[Clayart] Clayart Digest, Vol 50, Issue 43

[Snail Scott]

> On Feb 4, 2020, at 8:03 PM, Michael Wendt <mwendt at wendtpottery.com> wrote:
> Hi Snail,
> I have a funny thought...
> usually, freezing happens from one side inward as an object cools, proceeding at a pace controlled by how cold and windy it is..
> How about adding a 2" thick block of Styrofoam to one side of the item you are testing?


I am guessing that although clay has a fairly low thermal conductivity, it would probably go way up when wetted through. Not sure that there would be much of a gradient with typical ceramic thicknesses.  Good idea for thicker stuff, though, like brick. I note, observationally, than both cheap paver brick and concrete seem to spall to the air-contact side, even when the underside is as wet or wetter. We could presume that the ground-contact side is typically warmer, though that is conjectural.

A proper test of spalling under glaze seems in order, too. It seems to be that spalling sometimes occurs on a glazed surface, but not on the equally exposed unglazed surface of the same object. However, this is based on vague recollection, and as we know, ‘data’ is not the plural of ‘anecdote’.   ;)

 -Snail

p.s. A possibly irrelevant but anomalous observation of concrete: One day, I noted a hole in my front yard, in the rough location of my septic tank. It turns out that a cap on the tank had been replaced by an 18” concrete paver sometime in the past, and (of course) been buried ever since. The paver had spalled from the middle of the bottom side (where it sat over the open hole of the septic tank), until it spalled a hole right through to the dirt above it, which fell in and thus brought it to my notice. The spalling pattern was a dish-shaped recess, in the usual manner of such things, but had formed upside-down. So, did the dirt insulate the top side, while the uninsulated bottom was, for a change, the cold side, and thus subject to spalling?  It did only spall in one direction…why?