Making Molds

by thecastingfloor

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We’ve started casting and firing this week, with very promising results, but mostly we’ve been up to our elbows in plaster, making molds. Plaster molds for slip-casting are a means to an end. They are not the final product – just another tool, really. But we love tools. And we love making them. And we’ve gotten a little obsessed.

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First, we make sure our models – our plaster positives that we’re hoping to reproduce in clay – are pristine. Any nicks or air bubbles are filled with oil clay.

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Coddle boards are set up around the models.

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A clay wall defines the boundary of the first pour and keys (plastic buttons in concave/convex pairs that will later help register the mold pieces to each other) are set.

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Gently now.

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Sometimes the cleanup between pouring each piece can seem endless. But care at this stage pays off with mold pieces that fit together perfectly and produce a clay object that is as close to the original model as possible.

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Pieces that butt together are keyed using this round sureform.

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For pieces that sit atop one another you can use the plastic keys seen earlier, or simply make round indentations in the plaster. Some people use a spoon or a coin. We use a custom Ian Henderson key cutter (i.e. washer welded onto steel rod) in a cordless drill.

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The cleaned up, keyed mold piece is reunited with the model.

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And everything that will come into contact with the wet plaster of the next pour is covered with mold soap.

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Gently… Gently!!!

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The coddles are removed as the plaster starts to set but before it is fully cured.

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And the plaster is scraped away to the top edge of the model. In slip-casting the seams where mold pieces meet each other will always show up in the final product. By placing the mold seams on the corners and edges of the piece, though, they become essentially invisible.

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You can see why some cleanup is required between pours. The newly poured piece is sanded and keyed.

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And the whole thing is reassembled again, coddled again, soaped again. The next steps are simpler, though. The bottom piece of the mold is poured and left to cure.

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We had two going at once.

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Everything is reassembled and soaped in preparation for the last pour. As you can see, this part happened during respirator hour in the factory.

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The second half of the plastic key pairs are set in place, as are the red plastic ferrules where the slip will enter the mold. Tapered pieces of rubber are stuck on top of the ferrules. They will later be removed, creating holes where the funnels will fit.

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Ready to pour.

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All four mold pieces, plus the model.

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Lastly, the mold soap needs to be dissolved with vinegar and sponged off. It’s hard to see or feel with your fingers if you’ve removed all the soap – but the tongue never lies.

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The finished molds wait to go into the dryer with a bunch of Kohler ware. Daniel made these two and they came out looking great. A number of people in the factory commented on how nice they are. Which is all fine and good, but the proof, you know, it’s in the pudding…

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Success.

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Times two!

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Plenty of people do it differently – and better – than us, but that’s our process. So far it’s all working out astonishingly well.

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