Have you ever looked at someone’s layout and wondered how they did all the rock castings? There are basically two methods, dry and wet.
The dry method allows the modeler to mix large batches of plaster at a time. Have several molds ready and pour the plaster into molds until your supply is exhausted. This method eliminates the potential waste that can come from too much mix for the mold. Once the castings have set up, they can be applied to the layout in a jigsaw fashion. Simply dampen the back of the rock casting and the layout location. Then, apply a mix of soupy plaster to the back of the mold and hold it in the desired location until the plaster sets.
The wet method requires a little more planning. Since the castings are placed on the layout while wet, you probably can only cast one mold at a time. This “one mold per mix” presents a problem in determining how much plaster to mix per batch and avoid having either too much or too little plaster to complete the casting.
Over the years I have come up with the following method which insures ample plaster while avoiding waste. The key to this solution is to obtain the proper amount of water. Submerge the mold into the water with the mold cavity up. Pour the water from the filled mold into your mixing bowl. Dip the mold a second time, filling the mold to ¼ of its capacity. Add this water to your bowl and you now have the exact amount of water to make enough plaster to fill your mold. Finally, stir in plaster until it reaches the consistency of pudding, and pour into the mold.