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. In climates exposed to freezing temperatures, water soaked ceramic experiences a buildup of pressures within the material as ice formation proceeds in the pore space (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. While potters may never encounter this phenomenon, sculptors and mural makers need to confront it (as does the the construction industry).
Examples of clay porosity (these assume that the clays are fired at a temperature appropriate for each, under-firing will increase the porosity and reduce the strength):
|Red or brown burning stoneware clays||0.5-4%|
|White or buff burning stonewares||1-3%|
|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). Actually, that is not completely true, it is implied that the ceramic has been fired to a state of high strength. Obviously, terra cotta fired at cone 06 is weak (you can scratch it easily), it would not be able to withstand the stresses even if it could be densified enough to have a low 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 permits fired ceramics to survive freeze-thaw because the expansion of the water has somewhere to go. Thus, the simple addition of finely dispersed cellulose fiber to a clay body lacking closed porosity 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.
Of course, the only way to have confidence is to do some testing. Do not just test the clay at the temperature at which you will fire it, but higher and lower to get a profile of what it does (especially if your kiln varies in temperature throughout the chamber or you do not use cones to verify the temperature reached). 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 (assuming of course that it has good strength).
Of course, testing the body in actual freeze/thaw conditions would be ideal. But there are alot of complications. We all have freezers. Since modeling what happens in an outdoor climate would be extremely difficult it seems logical to simply subject the clay to repeated cycles of conditions more extreme than what it would encounter outdoors. However many cycles would be needed, theoretically hundreds (I have dried out, soaked and refrozen bars for half a dozen cycles and could not get spalling even on a terra cotta). In addition, it may not be true that rapid temperature drops or high penetration of water are the most harmful; possibly temperatures hovering up and down close to freezing with just surface water could be more destructive. It could also be true that glazed ceramic (with crazing) is more susceptible than non-glazed (see next section). Other factors are the thickness of the ceramic, how exposed to evaporation the back side is and if there is a density gradient (more dense surface with a more porous interior).
Earthenwares fired at low temperatures (e.g. cone 04-06) are especially susceptible to spalling. Although labels and advice might claim otherwise, it is likely possible that your red body/glaze could go to cone 02 and still be stable, even stoneware-like. Redart clay, a major ingredient in red earthenwares in North American fires to about 5% porosity at cone 02 whereas it has more than 12% at cone 06. However white low-fire talc bodies will not be much less porous at cone 02 than at 06 (they are known to spall and one of the poorest choices for outdoor work).
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). However a caution is in order: If the body is porous and the glaze is crazing it will be very difficult or impossible to get the sealant into the craze lines (but the water will get in and freeze there).
Of course, you can guarantee no freeze/thaw issues by using a smooth clay that will fire to close-to-zero porosity (grogged clays have high porosity). But this will bring other issues. It will warp much more easily on firing. It is more likely to dunt on cooling in the kiln and crack on drying. It could twice as much during firing. The surface texture will be boring (since only white and buff bodies fire near zero porosity). You could use colored slips but if they are not zero porosity also they may bond well to the body below, will not have the same fired shrinkage and will not likely match the thermal expansion). In short, this is not really an option.
Slips and engobes are often not bonded well to the body below, especially at lower temperatures. You might find that if you use slips they could flake off over time because of freeze-thaw.
There is a link to the C/B test procedure in the Digitalfire Reference Database (below). The procedure uses 10 mm thick by 25 mm wide by 120 mm long fired test bars and defines a 24 hour soak and weigh, then a 5 hour boil and weigh.
C = (v2 - v1) / v1
B = (v3 - v1) / v1
S = C / B
The saturation coefficient S should be less than 0.78 in order to pass CSA and ASTM specs for outdoor use.
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.
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.
If you are an artist or sculptor and are thinking about taking on a commission to do an outdoor piece think carefully about its vulnerability to spalling if it will experience freezing temperatures (see linked article below). The construction industry deals with this issue every day, take a cue from their experience.
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By Tony Hansen