Articles - Plaster of Paris
10/24/2017
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Plaster of Paris is a good plaster to use for casting, because it dries relatively quickly, and, more importantly, it doesn't shrink. You can buy this plaster at nearly any big-box home improvement store, art supply stores, or arts-and-crafts stores. For this article I am going to cast 3-foot by 3-foot slabs of plaster, which are going to be cut up into 3-foot cubes. These will serve as the foundation blocks to a coal tipple building I am building.
To make a cast, you need a mold. I built mine out of styrene. I used 0.040" styrene for the base, to give it its strength. The vertical sides were made from 0.020" styrene, so that it is possible to get the slab out of the mold. The mold's length is 7-1/4", but that was just because of what I had on hand. Testors glue was used to assemble the mold. Nothing fancy.
Plaster of Paris
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I had used Plaster of Paris before, but I wanted to make sure that I mixed it correctly. After some research on the Web, I found a video (by a model railroader) of how to properly mix plaster (see link). I followed his instructions exactly and it worked fantastically. The castings were nice and strong, and did not crumble. Basically the idea is to get cold water into a container, and then slowly add the plaster to the water, and not to start stirring the mixture until the water can no longer absorb any more plaster. The next photo shows the tools I used to make my casting. There is some plaster left over in the plastic butter container, which I used to test how well-cured the plaster was. The only thing the video did not show is that you can lightly tap the sides of the mold to get the plaster to settle smoothly and to release any remaining air bubbles. None of my castings had any air holes in them. By the way, don't throw the leftover plaster in the trash when it is dry; crumble it up and use it in your scenery as flat rock or rock debris.
Plaster of Paris
The first one I cast in my mold, I let sit overnight. However, on subsequent ones (I made five castings), I eventually got down to just about two hours before I removed the casting from the mold. None broke. All I had to do to remove the casting from the mold was run a razor blade in between the mold box and the casting along the long edges, and snap loose and bend out the end pieces of the styrene. The heavier styrene used for the mold box bottom held the sides in place, so my mold box is still in the same shape as it was when I started. Each time I re-glued the end pieces back to the long sides of the mold box before casting the next one.
Plaster of Paris
When I removed a casting from the mold box, I could feel that it was still somewhat wet, so I was looking for a temporary place where they could sit and continue to dry, while allowing air to circulate all around them. My eye caught my three empty flat cars, so I cut some toothpicks to the width of the flat cars, and then placed each casting on them as they came out of the mold.
Plaster of Paris
Cutting individual blocks wasn't as hard as I thought it was going to be, and was actually somewhat enjoyable work. The photo below shows all the tools I used to do the cutting. I used the flat file to make sure one end was flat and perpendicular. I then marked off a line on the top and on the front side of the casting, using the T-square, a scale three feet in from the end. Next, using the black-handled fine-toothed sawblade and the small metal machinist's square, and eyeing the lines I had marked, I carefully cut the 3-foot square block from the casting. A light pass or two over the flat file cleaned up the cut. Every so often, I cleaned the file with the metal wire brush you see in the photo. It is messy work, but I was able to get at least ten blocks out of each casting.
Plaster of Paris
And here are the 44+ blocks I need for my project.
Plaster of Paris