Monday, March 11, 2013

March 10th Hint of the Week

Outdoor Weather Resistant Ceramics from Digitalfire.com

How can you be sure that the porosity of your fired ceramic ware is low enough to prevent freeze-thaw breakdown in the winter?
 
Almost all fired stoneware and sculptural ceramic has some porosity. Porosity (also called absorption) is typically measured by weighing a sample, boiling it in water, weighing it again, and calculating the percentage increase in weight. This means that it will absorb water into the surface. In climates exposed to freezing temperatures water soaked ceramic experiences a buildup of pressures within the material as ice formation continues (water expands 9% as it freezes). If the ceramics cannot withstand this then scaling and cracking occur. Over a period of years the material can completely crumble. Concrete is susceptible to the same problem, this phenomenon is called Spalling .
Examples of clay porosity (these assume that the clays are fired at a temperature appropriate for each, under firing will increase the porosity)
Red or brown burning stoneware clays
0.5-4%
White or buff burning stonewares
1-3%
Porcelains
0-0.5%
Warp resistant sculpture clays
5-10%
Vitreous sculpture clays
2-5%
Red terra-cotta clays
10-14%
White or buff talc clays
8-10%
Thus, if climatic conditions demand it, outdoor ceramic installations must be able to survive the stresses of freeze-thaw.
Since any ceramic with more than zero percent absorption demonstrates water penetration it would appear that it is theoretically susceptible to freeze-thaw damage. For practical purposes however, this is not altogether true. The brick industry considers any clay having under 5% porosity resistant to freeze/thaw failure (regardless of its closed and open porosity).

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