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Hi, I'm trying to make a stop-motion puppet out of silicone. I've sculpted my character from Chavant sulphur-free clay, can anyone suggest which plaster to make a mould from please? In my research I've seen a lot of ultracal and hydrocal names mentioned but they are not available in the UK, any suggestions please? 

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Hahaha, thanks for the tip Simon :)

Simon Tytherleigh said:

And one more just for luck: Don't try adding water to dry plaster - all you'll end up with is a lot of lumps!

I've been looking for a UK equivalent to Ultracal 30 for a while and this Crystacal R looks good from what I've read. I attempted a mould with regular Plaster of Paris and I just found the stuff too brittle, possibly because I didn't reinforce it. Would burlap work ok as a reinforcement?

There is definitely more than one good hard plaster, not just Ultracal, and Crystacal seems to be the one to get in the UK.  

Yes, hessian (burlap) works.  It needs to be the more open weave, so the plaster can get into the holes, there is a more tightly woven hessian used for sandbags (to keep the sand in) that is not so good.  Personally I prefer the chopped strand fibreglass matting, but hessian and sisal fibre (sometimes called hemp, which used to be used once upon a time) are also good.  And probably cheaper.  Don't put reinforcing in the first 2 coats of plaster, let them thicken up before adding another coat with the hessian or fibreglass, so you don't get hairy bits coming through the surface of the mould.

Hi Angus. Yes, Crystacal R is a good one. What you are looking for is often referred to as dental stone. It is much harder than regular Plaster of Paris, with a much lower coefficient of expansion, meaning that it makes more accurate moulds especially for heating etc.

Try looking through the catalogue for these people http://www.industrialplasters.com/ , who supply lots of different plasters.

When using hessian, remember to dampen it a little before putting it into the plaster. You don't need to do this with glassfibres!

And always put the water into the mixing bowl first - about a third of the volume of what you will need. Then add the plaster, let it soak a moment before mixing. Bang the bowl a few times on the work surface to get the air bubbles rising to the top. Learn about the speed your plaster goes off, so you can get the reinforcement layer in at just the right time. Do not demould until the plaster has gone hot then cold again - and it gets quite hot!

If I have used a hard plasticine, I like to de-mould when the plaster has mostly cooled but is still slightly warm.  That way the plasticine is still a bit soft and will give a little.  Even with Chavant Medium, if it is cold it is hard enough to make the mould really difficult to pull apart if there are any undercuts at all.  And there usually are, with a puppet mould.  I have extra reinforcing around the prying points so the plaster can take being pried open before it is fully cured.   

Angus, don't use Plaster of Paris if you can avoid it.  Reinforcement will help, but it will not get as strong as the stones others have recommended.

Arrived today! Weighs a tonne, but I look forward to using it. Thank goodness for this thread!

Technically it weighs 0.025 of a tonne, according to the bag.  

It is finer ground, and more dense than a similar volume of basic casting plaster or plaster of paris.  Good luck with it!

Yeah I look forward to using it .

StopmoNick said:

Technically it weighs 0.025 of a tonne, according to the bag.  

It is finer ground, and more dense than a similar volume of basic casting plaster or plaster of paris.  Good luck with it!

That's the stuff! 

Remember the finer ground nature means that it is easy to get the mix too thin, by starting with too much water. Guesstimate about 1/3rd water for the final volume of plaster and you won't go far wrong.

Remember also to put the bag into a plastic bin liner and keep it dry. No wet hands/utensils in the bag!! Good luck.

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