[Clayart] Firing deep bowl-like sculpture

[Snail Scott]

> On Feb 17, 2020, at 4:24 PM, Kathy Forer <kef at kforer.com> wrote:
> 
> I’m firing a large bowl-like structure that has walls and planes inside and some perforations to the bottom outside. The overall shape is
> is an 18” shallow bowl. 
> 
> I will put sand underneath, thanks Snail, (regular sand, special fine silica sand?) and was considering a collar of a few supports, lumps of clay or kiln furniture, 1/3 of the way in from the rim. it pretty much can sit on its bottom. 
> 

A sturdy collar is a great way to go!  If you make it on the wheel, it can be a seamless cylinder, made a little thicker at the bottom to resist friction during shrinkage. A slab-formed cylinder works well, too. This eliminates one of the downsides to firing in a nest of sand: the tendency of the sand to insulate the already-slow-cooling contact area where the work touches the shelf. For most sculpture, this doesn’t make much difference, due to the already-irregular shape and thickness of the work and the likelihood that it is thicker than pottery anyway, and likely lacking in thin, fast-cooling edges. A bowl form, especially a thin one, may be more vulnerable to fast cooling edges if the bottom stays hot longer. A ring, much like the support ring on a stovetop wok, can be a handy assist with preventing pyroplastic deformation, too, by moving the support point outward (shortening the 'moment arm’ of the downward force at the rim).  Imagine holding a brick at arms’ length. Them imagine doing so while your elbow is propped on a support. Easier, no?Same concept.

A little sand under the ring, just to act like ball bearings, is always useful. Any heavy work, or work with multiple separate support points, or work which is wide enough for warping to have a visible effect also benefits, and it’s easy. Silica sand is a ’must’ for high fire, but for low or even midrange firings, less refined sand will usually suffice. You may notice some grains stuck to the work if the mineral content is high in fluxes, but it usually comes off. If the sand is under a support or firing slab, then it doesn’t matter at all. Even ‘iffy’ sand won’t stick to kin wash. Still, silica sand is cheap enough (and re-usable), so I wouldn’t resort to playground sand unless there was no option. If I’ve gotta store sand around the studio, having it be multi-purpose seems sensible.

Good luck on the project! Love your work.

-Snail