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How to make a plaster cast of a flat slab piece?

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#1 Linnea56



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Posted 03 December 2012 - 11:12 PM

I’ve been taking pottery classes for a year. I made some flat wall pieces for a special raku firing. They are flat slab built, about 4 ½ by 6 inches. I carved them extensively, in Frank Lloyd Wright style stained glass designs. I put about 4 hours work into each one.

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?

Thanks, Lin

#2 Kabe


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Posted 04 December 2012 - 12:15 AM

I don't remember making a cast from I fired tile, so I'm not to much help in that area. I know that there are some great discussions about plaster casting in the posts. You can buy mold release for plaster casts, I use Murphy Oil Soap and it works for what I do. I tryed vasaline once and it made the cast soft, also the plaster has to be able to absorb moisture and I think the Vasaline would hamper that. Maybe my oil soap does too never though about that part before. Potters Plaster is at least 5 headachs better than hobby grade molding plaster, It is not hard enough does not hold up. I don't know what you bought. If your carvings have under cuts you will not be able to get them loose from the cast no matter how much release you use. I don't know how deep your cuts are. That's the few things that I know. Try looking back through the old posts, there are some very talented and skilled artists out there. The next one reading this may have just the thing. Happy Firing

#3 Lucille Oka

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Posted 04 December 2012 - 01:15 AM

Do not use vaseline. Mold soap is better. It is difficult to tell you what you can do without seeing the original; because the piece has been carved and without plaster casting in mind. The best way I see this working is to make a bisque mold out of moist clay from the original. The original will be easier to remove from the clay, but you may still need to touch up the mold when the original is released. Give a photo if you can.

Maybe someone else here can be more helpful.

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#4 voceramics


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Posted 04 December 2012 - 02:51 AM

I found a book in my library that is a pretty great, it's called "The Essential Guide to Mold Making & Slip Casting" by Andrew Martin. It's better to read the whole book though because the author sprinkles fairly important hints throughout the text.
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#5 Marcia Selsor

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Posted 04 December 2012 - 10:04 AM

Casting a fired tile is a little tricky. Maybe put a little coil of wet clay around the edge so the plaster doesn't tend to grab it. Use a good release. mold release or soap as Lucille suggests.
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.


Marcia Selsor, Professor Emerita,Montana State University-Billings

#6 Joy pots

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Posted 04 December 2012 - 10:15 AM

I found some Magic Clay that is air dry clay that is sold for children's projects. It dries to a rubbery firmness and picks up detail easily and doesn't stick to fired clay. You could then make your plaster mold from it or just use it as your mold. I got it in a Dolleramma Store.


#7 perkolator


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Posted 04 December 2012 - 04:32 PM

if your originals are good to go in terms of no-undercuts and enough draft to release from the plaster, I don't see any issues with epoxy-ing it back together and just mold soaping the piece several times (or even lacquering) to clog the pores before casting it in pottery plaster #1.

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.

good luck!

#8 trina


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Posted 05 December 2012 - 01:11 PM

Hi there, I would make a plaster mold of the original then make a mold from that mold so you have the reverse and that second mold would act as a stamp to use on your flat slab. That way you dont have to fight getting the clay to release from a mold, your work stays flat and Bobs your uncle. T

#9 Linnea56



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Posted 05 December 2012 - 10:25 PM

Wow! So much useful information!Thanks to all of you. Being new to this, it will take me some time to absorband think of the right approach.

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.

#10 weeble


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Posted 06 December 2012 - 02:16 AM

I found a good pair of needle nose pliers worked pretty well for getting small tiles out of the kiln and into the reduction can, just be DARN careful...
Maryjane Carlson

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