores, if there is any absorption at all. But is that enough to call it
porous? Normally, vitrified claybodies are not referred to as porous.
Also, all the kinds of pores you mention will be filled with glass to some
extent during a high-fire glaze firing. If a fired clay surface is still
"full of holes," then I don't see how you could call it vitrified.
I'm certainly not trying to say that vitrified wares cannot still retain
some level of porosity, but I question whether it is enough porosity to make
thick architectural or sculptural work survive long-term in hard-freezing
weather.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
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