Metal cold-casting with Jesmonite

Metal casting is fun, but it’s not always practical due to the need for specialized equipment to melt metal, a well-ventilated workspace, and strict safety precautions.

Fortunately, if your main goal is to achieve a metallic look and feel rather than creating a 100% metal part, you can use cold-casting. This technique simplifies the process and avoids most of the challenges associated with traditional metal casting. It's also much less expensive.

Introduction

So, how does metal cold-casting work? Simply put, it involves mixing fine metal powder with resin or a similar medium, then post-processing the piece to enhance its metallic appearance.

For this project, I needed to create two small bear sculptures — one in bronze and the other in brass.

Since I wanted my pieces to have a dense, heavy feel, regular resin wouldn't cut it. Instead, I opted for Jesmonite, often referred to as eco-resin or eco-concrete. This composite material is primarily made from gypsum, bonded with acrylic resin. It’s easy to work with, captures fine details beautifully, and, as an added bonus, is more affordable than polyurethane resin.

Master models

Before we can start casting, we need molds to pour the mixture into. To create a mold, you first need a master, which can be anything from a clay or wax sculpture to a 3D print. For this project, I opted to use my 3D printer.

I found a nice set of simple bear sculptures online a while ago, but unfortunately, I can't locate the source anymore. If I find it, I'll update this post with the link.

Both of these were printed in PLA, at a size of around 11cm long. The issue with FDM¹ prints, however, is that they aren't perfectly smooth. Even with fine layer lines, the final result won't be smooth enough to achieve the shiny metallic finish we're aiming for. The same applies to resin prints — even though they appear smoother, they still require additional finishing. Resin prints also have another downside, which I'll touch on later.

So, unless you're intentionally going for that 3D-printed layer lines look, you'll need to do some post-processing.

I started by removing the 3D-printed supports and filling any imperfections with my go-to plastic putty (Revell's Plasto Filler), and then put in some elbow grease for the first sanding pass using 120-grit sanding paper & sponge.

Important: always wear a dust mask or respirator, gloves and eye protection when sanding or handling chemicals.

I then sprayed a coat of filler primer.

And more sanding, using 180, 240, 320, 400 and 600-grit sandpaper & sponges until the surface was perfectly smooth to the touch.

Silicone molds

Next, we'll create the molds using silicone.

Silicone can be quite expensive, so it's best to avoid using more than needed. One way to optimize costs is by making a sock mold. For very small pieces, you could use a high-shore silicone and simply brush it over the pieces. However, in this case, the models are larger and will be relatively heavy, so a basic painted-on sock mold would deform once the casting mixture is poured in.

Additionally, I prefer using low-shore, highly elastic silicone because it makes demolding much easier. The downside is that this type of silicone has a high viscosity, making it difficult to apply with a brush since it tends to drip off the model before it can solidify.

So, my solution was to 3D print cheap quality sock shells for my models.

I used a two-part platinum silicone (shore A 12), specifically Reschimica's R PRO 10. I like working with this silicone because it has a 45-minute working time, a 3-hour cure time, and equal parts A and B by volume, making it easy to mix by weight. Plus, it doesn’t expand during degassing.

Platinum silicone doesn’t react well with sulfur, so if your model is made from sulfur-based clay, it won’t cure properly. Additionally, 3D resin prints can inhibit the curing of platinum silicone. In either case, it’s better to use tin-cure silicone instead.

While optional, I prefer to degas my silicone to ensure it’s completely bubble-free. There are techniques for mixing and pouring silicone without introducing bubbles, but since I have a vacuum chamber available, I may as well use it to streamline the process.

As mentioned earlier, this particular silicone doesn’t expand during degassing, so I was able to use a container sized appropriately for the mixture. However, most silicones I’ve worked with tend to double or triple in volume during degassing, so make sure to account for that.

With the 3D-printed shells securely in place to prevent leaks and the models inside, I poured the silicone slowly and steadily.

A few hours later, I removed the shell and demolded the models. Since my models were simple enough, I didn’t need to create a two-part mold. I just made a single cut between the legs, hidden on the underside, to release them from their molds.

Tip: use a zig-zag cut instead of a straight one. This allows the mold to lock into place when closed, minimizing the chances of visible seams on your cast pieces.

Cold-casting

As mentioned in the introduction, I used Jesmonite (AC100 specifically) as the base medium for my cold casts.

Note: for this process, all ratios are calculated by weight, not by volume.

The standard ratio for Jesmonite is typically 2.5:1 (powder to liquid). However, since we’re incorporating metal powder into the mix, the formula needs adjustment. Using the regular ratio would result in an overly thick, unpourable mixture that would set too quickly. For metal cold casting, I follow a 2:1 ratio of powder to liquid and add 85% of that combined weight as metal filler.

Here’s a simple method to calculate the exact amount of Jesmonite needed for a mold (assuming a 2.5:1 ratio):

• Fill the mold with water and note its weight.
• Add 5-10% to account for waste.
• Divide the water weight by 2 to get the liquid volume.
• Multiply that volume by 2.5 to get the powder amount.

For example, for 100g of water:

• 100g + 10% = 110g
• 110g ÷ 2 = 55g of liquid
• 55g x 2.5 = 137.5g of powder

Alternatively, you can use the Jesmonite Calculator.

In my case, my molds hold 152g of water. After adding 10% for waste and rounding up, I used a total figure of 170g. Using a 2:1 ratio, I calculated:

• 170g ÷ 2 = 85g of liquid
• 85g x 2 = 170g of powder
• 85% of the combined total (170 + 85) = 216g of metal filler

To enhance the final color, I added pigments to the liquid before mixing. Relying solely on the metal powder for color can often result in a dull finish.

Note: the maximum pigment amount is 2% of the total Jesmonite mixture, exceeding this can interfere with curing and weaken the final piece.

Jesmonite cures quickly (within minutes), so I made sure to mix everything thoroughly and smoothly, pouring one mold at a time. To minimize bubbles, I poured the mixture in a thin stream from a height of about 20-30cm, pausing occasionally to gently tap the mold against the table and vibrate it to release trapped air.

After roughly an hour, the first model was ready to be demolded. It's important not to leave Jesmonite in the mold for too long, as it can develop sweat marks where the liquid couldn't evaporate properly, potentially ruining the cast. Demolding should be done as soon as the piece is solid enough to avoid breaking.

Note: during curing, Jesmonite undergoes an exothermic reaction, meaning it will warm up. While it doesn't get dangerously hot, it can become warm enough to be uncomfortable. Handle with care.

Jesmonite requires a full 24 hours to completely cure, so I allowed the piece to rest overnight for the final hardening.

Post-processing

Straight out of the mold, the models appeared rather dull, with none of the expected metallic shine from the filler. This is easy to fix, though.

I used 00 and 000 grade steel wool to buff both models, and within seconds, the metallic sheen started to come through.

To finish, I applied a basic polishing compound (Dremel 421) to give them a polished, refined look. In the picture, the model on the left is bronze, while the one on the right is brass.

And that's pretty much it! As you can see there are some defects on the feet of the brass model. This is what happens when you try to demold too early, it crumbles...