I have a mountain that needs some rock faces. I’m the type of guy that likes to go the alternative method instead of going the name brand model way. Plus my LHS is an hour away, lol. Went to lowes and got plaster of Paris and basically just mixed it according the instructions, poured it into formed foiled moulds, and let it set. Once set, took them out and let them air dry. Well, 24 hours later they’re still cool and very damp, almost wet. I’ve had a fan blowing on them for the entire time and didn’t seem to do anything. Can I bake these to get them to finally harden? Any other ideas?
Baking them may work but it may also cause some unwanted problems like powdering of the surfaces. Depending on how thick and large your castings are it may take a few days for them to dry completely. Be patient.
That seems like a long time. How thick are the castings?
Other than making thinner castings, I would recommend patience. If the plaster is still wet inside and you heat it, you will likely crack the casting as the water inside expands.
Alright thanks guys. Trust me, I have patience. ;-). I just never did castings before and didn’t know if it was a quick curing or took a while. I have other things to work on while I wait for them to dry, such as Ballasting turnouts, ugh. Haha. Thanks again. Will let you know
This can also be the result of adding too much water to the plaster when mixing. If they don’t harden and dry quickly, it’s often because there’s more water than necessary to transform the mixed plaster.
No biggie if that’s what it is, plaster is cheap and you didn’t install on the layout itself. I’d just go for a do-over if things don’t set properly.
I would suggest this method. Turn your oven on to a low setting. Let it warm up (not hot) then turn it off. The rack in the oven should be warm to the touch. Put you castings on a cookie sheet and place them in the oven. It shouldn’t be sufficiently hot to cause the casting to expand.
I have used a lot of rock using plaster of paris and have never had that problem. How is the humidity in the room where you are casting the rock?
I’ll give that a try when I get home. The humidity is pretty bad through the whole house. Constantly having to throw sugar and spices away as they lump up like rocks.
I make some rock castings the same way. However, after they set up for a little while, I plop them right onto the mountain face and leave the foil on until they cure, which is about 15 to 30 minutes. Then I peel the foil away and do some minor carving with a screwdriver blade around the edges to work then into a natural look that connects them with the castings beside it.
First, I use a 50/50 mix of water and plaster. Pour the water into the plaster for best results, then mix. I also don’t make the castings over 1/2 inch thick, and most times a little use less. Keep checking the mold often after you pour. You want them to still be soft, but start to show fracture lines when you flex the foil mold. Then you can peel back the edges and apply it to the mountain without the plaster running out of the mold. If you want to work faster, you can add about 1/4 teaspoon of salt to one cup of water. That will make the plaster set up faster.
If you make your foil molds the size of your hand with fingers spread, they will use one cup of water and one cup of plaster.
Well, don’t know what happened but the plaster of Paris turned to soup after it hardened! I’m making a second batch and pretending this didn’t happen lol. The 2nd castings set up rather quickly. I poured thin castings and a very equal 50/50 amount of plaster and water. I’ll let you know after these really dry and get set up how it goes! Thanks for the help
I’m not sure what you mean by that… did it somehow reliquify? I’ve never heard of that. If you use too much water, it will be crumbly rather than hard. Perhaps the humidity is too high.
Humidity is definitely the enemy of everything model railroadish. If you can, I strongly recommend putting your scale pike in a closed room and keeping a dehumidifier running constantly.
Alright, the 2nd batch of rock castings came out perfect. Nice and light yet very hard, not brittle. I guess that first batch was a bad mixture on my part. I’m having one issue though, they won’t stay glued to my plaster cloth mountain. I painted the mountain an earthy color as a primer with acrylics, and this was done a while ago so the paint is dry. The plaster rocks are dry, but everytime I try to glue these things on, the glue (ive tried all kinds : Hot glue, crazy glue, gorilla glue, white glue, you name it glue) picks up the surface of the castings and they fall off. Any ideas? I probably should have made the mountain out of foam or something, but supplies and $$$ are not unlimited. Any ideas on how to attach these things to a mountain structure?
Try mixing up some plaster and buttering the backs of the rock molds with it, then plopping them on the plaster cloth. The best way would have been NOT to paint the plaster cloth before gluing the rock castings in place.
The next thing to try if that doesn’t work is to paint the backs of your rock castings. Then try gluing them to your mountain with hot glue.
You should still be able to Cast your rocks in place on the painted mountain. (See my previous post.)
If you are using that mix to carve rocks in, I think you should try the method I mention in my first post. If you add a little vinegar to the water before you mix the plaster, it will slow the setting time and give you time to do your carving after you remove the foil.
At Lowes you can probably buy some Hydrocal plaster, which is much better for making molds, and is extremely strong and waterproof,(when it drys).Plaster of Paris is too brittle!
I let the pieces dry a bit more, painted on latex on the back of the pieces, hot glued them in place, THEN washed them. THe first rock I did I washed THEN put in place, making the back of the casting a little damp again. I have some trees coming tomorrow, will fill in the empty spaces on the bottom, some clump foilage to the bare side of the rocky hill here. I’ll post a completed hill soon. Thanks for ALL the help. It really did wonders!
Ben
BTW, this is my first model RR. So if it has rough edges (the layout that is) that’s the reason why. No clue what I’m doing but it sure does look like it, lol.
I have a mountain that needs some rock faces. I’m the type of guy that likes to go the alternative method instead of going the name brand model way. Plus my LHS is an hour away, lol. Went to lowes and got plaster of Paris and basically just mixed it according the instructions, poured it into formed foiled moulds, and let it set. Once set, took them out and let them air dry. Well, 24 hours later they’re still cool and very damp, almost wet. I’ve had a fan blowing on them for the entire time and didn’t seem to do anything. Can I bake these to get them to finally harden? Any other ideas?