Foundry - Polygons in Bronze: Part 7

During Spring Break, I spent the first half of the week finishing up my wax carving and adding the sprue system to the body of the sculpture. To keep this sculpture from being a solid block of bronze, I made sure to carve out the interior cavity to a more-or-less even thickness with some added material near appendages like the ears. The dome-like shape of the sculpture along with the thinner points of the ears and recessed details near the face/head made it fairly simple to choose the setup for my gating system. Luckily, because this was a ceramic shell mold, I did not need to worry about the venting of gasses. Ceramic shell is slightly porous and allows most of the gasses from the metal casting process to vent through the shell itself.

From the start, I knew I wouldn't be using a simple top-pour system, where the metal flows straight down into the mold like water into a pitcher. Top pouring can lead to surface imperfections or even air pockets due to the turbulent movement of the liquid metal pouring down into the mold.

I went with a bottom pour setup that allows the metal to flow straight down the main sprue before being forced upwards through the mold cavity where increased pressure and laminar flow leads the bronze into all the fine details and up through the vent holes, assuring that even the tips of the ears are properly filled.

The weight of the cat being off center of the main sprue did lead to some challenges but with some extra thick gates attached to the paw and the middle of the interior the whole structure was quite sturdy. The gates I added were angled up from the main sprue, this would ensure that the metal poured into the mold wouldn't flow into the gates before reaching the main attachment point at the bottom of the mold.

I would've liked to have taken more photos of the wax gating but I was worried about damaging the surface details on the cat.

For the second half of my break, and part of the next week, I was creating the ceramic shell that would encase the wax and become the mold in which metal would be poured. The first dip into the slurry was a thin "print coat" that is used to capture the details of the object. It then dried for about 24 hours. Then for the next 3 coats: after being dipped, twirled, and soaked in the slurry I had to sift fine silica powder over the entire surface of the mold. This silica serves as a binder to make the shell strong and able to withstand the casting process. For the next 5 coats: the process was much the same as the previous 3 dips, although these coats required a coarser grain of silica to increase the thickness and strength of the mold now that the fine silica layers were able to embed themselves to all of the fine details. For the final coat: just a dip in the slurry without any silica. Between each coat, I made sure to allow for at least 4 hours of drying time but no more than 24. I did a lot of research on the web about how the strength of ceramic shell is impacted by the amount of time between coats and found this to be the best range of time.

Once the last coat was dry, I cut the top of the mold with an angle grinder and then used a blowtorch to remove the pour cup and metal hanger. This is done to save wax and also to keep things less turbulent in the large cauldron of boiling water that we'd use to melt the wax from the rest of the mold. The art of lost-wax casting implies that we must, at some point, lose the wax... this can be done with a kiln, furnace, or other heat source. Anything that melts the wax without being so hot that it vitrifies the shell (we'll get there). For this class, we went with a boil-out. The ceramic shell was attached to a metal stand with some bailing wire, pour cup facing upwards, and placed into a large pot of boiling water. Once the wax melts, it flows up and out of the mold.

When wax gets hot, it expands. This expansion can cause a mold to crack, break, or even explode. This was the main reason for us to use a boil-out method. It allows the wax to slowly heat up and melt rather than heat up super fast and crack the shell.

My shell, either because of the thickness of the wax, turbulence of the water, thinness of the shell, or overall surface area of the mold led to the development of a large crack that circumnavigated the body of the cat.

If this shell doesn't work, then I will have nothing to show for my weeks of work...