Monthly Archives: December 2012

Front boxed

Here I’ve boxed in the front half, preparatory to pouring the plaster mother mold half. It includes a pour sprue and vents at all the high spots.

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It’s made from foam core, hot glue and packing tape. I think this is a way to work faster, and have a smaller plaster mold to deal with. I have usually made these boxes from wood, but this is easier to fit to the shape of the model. I hope it will be strong enough to support the weight of the liquid plaster. The inner surface of the foam core is lined with packing tape, and the whole clay surface will be sprayed with mold release before the plaster goes on.

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Vents must release the air from the mold at every high spot, or the poured liquid silicone will not be able to fill the mold entirely. I may still add a second sprue; While the silicone is forgivingly self-leveling, this mold has so much topography that it may be helpful. I can also thin the silicone to help it flow more smoothly.

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Front blanket

I’ve built out the parting line a little, to get a better sense of if this will work, especially between the body and the arms. This is all tentative. The real establishment of the parting line will take place later.

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I wrap the model in plastic wrap to protect it from sticking to the clay blanket too much. This will come off later before I pour the silicone rubber inner mold.

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And it goes back in the cradle.

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And I begin to cover it with a thin blanket of clay, about 3/8th inch thick. It is upon this blanket that the plaster mother mold will be poured. It is essential that there be no undercuts or catches which will bind the plaster to the clay surface, which itself will eventually be replaced by slicone rubber. As complex as my model is, I think this is evident to work OK.

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Along the way I create the pour spouts. Vents will get included later, when I’m doing the parting line. Still some ways from that now.

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Cradle

To work on a mold comfortably and productively, it is helpful to situate it in a position where the parting line, and therefore the halves of the mold, are laying horizontal. This way is easier to get your hands on it, and to see by light from above, what you want to work on. Also, when you pour plaster or silicone rubber, it is easier to make it flow and have a system of vents in the horizontal, and to cover the whole piece evenly. Therefore, it is my practice to build a cradle to hold the piece in the position I favor, which keeps the parting line generally facing upwards, and which is usually the case with figurative sculpture, having a parting line most often from the back and front. The construction of such a cradle can be complex and time consuming itself, but I feel it pays off well in results of the finished mold, and in the ease and comfort of working on it. You can see examples of figurative sculpture molds made in the vertical orientation, but the difficulty and discomfort of it out weighs the effort to make the cradle in my opinion. A craftsman does his best work in a comfortable, well lit and equipped situation. Since I recently received a gift of excellent fluorescent work lighting installed in the studio by my loving wife "A., I am appreciating this fact very much.

I begin wanting a surface which will support the weight of the model evenly and steady. I cut and trim and fit a sheet of paper which is creased and folded to fit the complex shape of the piece.

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I flatten and apply this template to a sheet of aluminum, the kind used for roof flashing. It is firm, but folds easily. I recreate the paper shape and fit it to the model.

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After some adjustments, I hot-glue some foam-core supports to stiffen and secure the shape I want. Foam-core is strong, easy to cut to any shape, and glue. I also get large sheets of it for free out of the dumpster behind the local university school of art at the end of the Spring term.

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Is it right? Fill it up with foam until it is as strong as you need it. Sides, edges, everywhere. Think: it will eventually need to support the full weight of a plaster mother-mold half which will be poured on top of it. And then add a little thin soft foam, automotive headliner is good for this, to cushion the model in a bed. You don’t want the model to deform under it’s own weight, during all the more work that’s to follow.

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That’s how you go through all the trouble to make a cradle that fits, and evenly supports the model in a position you can work on it easily and accurately. Trust me, it will be worth it.

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