I have just started using Woodland Scenics rock moulds, and am using plaster of paris. I am unsure of the correct mixing ratio for the plaster. The instructions on the box say to mix 1:1 with water, but when I do I end up with a white liquid about the consistency of milk. Of course , it pours into the moulds easily, but when setting forms a pool of water on the surface which has to be soaked up. On removing from the moulds, I have to be very careful not to break the casting, and thin casts just crumble in my fingers. Even completed casts remain very soft.
So, is there a generally accepted wisdom about what ratio to mix the plaster? What I am using seems very thin, espcially when compared to other scenery materials such as Sculptamold or WS Smooth-It. Are there any other caveats about using this material that would be helpful to know?
The internet suggests 3:1 ratio is “ideal” but there isn’t an ideal actually. It depends what you wish to use the mixture for. There is an ideal setting ratio chemically which is likely around three to one plaster to water. When mixing cement products generally you add water to the powder which makes it easier to reach the desired consistency. It’s because you’re pouring into a mould that WS is suggesting such thin plaster. It should still setup hard. Try a fresh container of powder, it doesn’t last forever unless you can keep all the humidity out.
If you find you like rock face and stone wall castings, you might also look at Bragdon Enterprises. They use a two-part chemical resin that sets up very well but can actually be softened later with a hair dryer to allow the castings to be bent around curves later. The castings are thin and light, and can even be cut with scissors. The Bragdon rock molds are very finely detailed and the foam material brings out all the fine details well.
If you have ever been to the casting room at the hospital as I have on many occasions[:-^] you will notice the Dr./Tech has a big apron and gloves to the elbow on and is often covered in plaster. Putting a cast on a broken leg or arm is a messy business and dipping the bandages in a vat of 1:1 mix ensures the material of the day is well impregnated with the PP. That being said I think the 1:1 mix suggestion is for dipping cloth in which I have done to make rolling hillsides. For use in moulds I think something a little thicker would be required and I am not sure how well it would hold together after coming out of the mould anyway. Hydrocal or some of the other suggestions would be a better choice I would think. Wait for Dr. Wayne to show up and see what he uses.
With all the reno’s I have done over the years and not wanting anything to go to waste, I have put leftover tile thin-set and grout that was in the bucket on this portion of the layout. It is super hard and once painted and gets a little greenery sprinkled on it, I think it will look great.
The different colours are just because they are different products.
Whenever I was tiling or grouting I had the drop cloths down in the train room pryer to starting.[(-D]
I’m sorry, but I’ve spent over an hour trying to compose a reply, but the Forum has been freezing every few minutes, and eventually simply deleted what I had managed to add, including photos.
I tried a mix of pancake batter, and the results are much more successful. It’s not self-levelling, but it sets well and comes out of the mould very well. I may experiment with maple syrup for my next effort.
Most instructions for mixing plaster are by weight, not volume. This easily causes confusion. If you use an inexpensive digital kitchen scale you can pretty easily get the weight the same.
By volume, you will use far less water than plaster powder.
I have never used the Woodland Scenics plaster, so I cannot say for certain what their instructions specify. All the commercial hydrocal and ultracal I have bought specified the mixing ratios by weight.
EDIT–
Just found the instructions on the Woodland Scenics website. They are now stating to use a 2.5:1 ratio by volume.
I’ve used plaster of paris since 1956 when I had a basement Lionel layout. I bought it for 5 cents a pound at the local hardware store. Moving up the the 1990s, I picked up several of the rubber rock molds and made literally a couple of hundred castings for my HO layout at that time. All I used was plaster of paris.
The trick was to make a mix thick enough so as to not run, but to make sure it was thin enough to fill the cracks, and to make sure there were no lumps. I never used a formula for the ratio, and the results were always good.
Some comments…
too thin a mix will lead to cracks when it eventually hardens
you will find the casting warming up as it cures. Be patient and pull after it starts to cool down.
make sure that the casting is well secured to a flat base.
any tinting or painting you do will end up darker than intended.
plaster of paris works well for the molds, but doesn’t do well for roads and the like. For that, I found Woodland Scenics “Smooth It” to work beautifully.
Hey, I’m not knocking other products, just saying plaster of paris is what I used for molds as it was what was available and I was experienced with.
The chemistry is such that the more water you add beyond 1.5:1 plaster to water by weight the easier the plaster is to flow into the mould but the faster it sets up and hardens. More water by ratio also leads to weaker plaster.
So, it appears you need to experiment. Start at about 1/3 water and 2/3 plaster. Then add water bit by bit to get the consistency you think is about right. Test the result by casting and adjust your ratio for next time. The actual chemical ratio is supposed to be 2/5 to 3/5 by weight but I doubt you need to be that accurate since you’re going to adjust anyway.
Adding water to plaster gives you more precise control over consistency without ending up with too big a batch. The bigger the volume the faster the set. The old surgical plaster bandage is deliberately light in plaster in part for this reason. Plaster can get quite hot as it cures.
Note the instructions say to add the plaster powder to premeasured water. Numerous other manufacturers, suppliers, etc, advise the same. For example, from DAP: Add DAP Plaster or Paris to clean water.
When the Hydrocal instructions say that working time is 5 minutes, they are not kidding. In my experience working with Hydrocal, you need to precisely follow the instructions and work fast.
