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Outdoor Weather Resistant Ceramics

Section: Clay Bodies, Subsection: General


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 flakes and scales are going to separate and fall away from the surface. 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, underfiring 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).

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 products firing to more than 5% porosity) in order to pass CSA and ASTM specifications for outdoor use. If you are buying clay, your supplier should be able to tell you what the porosity will be at the temperature you plan to use it. If it is over 5% (as noted above) then you should be aware of the Closed-over-open porosity also. If they sell the material for sculpture and know that customers are using it outside in freeze-thaw conditions, they should be able to tell you this figure.

The C/B test procedure is defined in detail in the Insight-Live 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.

A Specific Clay Body

We tested a specific cone 6 buff burning function stoneware clay body (Plainsman M340). At cone 6 its closed porosity is 2.5% and open porosity is 2.8% (or 2.5/2.8=0.91). This is a failing grade! However, since it is well under the 5% closed porosity trigger, it passes for outdoor use. At cone 5 it scores 3.2/3.4=0.95. This is actually a worse score, but it still passes since 3.4% is well under 5%. However at cone 4 it scores 5.3/6.4=0.83. This is a fail. If this body were fired to cone 3 or 2, the C/B score would likely improve and it would pass. That means a body with higher porosity can actually be better for resistance to freeze-thaw spalling.

A Secret Weapon

You can artificially make a failing clay suitable by treating the surface with a sealer or a repellent. While sealers have long been used, some companies now claim that a repellent is better because it enables masonry structures to 'breathe' while sealants trap moisture in and can actually exacerbate the problem. However if the clay is around or just above the 5% trigger, a sealant may be the best solution. If it has a much higher porosity, Siloxane repellent, for example, is said to not darken or discolor masonry walls and lasts about five years of protection (of course that means vigilance to reapply, so it may be better to just use a clay with a passing C/B score).


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.


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.


These relate to the problem of spalling.


Why you need to know about spalling

It is easy to find pictures of spalling bricks at google. This happens because water trapped in the pores of ceramic and concrete expands during freezing and breaks it down. This should be a concern to people making architectural and sculpture pieces for outdoors. How do you know if your ceramic will spall? It needs open and closed pore space if over 1% closed porosity. Measure the percentage gain in weight (of a test tile) after a 24-hour water soak, that is 'open porosity'. Then boil for 5 hours and it will soak up more water, measure that as the % closed porosity. The second number needs to be 20% greater than the first. Why? The closed space provides a place for expansion as the water is freezing.

In Bound Links

By Tony Hansen

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