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Casting with plaster

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On 11/09/2017 at 16:49, cocky05d said:

Hi ,got some 1/35 scale bricks moulds ,can anyone give me a rough idea about water /plaster ratio .

Thanks .


I hope this isn't too late for you

Different plasters have different mixing ratios.  Even then, standard weight ratios depend on how the plaster was stored, and how much water has been absorbed by the powder.

I make plaster moulds, mixing the plaster by eye.



Wearing a plastic glove is a good idea, I generally use one nitrile glove on my working hand (right in my case).  Not essential, but ...
Avoid breathing plaster dust!


Flexible bowles are easier to clean up afterward, the dried plaster will crack off, and any residue can be wiped.   Failing that, use a throw-away pot (e.g. old yoghurt pot), but watch out for unreachable corners.


Put about as much water in your bowl as you want to fill the mould - maybe a little less.

Have a plan incase you mix too much plaster - use it to form tree trunks or to support piles of rubble.


Never mix plaster in a dirty bowl, it just makes life harder.


Sprinkle the plaster in, a small handful at at time.  Keep adding until the plaster forms a dome above.

I'm told you should see it absorbing water and collapsing down under water - I never see that myself.  I generally add a bit more plaster at this stage


Let the plaster sit for a short while (30seconds, maybe a minute).

Then, with your gloved hand, start mixing it, feel for any lumps, and rub them smooth between your finger and thumb.


Again, let it rest again (not too long), before putting it into your moulds.

The more you mix plaster, the quicker it will set.  Heat (e.g. warm water) helps speed up setting too.
If the plaster is getting stiff, it can still be moulded, but you may trap air bubbles on the surface.


Pour your wet plaster into your moulds.  After a short while you may see water forming on top of the mould. If that worries you, you can pat it off with a paper towel. 


Leave it to set some more.

Gently test the back of your cast, the plaster should start getting harder after about 10-15 minutes.  If it takes 1/2 hour, then there was probably too much water/not enough plaster in your mix.


When the cast is hard, demould.  If it feels slimy, use less water next time.  If it feels crumbly, use less plaster.

The plaster will get harder overnight.  If you want to carve it, the carving will be easier when the cast is fresh.


Setting plaster gives off heat.  In small volumes that is an indication the plaster is starting to set.

In large volumes (e.g. body casting) that can cause burns.





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Hi ,thanks for the advice ,i did try casting bricks in a mould i bought from Diorama Debris ,the results were ok .

Some of the bricks were a bit rough around the edges .

Don't like mixing by hand ,i use coffee stirrers from mcdonalds and use plastic cups .

The casting plaster ,i store in air tight former ice cream containers in my shed ,don't know if i should protect it from frost .

The face in your photos ,what scale is it ?.

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