Casting Rocks

Model Railroader Layouts and layout building
1

Hi everyone.

I have a bit of a problem. I am trying to cast rocks using crumpled aluminum foil and plaster of paris. Everytime I try to glue it to the hillside, the plaster cracks and falls apart. Is there an easier way to do this or should I try something else?

Thanks.

2

My opinion? Try something else.

A few months ago I was in your same situation and I finally just bought preformed rock castings. They are made from a foam material that you can cut and shape. They aren’t even that expensive (about $8) and a HUGE time and mess saver.

I can’t remember the brand name, but I’ll check in my workroom and get back to you with the info…

3

Instead of letting the plaster dry, place it on the hill when the plaster is still in the mold and a little wet. Also, if you put a thin layer of plaster on the hillside, and wet the area where the casting will go you won’t even need to use glue. You could also try using Woodland Scenics rock molds instead of aluminum foil. They’re made of rubber and are very easy to work with.

Hope this helps!

4

JDog,

http://www.sceneryexpress.com/prodinfo.asp?number=FL1040

That’s a link to inexpensive rock molds that you can use to make your own castings or just glue to your layout and paint for a very realistic effect.

Good luck…

5

A method I use sometimes is to spray expanding foam in aluminum foil molds. It doesn’t take much and you get passable rocks, after you seal the tiny holes in it (white glue works good) and paint it, it looks pretty good.

6

There could be a few things going wrong here. First, plaster of paris is brittle when dry. If the plaster is old or has been stored improperly it won’t cure right and can crumble. You may be mixing and pouring right w/ good plaster but the castings may be too thin and as you press them on to an irregular surface it is cracking.
Personally, I don’t use the foil, I use rock molds and cast with hydrocal. The WS hydrocal is good for a few molds but very expensive. You can buy bulk 99# bags at a contractor supply. The hydrocal castings will show better detail and are much less brittle. When I cast my rocks, I use a masonry dye(powdered) and add it to the dry plaster to find a good base color then add the plaster to water, not water to plaster.
Try the WS casting plaster(hydrocal) with your foil molds and ck the results. The mold can be applied before it hardens (feels warm and is flexable) Apply the mold w/ a bit of fresh plaster buttered on the mold or the surface. Hold the mold, can contour to fit and don’t release until it will stay on it’s own. This works better with the rubber molds, but will work for the foil. don’t remove the foil until the plaster is dry enough to not pull off some of the detail.
Bob K.

7

I will echo Bob’s recipt with the hydrocal and rubber molds.I use the liquid masnory dye. The real secret is just right. You have to be just right putting it on an d just right taking the mold off. Then you will find that everything is just right. By the way I did try the plaster of paris but found it didn’t work near as well.
Later
Neal

8

I don’t use plaster of paris. I went to Home Depot and got an 8 lb bucket of plaster. Works fine. Obviously you can’t bend them when they are dry or they will break. Sculptamold works but doesn’t give you as sharp of edges on the molds but is ligher and will work fine. I use the Woodland Scenics rubber molds. Just make sure to spary the mold with water and a drop of soap before starting so you can get the casting out later.

9

If the plaster cracks and falls off it is too brittle.

Solutions:

  1. Make thicker castings and maybe add some gauze halfway through
  2. Use Art plaster or molding plaster rather than plaster of paris - it is much stronger. Dental plaster works too if you have a source
  3. Try to vary your mix ratio a bit and do NOT add coloring to the slurry.
  4. Don’t forget to wet the surface and the mold before 'plastering" it to the base. This gives better adhesion. Shouldn’t be an issue of you are using glue.
10

If you like to color the plaster, you can safely use a powdered masonry dye. These are formulated to color concrete, mortor and thinset. They have no effect on plaster curing or setting. Paints and some liquid colors can cause the problems that larak mentions.
I like to base color my molds, roads and top coat scenery plaster. It provides base color for final washes and prevents showing white if chipped or damaged especially for forground handling areas.
Bob K.

11

Suggestions on where to get the dye powder ? I am guessing that Lowes and Home Depot don’t carry them ?

12

The last time I needed concrete dye powder, I had to go to a concrete plant to get it and let me tell you, they’re proud of it.

13

Concrete color is a great idea!

My grandfather was a mason so he bought them by the bag at a masonry supply company. I wonder if I have any still sitting around …

Anyway, a real hardware store or home center should carry them. Our local true value (which is associated with a lumber yard) has them. I don’t know about the borg stores. Also try smaller hardware stores in out of the way places too. They often carry a wonderful mix of eclectic stuff.

14

The coloring found at HD and Loews is the Sacrete liquid variety. Some use it, but I don’t like the slight coloring for the large amount needed. The powdered masonry “dyes” should be found at any building supply, stone, masonry quarry or as jeff mentions a concrete plant. Colored concrete isn’t popular in all parts of the country but they should definately have it for the custom ordered mixes.
Bob K.

15

Thanks guys, this really helps and I’ll post pics after they’re done.

16

I use hydrocal and rock molds, but a little vinegar mixed with Plaster of Paris slows the cure time (for CSXFan’s suggestion of setting it wet).