Rock Moulds - Strengthen the Product?

[Sir TophamHatt]

Is there something I can do to strengthen the plaster of paris product when it comes out the mould?

 

I left mine in for an hour and they came out okay. A few hours later they still seem fairly fragile. Obviously leaving them overnight to fully cure but I wonder if there's some sort of glaze that I can put on them that will strengthen them further?

 

Or perhaps better quality plaster?

[Paul H Vigor]

1 hour ago, Sir TophamHatt said:

Is there something I can do to strengthen the plaster of paris product when it comes out the mould?

 

I left mine in for an hour and they came out okay. A few hours later they still seem fairly fragile. Obviously leaving them overnight to fully cure but I wonder if there's some sort of glaze that I can put on them that will strengthen them further?

 

Or perhaps better quality plaster?

Did you add enough plaster of paris powder to your initial mix? 

[Sir TophamHatt]

1 hour ago, Paul H Vigor said:

Did you add enough plaster of paris powder to your initial mix? 

 

I think so? 

Mixed it as per the bag instructions (1kg of powder, 750ml water), although elsewhere I read it should be 50/50 by weight.

 

Checked again now (so about 6 hours since the cast) and they're getting harder / dryer. I think they may just need an overnight cure.

 

Although I wonder what would happen if I mixed a bit of normal plaster mix to it, or a touch PVA glue. 

 

May get me experimenting!

[MikeTrice]

Can also try adding some PVA.

[PaulRhB]

I always add a good dollop of pva when using plaster for rocks in moulds or stippling plaster as weathered rock. 
It does lengthen the drying time and I’d always reckon on at least 24 hrs to harden. The rocks and snow here are plaster. 

Edited July 8, 2020 by PaulRhB

[Paul H Vigor]

3 hours ago, Sir TophamHatt said:

 

I think so? 

Mixed it as per the bag instructions (1kg of powder, 750ml water), although elsewhere I read it should be 50/50 by weight.

 

Checked again now (so about 6 hours since the cast) and they're getting harder / dryer. I think they may just need an overnight cure.

 

Although I wonder what would happen if I mixed a bit of normal plaster mix to it, or a touch PVA glue. 

 

May get me experimenting!

Takes me back to childhood when we used to make plaster castings of Huckleberry Hound, et alia. If you got the mix wrong, or tried to take him out of the mould too soon - his nose broke off!

[PatB]

You could try using paper plaster instead, for a bit of fibre reinforcement. It's commonly used in ceramics for making big moulds where maximising strength and minimising weight are important.

 

Just stir some toilet paper into the water, until it's well broken down, before adding the plaster powder, and then mix as normal. The surface finish won't be as smooth (probably not too important for rocks) but the finished product is much stronger and, as a bonus, lighter. 

[jukebox]

10 hours ago, Sir TophamHatt said:

Is there something I can do to strengthen the plaster of paris product when it comes out the mould?

 

I left mine in for an hour and they came out okay. A few hours later they still seem fairly fragile. Obviously leaving them overnight to fully cure but I wonder if there's some sort of glaze that I can put on them that will strengthen them further?

 

Or perhaps better quality plaster?

 

 

IMHO, the quality of the plaster is a big factor.    If you can find a supply of Hydrocal, or failing that, Densite, you should be able to produce crisp, strong castings.

 

I can recommend this video as a helpful tutorial:

 

 

 

Good luck.

 

Cheers,

 

Scott

[simonmcp]

I just used newspaper and PVA to 'line' the mould, three layers of newspaper was enough, and it produces very strong but extremely lightweight and slightly flexible rocks. I will be painting on a layer of plaster into the mould first, on the next batch as the PVA/newspaper tended to stick to the mould and I think might damage it if too many copies are made. Also nothing simulates rock better than plaster IMHO. I have no idea how long they took to dry though as they were made for and at a friend's house and we only went into the room once a week by which time they were fully cured. Simon

[shipbadger]

I use Woodland Scenics Lightweight Hydrocal when I need rocks.  Once dry I then saw the mouldings into smaller sections to either use as smaller pieces or to re-arrange them so that they don't all look as if they are from the same mould :-)  I've never had any break up whilst being sawn or used in the way.  I even have a box of bits for the 'come in useful one day' stash.

Tony Comber

[PaulRhB]

My preference to plaster in rock moulds is das clay, it can be used 2-3mm thick and as it already has fibres in it it doesn’t crack like the cheaper clay does. Very light once dry and if you take it out of the mould after 4-6hrs it’s still supple enough to form gently without getting fingerprints in the surface. The drying time depends on the ambient temperature, after a couple of hours I push a finger into the back to see how it’s going and keep checking it every hour or so until the top surface doesn’t take the fingerprint. 
You can get a lot of rock out of 2-3 packs  

Edited July 10, 2020 by PaulRhB

[PaulRhB]

It’s so durable I broke up the rock face on the layout above and reused bits on my next one!