Post #414053 by coruscate on Sat, Oct 18, 2008 10:38 AM

Glad to offer some help, you're doing great stuff.

I come at this knowledge through doing lots of wheel thrown and hand built work as a potter--but I apprenticed under two potters who were very experienced and generous with their knowledge and feel oblidged to share.

So, I hope this isn't too long, but here is some more clay knowledge sharing:

I forgot to note that if the clay gets really wet it starts to separate naturally and "slake" appart.
These types of cracks are rougher and look less like an S.
This rarely happens, but can when water pools at the bottom of a piece and you forget to sponge it out.

You probably heard this before, but it really helps to wedge your clay thoroughly so that natural alignments of the minerals through gravity and chemistry are mixed and also homogenized, as are the dry and wetter spots from the inside to the outside of the clay lump and any air pockets are usually compressed or released (air bubbles = bad)when wedged properly for mixing and de-airing.

Also, having thin walls that come down to thick bottoms means things dry at different rates and heat up and cool down at different rates and this can be too much for the clay and crack at the intersection of thick and thin to release the tension. When I throw a pot, the base is almost always thicker than the walls, so while it is still leather hard I trim a foot into it. The trimmed foot helps it suffer less stresses by making the base a similar thickness to the walls (therefore similar drying/shrinking rate) and the foot also adds functionally and aesthetically to the pot sitting level on a table. You might look at your hand built pieces the same way and trim out a foot or remove little plugs of clay, or simply poke a few holes out of the users view so that the thick areas dry and heat more similarly to the thin areas.

One of the wettest things I do is pull handles. If the clay is well wedged, then I can pull and stretch (like milking an extrusion) the clay and it still doesn't come apart. If you don't have the luxury of a cool damp greenware room, wrapping the newly made wetter handle attached on a slightly drier mug tightly with a bit of light plastic helps the moisture between two clay parts that are joined together equilabrate and stay together through the firing

When in the firing, the goal is to get the clay near vitreous. Kinda like a cube of velveeta cheese in the microwave. You want to get it so hot that it melts, but not so hot that it changes its shape and slumps or pools. Because clay is very flexible in this vitreous state, stress or tension in the clay tends to try to relax itself like a tired stretched muscle in a hot tub. Quickly and forcefully spun pots tend to unwind in the opposite direction they were spun on the wheel. Clay that is stretched and joined in hand building wants to relax back in the direction opposite from how they were stretched. If the bond at the location where those two stretched pieces came together was additionally weakened by stresses from unequal volume changes due to quick drying, they tend to pull apart and crack there in the firing.

Patience is not something you learn in clay work, it is thrust upon you --or else your end up with lots of Tiki-mosaic material :)

P.S. --Before I forget, see if you can go to "Feats of Clay" held at the old Gladding McBean factory near Lincoln California next June, I am often blown away by the non-Tiki ceramic work that is absolutely inspiring for the creative clayworker.