How to make a plaster cast of a flat slab piece?
Posted 03 December 2012 - 11:12 PM
They all got through the bisque firing just fine. But 3 of the 4 cracked during the raku firing. Not along the incised lines I made, I’m pretty sure it was because our instructor set them upon blocks to be able to pick them up from the kiln during firing, and they must have absorbed the heat unevenly.
Anyway…I want to make them over again, and fire them conventionally. But I don’t want to put in all that carving labor.
Studio staff suggested I glue the pieces back together, then make a plaster mold, and push new very moist slabs into the resulting mold. I bought casting plaster already that states on the box it is good for fine detail.
But studio staff were sketchy about how to make the plaster mold exactly. No one had ever actually done it. Especially considering the pieces I am molding are flat. How would I get the originals out again? Apply a film of something, like Vaseline?
Can you offer me any instructions?
Posted 04 December 2012 - 12:15 AM
Posted 04 December 2012 - 01:15 AM
Maybe someone else here can be more helpful.
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Posted 04 December 2012 - 02:51 AM
Vo Studio Ceramics
Posted 04 December 2012 - 10:04 AM
I would recommend firing the tiles upright in the raku kiln. Maybe setting them on some unglazed 1/4" coils so they are not on the shelf.
They can easily be picked up by tongs. I have special heat resistant glaves for removing large slabs, but 4 x 6 is not that heavy to lift by tongs.
Professor Emerita Montana State University-Billings
Posted 04 December 2012 - 10:15 AM
Posted 04 December 2012 - 04:32 PM
i'd probably roll out a slab of clay and leave 1-2" around the form, then just make some clay coddle walls 1-2" taller than the tile. Slightly embed your positive in the clay slab to keep plaster from getting underneath and to keep it from floating up when you pour your plaster. Mold soap the form several times - I use PureLube or mold soap from the ceramics supplier. Then cast it. I always mix plaster using the "island" method, but you can weight your plaster and water if you want to get the best results.
worst case scenario is your positive will get stuck and you lose the original, which was already broken in the first place. either way, it will still be a good learning experience since you have no plaster skills yet. keep in mind that your positives have already shrunk once, and that any casts from these will be even smaller (if this matters).
if you don't want to risk casting the originals then you will have to make a new positive or salvage what you can from the originals. for the latter, you'll need a negative (mold) to pull a new positive from. You can press it into clay and make a bisque mold like suggested - but might be hard to keep flat and also likely to pick up least detail since it'll have to be removed before it starts to dry and shrink/crack. You can also get something like the air-dry clay to make the negative. Even better might be something like Magic Sculpt or Apoxie Sculpt, which are 2-part epoxy clays. -- once you have your rigid negative mold, you can then press fresh clay into it for new positives, or use it to produce a good clay version to pull a plaster mold from. Either way will work, just depends on how many you need.
Posted 05 December 2012 - 01:11 PM
Posted 05 December 2012 - 10:25 PM
Carving detail is not deep; maybe2 mm maximum, with no undercuts. I took pictures but I’m not sure if they showany useful level of detail.
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