what Builders gypsum plaster is good for jacket mold?


New Member
Hello there , Sorry if this is the wrong place to question bout this but i'm clueless on this

Basically i need to know if i can use Builder type of Gypsum Plaster to apply on a silicone mold so it'll keeps a rigid shape that can allow some light handling

I know there's brands like Hercule / Hyrdocal but i'm trying to refrain from ordering online as i need this quick and local * UK South west based *
I've used plaster of paris /scrim before but it's just takes so much to make with results where it'll still crack with ease,
Got spare jesmonite but that's for my casts.
Would use resin/body filler but i live in a place with no open area to extract fumes .

Any help would be great


Well-Known Member
I don't know about builder's gypsum (I imagine it will be soft) but have you any dental suppliers near by? They usually carry much harder plasters. Also fibre glass suppliers usually carry hard plasters like Herculite. The other option would be to go with a fibre glass jacket which is much stronger and lighter than plaster... I made mine in the back garden.


Well-Known Member
I believe I heard some makeup effects folks in the UK use Crystacal as a replacement for Hydrocal?
The important thing to look for, besides how durable the material is when set, is the expansion rate, which you should be able to find on the tech data sheet. Compare the material you're looking at to the established plasters/stones, and see how it stacks up.


Sr Member
I would say yes, but am unsure of the strength of UK construction plaster to advise on the amount of reinforcement needed. I mean: You can use(the weakest) plaster of paris, and build a jacket, but something as strong and hard as Hydrostone is better.

Here is the U.S. Gypsum data sheet on Hydrocal: https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/hydrocal-statuary-gypsum-cement-data-en-IG1380.pdf

if you use the compression strength of 3000psi as a guide, there should be something similar in the UK.

here is the website for datasheets: https://www.usg.com/content/usgcom/en/products-solutions/products/industrial-specialty/art-&-statuary/hydrocal-statuary.html


Well-Known Member
I think yes,...i use it all the time.
For larger mold reinforcement i would advise to strenghten the plaster shell with pieces of fabric i don't know the exact name in english but i think it is called Jute fabric or burlap,...the fabric they make sacks out of ,....hemp or rough linnen.
If you follow the plaster mixing instructions exactly ,..the shrinkage will be not that extreme to deform or destroy your sillicone mold,especially when you let it dry with the molded object inside the mold.
It will not crack either.
I wouldn't use the construction plaster as a fine durable casting material though,..it is way to coarse i find,..but i have seen people use it for rough casting jobs with good results.
I also sometimes use insulation expanding foam as a shell for the silicone mold,....nice rigid and lightweight.
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