[Clayart] Best clay for outdoor sculpture

[Scary Potter]

Again Snail comes through with a thorough and thoughtful contribution!  
So I’m looking for a coarse, groggy clay that’s fully vitrified at Cone 5, roll the slabs 1/4-3/8” thick, glaze the outside only, and enjoy outdoors for a nice long time. 

Thanks so much, 
Sharon 

S.Cary -
Make something beautiful every day. 
Sent from my iPad

> On Feb 2, 2020, at 9:18 AM, Snail Scott <claywork at flying-snail.com> wrote:
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>> On Jan 31, 2020, at 11:03 AM, Scary Potter <scary.potter at yahoo.com> wrote:
>> Thanks, Ric and Vince. You seem to have the opposite opinion on vitrification but I can see the merits of both. 
>> Now I’m worried about the unglazed bottom in contact with the wet soil. Since this will likely be a hollow cone shape should the inside be glazed or better left unglazed?  Also, would thicker clay be sturdier in this regard than thin slabs?
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> Strength conferred by thickness will help if the force you worry about is expansion of water from inside the form, such as happens with flowerpot full of damp soil or sculptural forms with ‘scoop' shapes that don’t drain. Thickness will not help if the expansion is due to water inside the clay wall itself, as it will still spall as the ice expands. (This is based on personal observation.)
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> This is analogous (though definitely not identical!) to the way damp clay is damaged by steam explosions, compared to the way it is damaged by internal air pressure buildup.
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> If there is no reason to glaze the interior, I would leave it bare.  Many fired-but-porous clay bodies will release moisture through an unglazed surface as ice expands, while a fully glazed surface can trap it even within an otherwise suitable clay body, much like dampness building up in an old cracked sealed-beam headlight - it works its way in gradually, but has a harder time escaping quickly. Unlike a headlight, the clay can get fully saturated, with nowhere for the water/ice to escape if it’s tightly covered up.
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> As was mentioned by others, strength is critical in resisting this ice pressure! The ideal, in my mind, is NOT an undermatured body, which lacks it potential full strength, but a fully matured and vitrified body which still retains micro-channels within its structure. This may register as undervitrified to a simple absorption test, but while water may penetrate, the clay itself is fully fused. The reason, as I see it, is grog.
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> Imagine a small rock, wrapped in a layer of soft clay. What happens as the clay dries? We all can visualize this: it cracks!  This must be happening to grog as well, which although smaller than a rock is still a thousand times bigger than a clay particle. It will inevitably be a source of microcracks that permeate coarse clay bodies even when fully vitrified, and serve as the ‘escape hatches’ for expanding ice even while allowing the moisture to penetrate in the first place. 
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> All the clay bodies I have used which are reliably frostproof are also very coarse, with lots and lots of grog. They consistently glaze ‘badly’, too, as the first layer of any glaze seems to get wicked down into the clay when fired, requiring a far thicker application than on a ‘tight’ clay body.  
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> The correspondingly low shrinkage is a bonus!
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> I wouldn’t worry about the soil-contact surface one way of another - do whatever is easiest. Moisture will still find its way into the clay without ground contact at all. I’ve had animal-water basins on the ground for decades, unglazed bottoms and all. I do glaze the interiors where water is held, though, so that ice won’t bond to the sides and can float freely up. With enclosed hollow sculpture, I only glaze the outside if I glaze anything at all, and I never glaze the bottom unless it was fired upside-down anyway.
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> -Snail
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