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Could be that your plaster has been exposed to moisture, too much water in the mix, or there were contaminants in the mixing bowl or water.
 

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Oh... IMO I think the problem is in the mix... Can you add more power? But that's weird because the water in the mix should evaporate. If its really hot outside, did you tought about taking it outside?
 

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Plaster of paris is a chemical reaction not an evaporative process. True, plaster will give off water vapor as it cures, and will shrink ever so slighty.
But from the description, it could be that the proportions of water to plaster are incorrect.
Remember that water represents the volume (amount) of plaster that you need. Usually, the ratio works out to 1 part water to 3 parts plaster, give or take.
If I'm making a casting, (after prepping my mold) I fill my mold with water, then pour that water into my mixing container, plus about 1/4 cup extra. Then I start adding the dry plaster. Spinkle it into the water. DO NOT STIR JUST YET.
Keep adding plaster until all the water has been absorbed. (This takes more plaster than you think) Once all the water has been absorbed THEN you can begin stirring. Stir slowly to avoid air bubbles which will weaking the plaster. Once the plaster is throughly mixed, pour slowly into you mold.
If you star with cold water or even better, ice water, you can increase your working time.
It should take about 30-45 minutes for the plaster to set. It will give off heat during that initial setting process. To test readiness, scrape/scratch the back of the casting. If your scratch tool come out clean- like testing a cake- then you can de-mold, if not wait another fifteen minutes and test again. If you casting is really large, it may take over an hour.
Once your casting has set, remove it from the mold right away. This allows the chemical reaction to continue and give off water vapor. It can take about a week for some pieces to cure. As its curing, remember to rotate the piece so the surfaces are evenly exposed to the air. You'll know it's cured when you can tap on it and it "rings" (tap on a ceramic dish for comparison). If it "thumps", it's still wet.
Hope this helps
 

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Thanks for all the info tiny terrors! I've only used plaster once, but had no problems. The only thing i've ever been told is to use cold water. And dip your finger into the plaster, pull it out and if a drop forms on the end of your finger but doesn't fall off then you have a good consistency.
 
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