|Monthly Tech-Tip |
Clay products both structural and artistic which will be used out-doors must be able to survive the stresses of freeze-thaw. Porous ceramics absorb water and freezing temperatures which cause this absorbed water to expand tend to break down the matrix, eventually crumbling it.
Some technicians assume that if absorption is below a certain amount that a fired clay is durable for outdoor use. However, any fired clay with more than zero percent absorption demonstrates water penetration and is theoretically susceptible to freeze-thaw damage (although for practical purposes if a body is under 0.5% normal porosity there is usually no worry).However, a fired clay has both absorbency and porosity. A fired piece will naturally absorb a certain amount of water to fill the pores (open porosity). However clay matrixes also have capilliary-like networks which 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 (assuming of course that they are strong enough to withstand the stresses of channeling the expanding ice).
This test is based on the principle that a sample of clay boiled in water will absorb more water than one that is soaked, 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 specs for outdoor use.
This test depends on bars as made in the SHAB test. Prepare bars using the SHAB standard procedures altering the boiling and weighing aspect as follows:
Weigh the bars after a 24 hour soak, boil them for five hours and allow them to cool to room temperature, then weigh again. Be careful to blot the bars well on an absorbent towel. In the case of some dense bodies it is possible for the boiling process to clear the pores somewhat so that the boiled bar will be lighter than the soaked one. Such cases are considered a pass.
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.
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.
This is a calculation of the C/B value, the cold water absorption divided by
the boiling water absorption.
The weight of a dry test specimen of the fired clay.24WW - Value (V)
The weight of a specimen which has been soaked for 24 hours in room
temperature water and wiped clean of all surface water.
The weight of a specimen which has been soaked for 24 hours and boiled for 5 hours and wiped clean of all surface water.
Tests conducted on bodies made from materials, as opposed to the materials themselves.
Outdoor Weather Resistant Ceramics
How can you be sure that the porosity of your fired ceramic ware is low enough to prevent freeze-thaw breakdown in the winter?
|By Tony Hansen|
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