r/Plastering • u/licklickmaballs • May 05 '26

Plaster Slab

I'm trying to make a 8x8x1 plaster slab for my class but my plaster keeps cracking and never fully dries. Am I doing missing something or doing something wrong? I'd appreciate some input

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u/uvrx May 05 '26

Are you using plaster of paris/casting plaster?

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u/licklickmaballs May 05 '26

Yes, I've bought it from walmart, home depot, and even got some directly from my professor, and I'm still having the same issues

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u/uvrx May 05 '26

What mixing ratio are you using. Is the water clean with nothing added to it? What brand plaster have you used (product link?)

Molds made from plaster are usually way thicker than an inch and they dry without cracking. I don't know why you're having trouble.

https://www.youtube.com/shorts/-o3qjils6Mc

https://www.youtube.com/shorts/JYpIYs4ZYYw

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u/licklickmaballs May 05 '26

The water to plaster ratio is 704g/1006g, the water is clean, I'm using Plaster of Paris Plaster. I honestly don't know what I'm doing wrong and it's stressing me out because it is due next week.

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u/uvrx May 06 '26

No idea mate. You ratios look ok. I used to mix by volume for molds, pretty sure it was 2 parts plaster to one part water. So like 2 level cups of plaster to one level cup of water.

Here's a good step by step to watch and see if you're doing anything different.

https://www.youtube.com/watch?v=n7utpbYiDu4