Showing Age

by thecastingfloor

The Kohler Pottery’s slip pumps are only turned on for an hour in the morning. Kohler’s production molds get cast once a day, and only on weekdays, so they don’t need pressurized slip any other time. We’re casting our molds, all twenty-five of them at last count, twice a day, seven days a week. We therefore store up slip in a forty gallon container and pour our molds with buckets in the afternoon and on the weekends. Here Andrew demonstrates the proper technique for filling the storage vessel.

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We have been working our molds hard. They are champs.

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But they are showing their age.

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Plaster is porous, and it’s the capillary action of those pores that makes slip-casting possible. The water, in a slurry with the clay, is drawn through the plaster, but the clay particles are too big to pass through the pores and so build up along the inside faces of the mold. Water is an erosive agent, though, and with each casting our molds break down a little more. Here you can see up close the deterioration of the molds.

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The cast object is always true to the mold, so our cast objects are coming out with more flashing at the seams (which are especially prone to wear) and more rough spots on the surfaces.

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So our pieces are requiring a lot of sponging and other care. The molds may decide to stop casting altogether before long. We’re replacing them as we have extra time, which is almost never.

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For the ceramics geeks out there, here’s a puzzle. Our first tiles came out of the kiln with a hard, shiny covering on the high spots. It looks alright, and we tolerated it at first, but we also asked around for advice. It turns out some of the soluble materials in the slip were building up as the water left the tile. I’m still not sure why it only happened at the high spots. Maybe because those are thicker parts of the tile with more water leaving those areas, but the glassiness hewed exactly to the peaks of the facets.

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Our solution came from Shari McWilliams, our beloved Pottery Technician. On her recommendation we started drying the tiles upside down and the problem disappeared – presumably because the bulk of the water traveled upward out of the bottom of the tiles, though we’ve never seen any glassiness on the bottoms. In any case, we made a soft area in the corner of the studio for our tiles to rest.

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And when it’s free we sometimes use it for other things, too

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Like our molds, we’re working hard. And showing our age.

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