Cold casting is a newer technique for sculpting and jewelry making compared with the traditional lost wax method. The lost wax method allows for an extremely accurate, hollow copy of an original wax or clay model to be created with a variety of possible metals. Meanwhile, cold casting is the process of mixing metal powder with resin to create pieces that give the appearance of solid metal, but are not actually made of pure metal. As a result, the cold casting method is faster and less expensive. This is also sometimes referred to as “bonded metal” —for example, a bronze sculpture created with this method would be referred to as “bonded bronze”. Here, we break down both of these processes in order to allow you to compare and contrast which one would work best for the needs of your own personal project.
The Cold Casting Process
1. Prepare the Mold- The most common molds are made out of high-quality silicon rubber, latex, or a polyurethane compound. Refer to the article titled, “Molding Process,” for more on creating the molds themselves. Once you have your mold established, however, start by coating the mold with an “Ease Release” (think of it like oiling or buttering a pan in baking) so that the resin can be released easily from the mold once it is dry without damaging the piece.
2. Mixing & Dispensing- Combine the resin, tint, and type of metal powder you want to use together. Be sure to continually scrape the sides and bottom of the mixing container so as to not allow the powder mixture to start to settle. Once it is thoroughly mixed, pour it into the mold. Brush the mixture around the mold immediately so that it starts to form a “gel coat” on the sides of the mold. As soon as it starts to stick to the edges, stop brushing and allow it to cure for five to ten minutes.
3. Backfilling- Once again, in a separate clean container, combine the resin, tint, and your chosen type of metal powder. After the “gel coat” has cured, you now need to pour the new mixture into the mold and fill it completely this time. A wire or bent piece of metal can then be placed into the liquid as the resin is drying to use as a mounting hook. You only need to wait for approximately one hour for the resin to be fully dried and able to be removed from the mold.
4. Finishing– You are now able to carefully remove the resin from the mold. It might be a bit plain at first as it still needs to be polished to get the full metal effect. First, wipe down the resin with mineral spirits to soften the surface and prepare it for the coloring. Then, take steel wool and rub the entirety of the piece of resin. After that, take a shoe polish of your choice that coordinates with the color metal you are working with, and gently rub the surface of the piece. Once it dries, give it a quick buffing with a polishing cloth. And finally, add two coats of acrylic spray to help the finished piece retain its sheen.
The Lost Wax Process
1. Wax work– The very first step of the process is to create your intended sculpture, either directly out of wax, or, to start with clay that you would then create a mold of to finally pour the wax into in order to get a wax replica. Add sprues– Wax rods, known as sprues, are attached to the wax. They serve as a network that allows the molten bronze to enter and excess gas to escape.
2. Weighing them out– This step is sometimes forgotten, but is very important. How much the wax weighs determines how much metal to pour. The number you multiply the wax by has to relate to the specific type of metal. For example, sterling would be multiplied by either 15, or by 10.4 and then an added Troy ounce. Attach to sprue base– After the wax is finished being weighed and the metal calculated, it is time to attach the wax to the sprue base or button.
3. Place flask on base– Next, lower the flask over the wax, making sure there is about ¼” of space. Setup for investing– Calculate the size of the flask and the amount of investment you will need to use. Be sure to measure the water out first and then place it into your mixing bowl.
4. Weigh investment– Once you know the amount of investment you need, it is time to weigh it out. Any type of scale should work fine— it doesn’t need to be expensive. Do be sure to wear face protection when measuring as to not inhale any of the silica in the investment. Time it and mix it– Now is the time to mix the investment into water for about three minutes, followed by vacuuming it at the five-minute mark. Overall, you don’t want to disturb the investment for longer than eight minutes total.
5. Vacuum investment in bowl– Start by vacuuming the investment for two minutes. Seeing bubbles in the mixture is a good sign. Pour investment in flask– Once the vacuuming is complete, it is time to pour the investment mixture into the flask. Be sure to solidify the top of the flask with tape so that the mixture doesn’t spillover accidentally. Try your best not to pour the mixture directly onto the wax itself so as to not disturb the design.
6. Vacuum investment in flask– Next you want to vacuum the flask itself. Be sure to mark the flasks in a way that differentiates them if there is more than one—chalk works great for this. Vacuum the flask for approximately one and half minutes as it’s still important to try not to go over the eight-minute mark. Afterwards, leave it to sit for one and half to two hours before putting it into the kiln for the burnout process.
7. Kiln– After you have waited the appropriate amount of time, you can now put the flask into the kiln to start the burnout process. Usually these last between five to 12 hours depending on the size and the number of flasks. If you don’t have an automated kiln, you will have to pay attention to adjusting the temperature as you go along.
8. Heat the crucible– Once the kiln has been going for about an hour, you should start to heat up the crucible and the metal. Be sure that the crucible is literally red hot before adding the metal.
9. Flux & stirring rod– As soon as the metal is fully melted, add the flux, and stir it with a pre-heated carbon stirring rod.
10. Flask out of kiln– Retrieve the flask from the kiln. Check for ashes so that they do not accidentally get stuck in the piece. Always be sure to have the utmost safety measures in place— in this case a large fireproof glove. Place flask and turn on the vacuum- Next, put the flask upside down on the casting table with the holes facing up, and go ahead and turn on the vacuum pump.
11. Pour– Now it is the time to pour the metal. Start by positioning the crucible over the flask, making sure to keep the torch on the metal to retain consistent heat.
12. Cool it– You can go ahead and turn off both the vacuum pump and torch in the cooling process. Wait until the metal is no longer in a red-hot state before moving onto quenching it.
12. Quench it– If you used sterling for your piece, after waiting approximately two to five minutes until the metal has cooled, you can begin to quench it. Be sure that the flask is completely underwater and that it is bubbling. If you used stones or alloys in your piece, you will need to wait until it is completely cool to complete this step as to not crack them— for about an hour or so.
13. Clean the sculpture– Once your piece is complete, it will have leftover investment on it, giving it a dirty appearance. A simple toothbrush is a great tool in order to get the excess off. You can also use an ultrasonic cleaner, or even good old vinegar as a cheap alternative. After the investment, it’s time to remove the sprues, file, sand, grind, add patina, and then apply a final polish to make it into its final, beautiful version.
14. Clean your work area– And of course, albeit perhaps the least fun part of the process, it’s always good to thoroughly clean your workstation in between pieces to help prevent contamination of future pieces and for the sake of safety.
Compare and Contrast
It is evident from the comparison of these two processes that cold casting is clearly a much faster, cheaper overall process. However, it does not contain the solid metal, but rather elements of metal powder, that the lost wax process does. The creative process is of course unique for each individual artist, so choosing between these processes depends on what you are looking for in the piece you are creating, as well as the logistics (such as time and budget) surrounding the project. No matter whether you choose cold casting or lost wax casting, the above breakdown of the processes should help provide a solid overview of this these art forms that are utilized all over the world.