Eleanora Eden on sat 18 jun 05
I don't have Peter Coates' original post so I hope this gets to you,
Peter. This is the directions for testing a clay body for outdoor
use from the digitalfire index.
Eleanora
Closed and Open Porosity
More porous ceramic actually has both absorbency and porosity
(technically they are not the same). A fired piece will naturally
absorb a certain amount of water to fill the pores (open porosity ).
However more porous clay matrixes also have capillary networks that
normal soaking does not fill (closed porosity ). This auxiliary
network allows fired ceramics to survive freeze-thaw because the
expansion of the water has somewhere to go. The addition of finely
dispersed cellulose fiber to a clay body could theoretically improve
the capillary network.
It is possible to perform a simple test based on the principle that
a sample of fired ceramic boiled in water will absorb more moisture
than one that is simply soaked. This is because for the former, the
entire network is filled, for the latter only the pores. This test
compares the cold soaking absorption or open porosity (C) of a clay
with its boiled absorption or closed porosity (B) . The structural
ceramic industry requires a C/B result of less than 0.78 in order to
pass CSA and ASTM specifications for outdoor use. If you are buying
clay, your supplier should be able to measure and provide this
information for you. They will want to know the temperature and
rate-of-rise of your firing.
The C/B test procedure is defined in detail in the FORESIGHT
software Test Information database. The procedure uses 10 mm thick by
25 mm wide by 120mm long fired test bars and defines a 24 hour soak
and weigh, then a 5 hour boil and weigh.
Variables
Dry Weight - v1
The weight of a dry test specimen of the fired clay.
24hr Wet Weight - v2
The weight of a specimen that has been soaked for 24 hours in room
temperature water and wiped clean of all surface water.
5hr Boiled Weight - v3
The weight of a specimen that has been soaked for 24 hours and boiled
for 5 hours and wiped clean of all surface water.
Calculations
24hr Absorption - C
This is a calculation of the C value for this test, that is, the
absorption of a clay sample if soaked for 24 hours in cold water.
C = (v2 - v1) / v1
5hr Boil Absorption - B
This is a calculation of the B value, that is, the absorption of a
clay bar if soaked for 24 hours and boiled for 5 hours.
B = (v3 - v1) / v1
Saturation Coefficient - S
This is a calculation of the C/B value, the cold water absorption
divided by the boiling water absorption.
S = C / B
The saturation coefficient S should be less than 0.78 in order to
pass CSA and ASTM specs for outdoor use.
Sealing the Surface
Many products are available from building supply stores to seal the
surface of concrete and masonry. These just soak in and harden to
plug the pores. The use of these is standard practice in
construction. So even if your fired ceramic does have a high porosity
you can just seal all surfaces.
>On Wed, 15 Jun 2005 17:21:22 -0500, Peter Coates wrote:
>
>>Hello All...
>>
>>I recently mixed up a few hundred pounds of a homemade clay body. Out of
>my test clay, I have made a tile that measured 10 centimeters when wet. The
>tile is currently being fired to cone 10. When I unload the kiln, I will
>weigh the tile, then boil for an hour, then weigh the tile again. Is this
>the correct way to conduct an absorption test?
>
>Yes,
>This is how to test absorption. The math is also posted in the clayart
>archives. Thank you Ron Roy.
>
>>
>>Also, is the shrinkage of the clay measured with the 10 centimeters test
>the measurement that I would use to determine if glazes will fit correctly?
>
>
>Well, Ron will tell you that clay bodys can have a measured shrinkage but
>glazes are measured for coefficient of expansion. Others will have to advise
>on matching a body to a glaze. It is beyond me atthe moment.
>
>You are on the right trackt though. The body must be stable and vitrified
>for strong ware.
>
>I am doing some of this testing myself right now at cone 6. Will post my
>results.
>
>Ben
>
>
> Any advice on how to test a homemade clay body would be appreciated.
>>
>>By the way, the clay is an off white, no grog, cone 10.
>>
>>thanks
>>Peter Coates
> >Oklahoma City
>
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