Plaster of Paris or Hydrocal

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Newbie Switchman
I have some mountain sides that I am putting on an incline next to a track.
These sides will be about 6" to maybe 8" tall.
I have several rock molds I made many years ago.

I just bought 25 pounds of Hydrocal.
It seems to work well just takes a while to setup.

When making rock molds and such what do you fellas use?



Hydrocal is very strong, which is why I prefer it. It also takes detail really well. I also used some guaze when casting rock molds to act as a sort of reinforcing. "Back in the day" we used steel window screens, but they eventually rust. I suppose you could do well with the vinyl or whatever it is they make screens out of these days, though I haven't actually tried it.

However, you'll find plenty of folks who like either one, and they all have reasons why their product is better.

That said, the only problem I've ever had with Hydrocal is that it's expensive.
I like hydrocal for casting and details. If you want to carve details in the casting like bricks then hydrocal is the way to go. It will stay green for a long time, days. During this time you can do some carving and details. Very good for that. I purchased a large bag for list $25.00 and it was about that much to ship so over all it was about $1 a pound.
A good tip is add some color to the mix so if or when it chips you don't have what looks like a snow pile on your layout.

I have also used dry sheets for backing to your castings. They are used first. And they make your layout smell nice and fresh. LOL They do seem to work well for adding strength.



A good tip is add some color to the mix so if or when it chips you don't have what looks like a snow pile on your layout.

Excellent tip! It chips and you can never match the color... Pick a neutral color, close to what the rock or whatever you're casting will look like, so it blends in well. Then you can touch it up a bit if needed, maybe a dash of India Ink for weathering for example, but all in all, it will be a lot less noticeable.


Thanks modelbob.
I use gauze for the mold it's self but not on the molded rock.
I use it in the plaster for the same reason. I did reinforced concrete QC and inspection for my first job out of college, and when you get right down to it, plaster is a lot like concrete. They both do well in compression, but suck at flexing, bending or tension. So having something in the plaster that will do well in tension helps. As I mentioned, years ago, it was metal window screen. I'm pretty sure the plastic stuff would work just fine too. Or the gauze, same idea. You're going to see some cracking, your layout will move over the seasons as the wood expands and contracts and gets wetter or drier. So having something that helps limit the spread of cracks is useful. The nice thing about Hydrocal is that if you dampen it, and then make up a thin slurry, you can seal the cracks up nicely. Let it set a while, then carve to match as needed.

Then again, if you're modeling rocks, for many rock formations a crack would be prototypical, so depending on how big it is and where, you may be able to simply add a touch of weathering and ignore it, if you're lucky enough to have it flow in a natural looking way.
I pretty much have used Plaster-of-Paris for my scenery. Although I don't use rock moulds, I have used P-o-P over plaster gauze and screen wire. If you can find a hobby store or artist supply store that carries powdered tempra paints, mix a little in the dry plaster or Hydracal, before adding the water. You may want to experiment with the proportions of tempra to plaster. A little tempra goes a long way. I use Raw Sienna and Burnt Sienna and Raw Umber and Burnt Umber and plain old Brown. Do NOT use RIT dye! The plaster in which I tried that many years ago, did not harden, no matter how long I let it dry!
Do NOT use RIT dye! The plaster in which I tried that many years ago, did not harden, no matter how long I let it dry!

Now that's funny! I swear the old articles in Model Railroader always suggested RIT Dye? Wonder if the proportions were off, of it doesn't work with Plaster of Paris but does with Hydrocal or what? Odd...


Whiskey Merchant
My choice is also hydrocal. Extremely strong and like was mentioned is great for castings. The problem is locating it. Many folks have tried places like Home Depot and Lowes but I went to a building supply company and picked up a couple of fifty pound bags. For scenery, I usually used fiberglass screen material, supported, and used hydrocal soaked paper towels over the screen to get basic contours established. Then I'll go back with additional coatings until the terrain is the way Iwant it. The extra layers make it extremely strong. I have leaned on it while working on scenery and not had any problems with it cracking or breaking.

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I use Hydrocal. Mostly because I started using it in 1966 for hardshell scenery per Bill McClanahan's Scenery For Model Railroaders and I'm comfortable with it. Classic case of the "inertia effect".
I dunno. Although it has been many, many years since I tried using RIT, I do know what happened. The only problem I've ever had with powdered Tempra paint is...finding a good, convenient source of supply for it. Not every place carries it nowadays!

When I was a kid, not only could you mix the Tempra powder with water to make watercolor paint, but with linseed oil to make artists' oil paints. Mixed with Plaster-of-Paris, it makes great scenery! I can't really describe the proportions, except that a little goes a long way, depending on the shade of color you want. Once the plaster has set up, I use acrylic paint in various earth tones, applied fairly heavily, with whatever "grass" ground foam or other medium sprinkled on the wet paint. Wherever the paint is a little thin, or where I need to break through, say to modify the scenery (too close to the track or where I need a unplanned tunnel opening, etc., the colored plaster doesn't show white.


Newbie Switchman
Well I tried hydrocal.
It works well and fairly easy to work with.
It is near impossible to do anything with it after it dries.

Got some Plaster of Paris to try.

BTW, I found a place to buy plaster and plaster cloth.

Free shipping.

Looks to me to be fairly good prices.

Well I tried hydrocal.
It works well and fairly easy to work with.
It is near impossible to do anything with it after it dries.
That sums it up really well. It dries hard. Really hard. That's great, unless you want to change something. You can't do much carving with say a dental pick. We're talking Dremel tool, saw, small hammer or chisel. Once it's hard, it's pretty much done.


Section Hand
Either product works great as long as its fresh or properly stored. And, don't add too much water when mixing either one.

Don't forget about using Sculptamold from American Clay. Works well and dries strong.

Tempra paints?
Yup! Tempra powder, mixed dry with plaster of Paris, (you have to experiment with how much to add...a little goes a long way), colors the plaster so you don't have stark white dings or modifications showing. The only thing to watch for is getting the same amount of tempra in each batch of plaster. Probably works with Hyrocal, but I've not tried it.
Plaster of Paris, USG Hydrocal, and gypsum cement are all beta gypsums, with similar characteristics. Alpha gypsums, on the other hand, are much harder and stronger than beta gypsums, and are often referred to as "stones." These are the types of plasters used in dental applications, called "dental-stones," or "die-stones." They're rated in strength from Type III to Type V, with Type V being strongest.

For modeling applications, Type IV is best (Type V is slightly stronger, but costs three times as much). A 25-pound box of Type IV dental-stone can be had for only about $20. Find this locally from a dental laboratory supplier in your area. Their quick setting times and superior strength make them excellent plasters for pouring finely detailed rock-molds. These are the same types of dental-stones sold under the brand, "Merlin's Magic," which offers a variety of dental-stone plasters in model-friendly colors; however, the shipping costs make the product costly. It's much better to buy this locally. If you can't find a dental lab supply nearby, your dentist can order it for you.

Here's the relative strengths of the various plasters, measured in PSI of compressive strength, according to Hirst Arts:

Plaster of Paris: 2,000-5,000 PSI
Hydrocal: 6,000-8,000 PSI
Type IV dental-stones: 12,000-14,000 PSI
Type V die-stones: 18,000 PSI

Plaster of Paris, Hydrocal, Sculptamold, and even Celluclay are all easily colored using Quikrete-brand liquid concrete colorants. They're available from Home Depot in a variety of earthy colors. They're so concentrated, you only need very little colorant to color your rocks. Tempera or latex paints typically require much more material to achieve the same depth of color, and can significantly compromise your plaster's setting strength.
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