Casting rocks - problem

I'm using a couple of Woodland Scenics rock moldings & L/W Hydrocal. Following the mixing instructions to the letter including spraying the molding w/water & liquid soap, I'm having trouble getting the rocks out of the mold w/o breaking them. I've left them overnight but still no joy. The mold material is so thick that extra pressure is required to pop them out and that's when they come out in pcs. As a last resort I filled the molds just partially but still getting pcs. I guess the good news is that these pcs can be dressed and used.
Any/all tips appreciated -
Thks

 

I use plain old cheap, by the 5 pound box, Home Depot plaster, and other than just pour it in the mold and let it set for an hour then pop it out I do nothing else.

 

Thanks, just to be clear is that Plaster (dry powder to be mixed) or drywall joint compound?

 

Plain old plaster, not joint compound. Since it will set quite fast you cannot delay much in making a pour. If you know you are going to have some slight delay in say pouring several molds I just make a little thinner mix of dry plaster to water. Also it can be tinted some with water based colors. The color will dry lighter than when wet. Also another trick when using plaster castings is to wet them before applying to the scenery, say the cliff face. The dry casting will quickly absorb the water from a thincoat used to set them, sort of like laying bricks or mudset tiles, and then you will have just a thin film adherance.

 

You can use just about anything for casting. I like HYDROCAL, full strength not the light weight stuff. And I don't buy WOODLAND SCENIC brand either. You can get REAL Hydrocal at your local Plaster & Drywall supply store. DON"T try use Joint Compound, it will crack when it dries and it's not that strong either.
When you are casting, especially if it's a big casting, try putting a couple pieces of GAUZE in the casting. Gives strength to the casting, sort of like RE-BAR does in concrete. I've used gauze, clean burlap, clean torn up t-shirt pieces for added strength in castings. Just make sure your added material is completely covered by the plaster/hydrocal.

 

I use lightweight setting type joint coumpound available at any Home Depot, Lowes, etc . It comes in a 20 lb. bag and is just like plaster (you mix it with water) for less than 10 bucks a bag. It comes in three different drying times, 20 minutes, 45 minutes, and 90 minutes. I use the 20 minute stuff and I spray the mold with silicone spray and get it real wet. Mix it up until you can pour it in like cake batter and let it sit until it is hard and just pop it out. I think Hydrocal is a waste of money when this stuff works just as good.

 

Thanks to all for those great tips. Appreciate the economy side of it also. More and more I find myself returning from the hobby shop w/ a small plastic bag and I've spent $35!

My layout is small - 40"X72" and I hoist it up into the rafters on a pulley system when I'm not working on it - I try to minimize the weight added as much as possible.

 

Consider this as an alternative to casting: http://www.cripplebush.net/

It's a bit pricey, but for smaller layouts it's not that bad. It's really lightweight and easy to work with--I have a number of clinics on using it:

http://ganddinz.blogspot.com/2011/03/lets-rock.html
http://ganddinz.blogspot.com/2011/03/rock-on.html
http://ganddinz.blogspot.com/2011/03/daphetid-tunnels-part-deux.html
http://ganddinz.blogspot.com/2011/03/rock-around-clock.html
http://ganddinz.blogspot.com/2011/04/last-of-rock.html

 

Instead of plaster/hydrocal I use Celuclay. It is also sold as Scultamold. The paper based product is lighter more flexible and easier to work with than plaster. Instead of packing the mold full I just put a quarter inch layer making a hollow casting. I found it dries faster than packing the mold full. The hollow back conforms better to an irregular surface than a rigid hunk of plaster would.

Sculptamold comes only in white but the Celuclay comes in gray or white. The gray looks great without any painting. The Celuclay does not chip or crack as plaster would if struck or subjected to a shock. I've used water based paints to add color. It works like concrete when using it. The more you work the surface the smoother the surface finish can be.

The Celuclay will not produce a drippy mess that will ruin anything when it spills.

 

Is this more of an "artsie/craftsie" type of item or would you find it at a big box (Home Depot) ?

 

The Sculptamold is found in the Hobby Shop.
Celuclay is found in the Arts and Crafts Store.
LOOK CAREFULLY Michael's puts the larger package quite away from the smaller ones. There is a bit of savings with the bigger package.

Here's a pic from the layout.
At the lower edge is a road surface, behind the loco is some vertical surface.

 

Good old (actually new) plaster is what you need.

There is moisture absorbtion if you have an opened bag of plaster laying about for a long time.

I use plaster with the same molds and have never had a problem.

If it is getting too heavy to hoist, you may want to look into a small block and tackle system as used on old sailing ships.

 

Celluclay and Sculptamold are two different products, incidentally. Celluclay is made by Activa Products, Sculptamold by Amaco. They are similar but not quite the same. I've found that Scultpamold is stronger. Both can be found in most hobby shops and art supply stores.