[Clayart] Clayart Digest, Vol 50, Issue 43

[Snail Scott]

> On Jan 31, 2020, at 10:33 AM, Alice DeLisle <wanderland at att.net> wrote:
> ...Most potters assume that the best clay body for outdoor work is the one with the lowest absorption…



This is often what's taught, because it’s easy to convey in a brief lecture, and easy to test for. But it is a flawed model.

All the best research that I know of seems to have been done in the 30’s, when architectural terra cotta was big business, and it makes clear that a clay body with open channel structure allows ice crystals to expand into the structure without damaging it, and entirely vitrified clay body prevents intrusion of water altogether, but the worst circumstance is a nearly-vitrified clay which allows some penetration, but is a partially closed structure which traps expanding ice in blind alleys, where expansion can only be relieved by spalling. The testing for this is not as easy and clear-cut as finding an absorption number, as this does not correlate to the structure of the pores. It’s actually rather complex and non-obvious, which is why so many people revert to describing frost-proofness as a matter of absorption percentages.

In spite of reading, I cannot claim anything like sufficient understanding to predict in advance whether any given semi-vitreous clay body will be frostproof. So, I rely on empirical testing, which is time-consuming and annoying, but has given me good results up until now.  I currently use a commercial clay body with a multi-decade track record of outdoor use, which supplements my own testing to a far more reliable statistical sample in actual conditions of use. My past clay body has also survived well outdoors for decades, but was designed for reduction, which I’m not presently doing. Any new homemade clay body would require extensive testing, and I like being able to represent to a client that my larger work can go outdoors, with evidence beyond my bare word. I am presently developing a new body, but I will use it only for indoor work until it has a well-tested track record.

Cautionary tale - a nearby location of the ‘Del Taco’ fast-food chain had red and green tiles embedded in its stucco exterior, superficially identical except for the color of the glaze, and presumably spec’d by the contractor for outdoor use nationwide: hundreds of locations.  A year later, the green tile looked great, while the red tile had spalled down to its bottom layers as moisture trapped behind the glaze froze and expanded. Same brand but different factory? different firing conditions? or a different batch lot of clay?  Would the bare clay have survived OK without glaze on top to trap the moisture?  

The hundred-year-old ceramic architectural details in the old loft district of St. Louis all looks great, though, and the old Majestic Theater in East St. Louis - abandoned for decades, with no roof left and trees growing from every crevice, retains its nearly flawless ceramic facade. They knew their shit, back then. 

       -Snail