So, just how does a clay original become bronze or bonded bronze? Today I will show you the process for bonded bronze. The process for casting molten bronze at a foundry is different and more complex, but stay tuned! I will get to that in a future blog.
What exactly is bonded bronze? Bonded bronze, sometimes called cold-cast bronze, is exactly that. Normally, bronze must be heated to 2000 degrees, melted and then poured into ceramic molds that have been made from wax castings created from a different set of molds that are made much like I will describe today. It's a bit confusing I know, and the process requires many man hours, but the end result is a piece of art that will last forever.
Bonded bronze is a method of getting a bronze finish on our sculpture without the process intensive methods necessary for foundry bronze. It is particularly suitable for smaller sculpture such as The Bergin Six. Creating bonded bronze is simple. I have the bronze pulverized down to a baby powder consistency and then mix it with a polymer resin that "bonds" the bronze particles together. The final piece is about 85-95% bronze, so it can be finished in similar fashion to foundry bronze, with chemical patinas, paints, glazes and waxes. The bottom line difference between foundry cast bronze and bonded bronze is that the bonded bronze is mildly more fragile than the foundry bronze.
I could go on about the two forms of bronze, but this is about making molds, so let's get on with it! The first step is to mix up some liquid silicone rubber and apply a thin layer to each figure making sure to get in all of the nooks and
crannies to get all of the detail. Silicone rubber will grab detail right down to a fingerprint, so the sculptor must be satisfied that the surface of the sculpture is what they want, because what ever texture and or marks are on the surface of the clay figures WILL show up in the silicone mold.
The silicone being used for this mold is formulated to be fairly thick so that it sticks to the vertical surfaces of the figures. Silicone is amazing stuff. It will bond to itself leaving no seams, so if you don't mix enough to cover a figure completely, no worries, just mix up some more, even after the first batch has completely cured, and it will all become one!
For this silicone, we let it cure for about an hour before applying the next layer. For most figures, 4 thin layers, totaling about 3/8" is enough to yield a strong yet flexible mold that captures all of the details of the sculpture.
Another nice thing about silicone is that it accepts pigment very well. So, as we add layers to the mold, we change the color of each layer so that we can ensure that we have covered every spot, creating a more consistent thickness over the entire mold.
Normally only one color is needed and we alternate
layers between the natural color of the silicone and the pigmented layers, like this vivid blue. We always want to choose a color that will stand out against the natural peach color of the silicone...I think this works, lol.
Next we have to do something about gaps between arms and torsos, and between the legs in some cases. We want to fill in small gaps between body parts with silicone, but how do we make a liquid rubber do that? (see next picture)
To accomplish this, we will add a thickening agent to our silicone, which makes it more like cake icing and it can be spread into the gaps with out sagging or sliding down the figure.
So now we have all of the figures covered in silicone and if we have been careful we now have perfect molds that capture every bit of detail on the sculpture. But we aren't done yet! We have to have a way to get in and out of the mold, to remove the clay original figures and be able to remove the final sculpture castings after we pour bonded bronze into the molds. What to do?
And the answer is...we create an extended strip of silicone and apply it around the outside of the molds. We use more thickened silicone to "glue" the strips in place and secure them with long stick pins to hold them in place while the silicone rubber cures.
How does this help us get into the molds? Hold tight! First we have another step to complete...making the "shell" or "mother mold'.
So, rubber being rubber, we need a solid shell around it to help it hold the correct shape when the mold is empty. It's plenty sturdy with the clay figure inside, in exactly the way we want everything to be, so while the clay is still in there, we construct a two-piece hard shell on top of the silicone rubber. There are many materials that the shell can be made from including things like plaster, fiberglass or urethane paste, but my favorite and what we are using here is an epoxy putty called Free Form Air. It works like clay, can be kneaded and spread and holds its shape. As indicated by having 'Air' in the name, it is hard as stone when cured and is as light as a feather, which becomes important later when casting the final bonded bronze. We form one half of the mother mold on the silicone rubber, let it cure and then create the second half of the mother mold. a small piece of kitchen plastic wrap is inserted around the edges to prevent the two halves of the epoxy mold from bonding to each other.
After a few hours, the epoxy has cured enough to remove the now hardened mother mold and work on finding our way into the rubber mold. This is where the strip of silicone that we attached on one side of the rubber mold comes into play. Using surgical scalpel blades, we start at the bottom of the rubber mold and cut our way to the top. You will notice that I cut a very rough, zig zag line all the way up. This is very intentional. We want our mold cut to always mate up perfectly to prevent a seam line from appearing on the sculpture. If it doesn't seat perfectly, when you pour the bonded bronze casting, it will have a seam line that will have to be sanded or ground away. We want to avoid this, and this technique is SO good that in most cases there is no seam line visible on the castings AT ALL.
In the picture to the right, you can see a still-closed mold behind the open one. These molds will only be used once or maybe twice, so I didn't take any time to make them look nice, but for molds that will be used for many castings of a limited edition, I take the time to
'dress' the molds. I offer mold making services to other sculptors, so the molds need to look, work and feel professional if they are leaving my studio. This is one I recently completed for sculptor Maria Kirby-Smith. This mold will be used to create many wax figures for conversion into foundry cast bronze.
Well, that is mold making in a nutshell...a very small nutshell, lol. These are simple molds. As the complexity of the sculpture increases, so does the complexity of the molds. When making molds for foundry cast bronze, the rubber molds we make are for wax castings. The wax castings are then encased in many layers of ceramic material, a ceramic mold, which is then fired in a kiln where the wax melts out and then molten bronze is poured into those ceramic molds. It's an amazing process which I will cover here soon. We will take a field trip to Carolina Bronze foundry in Seagrove, NC where incredible bronze monuments are created that end up all over the world. Until later....