Plaster of Paris:

[DONNZ]

After two help me moves, two garage cleanings, and a computer crash, I'm back, but with a weak connection. Working on this.
Now have time to catch up on the things I like to do.

TOOLS: (1)
Putty knife, misc. knifes, round file, piece of scrubbing pad (that red thing)(also handy for cleaning slip from mold), fine grain foam backed sanding pad, adjustable spray water bottle.
6" square Lazy Susan , Lazy Susan with a steal plate, fire brick and crucible. Just my thought on rotating while torching. Sounds doable.
Mill my wood, makes it easier to pick up with a steal plate or other heavy object sitting on top of wood. Top two need an bit more milling. Just scraps pieces laying around the garage or someone else's garage. And a NO particle boards allowed of any type in my wood stash.

FORMS: (2)
Top form, two plastic pieces placed on a piece of glass. 60 pounds of air from the compressor moved the center out of the plaster, otherwise it would still be there.

Bottom pic, a small butter and yogurt container made this plaster form. Had to cut the butter cup off the plaster. This was done before I remembered I have a air compressor.

FIRST POUR: (3)
Left pic, not the original cups but for show and tell.
Right pic, first pour. Using slip to check shape.
Dry plaster can be shaped, sanded, dabbed on with a brush (thinned down) and layered with more plaster. It sticks to itself and does so very quickly.

THE START: (4)
The finished mold and the first pour.

THE FINISH: (5)
The finished mold, the first pour before the mold was finished. The progression is bottom left to right then back to mold. All done by hand with the tools listed.
Don't need but a couple molds. If I damage one I now know how to fix them.

 

[Palladium]

Welcome back Donnz.
Another informative post! Thank you.

 

[MysticColby]

I've tried this before! very similar to you but much less straight-forward I kinda wandered from one step to the other.
thickness of the mold can be very important depending how you'll use it. thicker = can cast more crucibles before needing to be dried again
my one question: what did you make the slip out of? I tried 2 different types of clays and both cracked to 20 pieces when pre-heating

 

[DONNZ]

MysticColby,

Clay slip used and will be recycled. I used this just to tweak and refine the shape. Clay will not hold up under a torch. Updated will be posted.

 

[Geraldo]

I seem to be misunderstanding here. Are you suggesting you are casting gold or silver in plaster of paris molds, and that the molds would be re-usable? I would have thought the plaster of paris would degrade pretty quickly at those temperatures. I know that people make small, intricate plaster of paris molds for casting gold jewelry and such, but I thought those were essentially disposable molds?

 

[DONNZ]

Also make this two part mold.
A mold is just the shape of the crucible or dish to come.
I used only the bottom part of this mold to pour the dish using clay slip.

The final formula used could look like this.

Grog (20 to fines): Grog is a sand brown color gritty stuff, crushed calcined clay.

Tennessee ball clay, old mine #4: a medium body clay, grayish. Ball Clay Al2O3:2SiO2:2H2O*

Georgia kaolin: a very white fine clay.

Edgars plastic kaolin (EPK): adds to thermal shock resistance

Potash feldspar: acts as a flux Feldspar K2O or Na2 Al2O3:6SiO2*

Spodumene: reduces thermal expansion Li2O:Al2O3:4SiO2*

Water

This is only one of many formula.

 

[Geraldo]

Ah gotcha. Sorry, I didn't get at first that the plaster of paris molds were there to cast ceramic crucibles into. Thanks for the clarification!

Best Regards, Geraldo