Casting Coal With Epoxy Resin and Silicone Molds

Epoxy is amazing stuff. Since it bonds to practically anything, you can mix it with granules of almost any material and end up with a substance that can be formed or cast into whatever shape you desire. With this in mind, I was interested in forming a relief panel from a material that wouldn't seem capable of taking such a shape: coal.

In this Instructable, I will outline the process for 3d modeling a relief, machining a foam positive, creating a silicone mold from that positive, processing coal into powder, and casting the final piece with epoxy resin.

Step 1: Modeling Relief

There are many ways to go about modeling a relief, but I will cover my preferred method here using Rhino. First, run the Heightfield command. Heightfield will use a grayscale input image to drive the creation of the relief in which light colors define high points and dark ones define lows.Select the image from which you wish to generate the relief. Next, set your origin. You will be asked to enter the length of the heightfield. Set the length of your desired relief. Then you'll need to set the number of sample points, total depth, and the desired geometry output. The sample points will define how closely your output surface will conform to the image colors. Experiment with this setting until you find something that achieves a resolution that satisfies your needs. I prefer to output a mesh with vertices at the sample locations as this will allow you to create a fairly high sample rate without bogging down the computer. You will get a jagged mesh initially, but this mesh can be smoothed out using the free WeaverBird plug-in with the wbCatmullClark command - a mesh subdivision algorithm. You can repeat this command on the mesh until you achieve the level of smoothness you desire. You will need to export this mesh and bring it into your CAM software of choice in order to toolpath the model.

Alternatively, you can create your relief directly in some CAM software. ArtCAM has this capability and if you plan on working with relief often, it might make sense to explore this. You can use grayscale images as outlined above, or import other 3d models and ArtCAM will convert them to reliefs which can be something of a pain to model directly if you're working often in re

Step 2: Milling a Foam Positive

Urethane foam is great for prototyping for pure form - which is all we really need in order to make a silicone mold. It is light, homogeneous, relatively affordable, machines easily and nicely, and comes in a range of sizes and densities to suit the needs of your particular project.

You can order high-density urethane foam from a number of sources, and it goes by a few names, so you may have to do some searching to find the supplier that makes the most sense for you - likely the one located closest to you. You can order it through McMaster-Carr if you're in a hurry. Freeman Manufacturing & Supply makes it under the name RenShape in a variety of sizes and densities. Even better, they include a machining guide for the various types available.

If you're using a lower density foam (15 lb./cu. ft. or less) and the depth of cut is shorter than the cutting length of your bit, you can likely skip the roughing pass altogether and get nearly identical results. The chances of damaging the machine are low with such soft material. This can translate into a big savings on machining time.

Step 3: Casting Silicone

You can read about the general procedures for casting silicone in a previous Instructable I've written. One critical difference here, however, is that I used a tin-based silicone as opposed to a platinum-based one. The urethane foam can inhibit the cure of platinum-based silicone, so I didn't want to risk it, as silicone rubbers are very expensive.

I constructed a small box to contain the foam positive, leaving some room on top where I would pour the silicone. I could have sealed the foam with some kind of spray finish, but I didn't bother since the material I was going to be casting (coal dust) wouldn't have the resolution and detail of something like a plaster or pure resin cast.

Following the manufacturer's directions, I mixed up parts A and B. Though it probably wasn't necessary, I decided to de-gas the silicone, which required using a vacuum chamber. After the silicone was prepared, I poured it into the mold at the lowest point. It's important to resist the urge to spread it around as you're pouring, and instead allow the silicone to slowly spread and find its level. This will help prevent any air pockets from forming in your mold.

The video here only shows the first pour, and it took about 5 to reach the necessary level. For this reason, it was good to have a silicone with a long pot-life. I was limited mostly by the available mixing container sizes, but it was also nice to mix only precisely what I needed for my mold and avoid wasting the expensive material.

Step 4: Processing Coal

WARNING: Coal dust can be very dangerous. A Google image search will offer related images for terms like 'pollution', 'explosion', and 'black lung'. In fact, coal dust explosions are responsible for some of the biggest mining disasters in history. For these reasons, please handle with caution, use protective gear to minimize dust exposure, try to minimize total airborne dust, and keep away from any possible ignition sources when working with the material. Also, make sure to use adequate protection when mixing the resin. Epoxies are dermal sensititzers, so be particularly careful with skin contact while the resin is in its uncured state.

Due to the relatively brittle nature of coal, dust isn't difficult to create. In fact, if you happen to know any blacksmiths or anyone operating a coal-fired forge, you might be able to scrape together enough dust for a meaningful project. They would likely be more than happy to be rid of it than have to dispose of it some other way.

If you need to make your own dust, the first step will be acquiring some coal. I ordered my coal from the Metropolitan Coal Company and received it about a week later.

How you determine to process the coal will depend primarily on the degree of fineness and total volume required. If I had needed a very large amount, I probably would have developed a more automated way of creating the dust. Since I didn't need much more than a small bucketful, though, I decided to handle the dust creation process manually. With a 10 lb. sledge, I was able to break up the coal into smaller, more manageable pieces. These were then processed in a small blender into the dust I was looking to create.

Using the mold I had created in the previous step, I was able to pour the dry coal mixture into the mold and measure to see if I had made enough to cast my piece. Once I did, I mixed the entire batch of dust with epoxy resin until the coal looked just slightly damp. You don't want to add too much or the resin will be more visible in the final panel, giving everything a 'plastic-y' appearance.

Step 5: Casting Coal

Once the coal and epoxy were mixed thoroughly, I poured the mixture out into the mold. The rest is about as straightforward as you'd imagine it. I leveled out the coal in the mold, making sure to pack everything in nicely to prevent voids in the mold. To ensure that the dust was tightly packed enough that the epoxy would bond everything together, I cut a piece of plywood to the exact dimensions of the final panel, and used it to apply even clamping pressure over the entire panel.

With everything clamped up, I allowed the epoxy to cure. I was using a very slow curing 24 hour epoxy to give me the most working time with the pulverized coal. While I didn't use any special release agents on either the silicone or the plywood mold box, I also didn't have any trouble de-molding, so release is probably not necessary if you don't use too much epoxy in the mixture.