DianeS1234@aol.com on sat 12 apr 97
Hello all:
I am planning an outdoor project where I will be (ultimately)
thin-setting clay pieces to a concrete background. My question is: Where
would I find technical information regarding the strength of clay bodies?
I'm in a "discussion" concerning what ^ clay to use.
From what I've been able to gather so far, vitrification is the key to a
strong piece. Low fire earthenware is too porous and is not considered very
strong in relation to say, ^4-6 earthenware. Also, I've read that the only
reason to fire at ^10 (strength-wise) is if you're using porcelain. Is this
correct?
It seems to me that as long as I'm affixing the fired clay to a concrete
background, the strength of the clay piece is rather irrelevant, assuming
that it's strong enough to not crumble, or be vandalized by "initial
scratchers." Can low fire fit this bill? I'm planning on glazing with a
matt clear glaze. I know that, if there is a good fit, that greatly
increases the strength of the piece.
At any rate, can someone steer me to the correct technical paper/book/
whatever, so I can find what I'm looking for? All help greatly appreciated.
TIA, Diane in Tucson (where allergies are in FULL bloom)
Gavin Stairs on sun 13 apr 97
At 01:27 PM 12/04/97 EDT, Diane in Tucson wrote:
....
> I am planning an outdoor project ...Can low fire fit this bill?...
I'll let the clay guys handle the strength issue.
I would like to note that low fire (terra cotta) bodies have been filling
this bill for centuries. There was a recent article in Ceramics Monthly
about terra cotta building restorations and new work, I seem to recall.
There's the usual discussion of a standard pore size testing method for
durability of frost prone exterior bodies in several standard texts,
including Val Cushing's Handbook, and Lawrence and West, Ceramic Science
for the Potter.
I can't remember who says that the ratio of change in cold wet mass to
change in boiled wet mass must be .78 or less for durability in frost. I
suspect it was Val Cushing, since I seem to recall that the work was done
at Alfred, but I may be wrong. The reason for this ratio is that the key
is that the pores in the clay, however dense, may not be filled up
completely by rain, etc., or they will be cracked open when the water
expands on freezing. However, if there is extra room for the ice to expand
into, the piece will be durable. The boiled sample represents the whole
pore being filled, while the cold sample represents what rain and ordinary
atmospheric exposure can fill.
I won't reproduce the whole procedure here, but if you can't find it
elsewhere, send me a note.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
Tony Hansen on sun 13 apr 97
> I am planning an outdoor project where I will be (ultimately)
> thin-setting clay pieces to a concrete background. My question is: Where
> would I find technical information regarding the strength of clay bodies?
If a clay is vitrified (under 1% porosity) it can be strong at any
temperature
above cone 02. I've had terra cotta bodies fire to 10,000 lbs/sq in
strength at
cone 02 and iron bodies fire to 2,000 at cone 10. Hand extrude some
pencil
thickness bars, fire them, and simply break with your bare hands. Strong
bodies
are very difficult to break. Measure the porosity, the lower the
stronger
(provided the body is not overfired and porosity is a result of
expansion
and melting activity). In the brick industry, anything under 5% porosity
is
usually considered resistant to the elements.
--
=================================================================
Tony Hansen, IMC thansen@mlc.awinc.com
INSIGHT5/Magic of Fire II demos at www.ceramicsoftware.com
Tony Hansen on mon 14 apr 97
Gavin Stairs wrote:
> I can't remember who says that the ratio of change in cold wet mass to
> change in boiled wet mass must be .78 or less for durability in frost. I
> suspect it was Val Cushing, since I seem to recall that the work was done
> at Alfred, but I may be wrong. The reason for this ratio is that the key
> is that the pores in the clay, however dense, may not be filled up
> completely by rain, etc., or they will be cracked open when the water
> expands on freezing. However, if there is extra room for the ice to expand
> into, the piece will be durable. The boiled sample represents the whole
> pore being filled, while the cold sample represents what rain and ordinary
> atmospheric exposure can fill.
Check the web page at
www.ceramicsoftware.com/education/clay/outdoor.htm
I changed it slightly. It turns out that if your fired clay is below 5%
porosity, you don't need to worry about all of this.
--
Tony Hansen, IMC thansen@mlc.awinc.com
Denis Whitfield on tue 15 apr 97
The following is something I forwarded some time back on the topic of
porosity measurement. It might be useful to those following this thread
about outdoor bodies.
.........................................
I have had some messages that suggest my attempt to elaborate on the
porosity question has not been able to be read.
I need to give a more precise formula for calculating the porosity of a
fired body.
%apparent porosity = (B-D)/(B-S) X 100
where B = weight of the test piece in GRAMS after it has been boiled for
one or two hours and then allowed to stand to soak for another twenty four
hours. The Boiling drives out the air that is trapped in the small pores of
the fired clay and allows the air to be replaced with water.
where C = the weight of the SAME piece in GRAMS when it was dry (do this
weighing first).
So now what we have (B-D)is the weight of the water able to be absorbed by
the fired clay body - more importantly, since 1ml of water weighs 1 gram
(just one of the nice things about the metric system)we have the VOLUME of
the water in the body and therefore the VOLUME of the pores.
where S = the weight of the SAME piece of body used for the B measurement
(ie boiled and soaked)when it is immersed in water so it is completely
covered but not touching the sides or bottom of the container (use a beaker
to be able to check this easily).
To do this weighing you suspend the piece from your scales with a short
length of fine nylon fishing line. At our University we use an electronic
balance that allows the line to be attached to the balance below the scales
and the line passes through a hole in the table on which it located and then
to the piece being weighed which is below the table in a beaker of clean water.
"S" will be less than "B" becuse there is an apparent loss in weight when
things are in water (as you will know if you have lifted someone while
swimming).
So B-S is the apparent loss of weight but, more importantly, the apparent
loss in weight is equal to the weight of the water displaced by the piece
and therefore the apparent loss in weight IN GRAMS is equal to the volume of
the piece in ml. Because the piece was boiled and soaked first and is
therefore full of water B-S is the volume of the piece as if it were
completely solid.
So the expression (B-D)/(B-S) compares the volume of space filled with water
(top line)with the volume of the whole piece (bottom line) and "X 100" gives
us a percentage.
You need to use an electronic balance for these weighings as an accuracy of
1/1000 gram is necessary. The scales you use for weighing clay is not good
enough. Dick Smith (is he over there?) sells useful electronic scales for
around $80 Aussie.
Bodies for outside use, especially in places where you have freze/thaw
conditions, should have low apparent porosities. You should look to have
more vitrified bodies - fire higher, fire longer, or change you body
composition.
In Australia we don't have the freeze thaw problem so much. The saying of
one of our states which is equally true of the country is that the weather
is glorious one day and perfect the next.
..........................
****************************************************************************
DR DENIS WHITFIELD
Senior Lecturer
Co Director, Centre for Ceramic Research, Design and Production
Department of Visual and Peforming Arts
University of Western Sydney, Macarthur
PO Box 555
CAMPBELLTOWN NSW 2560
AUSTRALIA
email: d.whitfield@uws.edu.au
phone: 02 97729345
fax: 02 97723244
****************************************************************************
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