Metal Pouring Time!
For today’s episode of “You Can’t Do That In Your Brooklyn Apartment!” I have a throwback to work I could make when open space and open flames were bigger parts of my life. The topic du jour is lost-wax bronze casting, just like humans have been doing for almost 6000 years.
No matter what you’re planning to cast, you always start the same way, by making the original model. Some people like to work directly in wax, but the model can be made from just about anything, although in many cases a wax copy of the original may prove necessary.
In this case, I mostly skipped the wax since my original model was made with acorns. Although they don’t melt as cleanly as wax, acorns won’t survive the burn out process, so I can avoid having to make what would end up being the fairly complex multi-part mould necessary to produce a wax copy of the sculpture. The model, however, needed some additions in order to get it ready for casting.
If you’re planning to pour liquid metal into a vessel, you should probably make sure there’s an opening to pour into. Additionally, unless you’re pouring in a vacuum, your shell isn’t actually empty; it’s chock full of air, and it all needs to go somewhere else if you want to fill the whole thing with metal. Unfortunately, because molten metal cools very quickly once it leaves the crucible, there’s no time for air bubbles to rise to the surface and escape naturally, so the metal needs to force the air out of its way in order to prevent bubbles from ruining the casting. In order to facilitate getting the metal in and the air out, shells need to include a large cup to pour into, as well a series of tubes, called sprues, runners, and gates, which guide the metal from the cup to the primary cavity, and also allow the air to be pushed out through vents.
Wax will adhere well to most surfaces, acorns included, but proper technique is important in order to create a strong weld; in particular, the surfaces need to be evenly heated. In order to weld wax to wax, I briefly heat a thin metal blade or putty knife under a flame, then place the blade inside the joint so that both wax surfaces heat evenly. Quickly and smoothly removing the blade brings the two pieces into contact, and if done well they will weld together and become a single piece. Acorns aren’t quite as easy to weld to, since the irregular surface is difficult to heat evenly. In order to connect gates to the model I had to use a heat gun to flow wax into the crevasses. At this point, the model is entirely finished and is ready for shelling.
Shelling refers to the process for creating the negative vessel for the bronze. The shell is created by dipping the model into a silica slurry and coating it with silica sand of various grits. This is repeated over the course of several days in order to build up a ceramic shell at least half an inch thick, still leaving the top of the cup open.
Once the shell is sufficiently thick, it’s finally time for the lost-wax part of lost-wax casting. The dried shells are placed upside down in a kiln (we refer to the cup as the top of the mold), and heated for several hours. The burnout process serves two purposes: First, the wax is literally lost as it melts and flows out through the cup. Second, the kiln bakes the shell, drying and hardening it. Notice how the shell changes from yellow to white. Once burnout is complete you are left with a sturdy, hollow shell, ready for casting.
(Sometimes burnout doesn’t go perfectly. My acorns burned nicely, but ash isn’t a liquid so they didn’t all make their way out of the shell the way they were meant to. Luckily, it wasn’t something an air hose and some patience couldn’t solve.)
From there, it’s time to pour. Or at least start the furnace. Bronze melts between 850º and 1000º C depending on the alloy, and when you’re working with that sort of heat it’s best not to rush. In particular, the enemy of a good pour is moisture. At such high temperatures, water boils instantly, and the rapid expansion can damage the shells, the equipment, and of course anyone unfortunate enough to be standing nearby. Before it even goes into the crucible, the bronze must be heated to get rid of all traces of moisture; even the ambient moisture is enough to be dangerous. The shells are also reheated in the kiln and stored there until the last moment to keep them dry.
At Skidmore we had a relatively elaborate setup for casting, including a motorized gantry to hoist a much larger crucible than we could use otherwise, but the principles are the same no matter the scale. Before pouring, we scrape the slag off the surface of the molten bronze. Slag is a mixture of impurities in the metal, and in the melting process it accumulates at the top of the crucible. It’s unsightly, and of course we don’t want to get any in the molds.
It’s important to pour from an appropriate height. The bronze cools quickly, and if a pocket of air gets in its way then the result can be a void or an inclusion. Pouring from an elevated position utilizes gravity so that the bronze moves through the mold fast enough to prevent inclusions.
Once the molds have cooled, the finished models can be removed. The release process is sort of like opening a present, but with a hammer. Once the shell is all gone (depending on how complex the model is, this can take quite a while) the sprues, now solid metal, are cut off as waste. Sometimes sprues can be cleverly concealed on the model, but typically they leave ugly spots that need to be cleaned up. The clean up is called chasing. Using a variety of tools, particularly die grinders, you work the surfaces until the marks from the casting process are gone. If there are voids or inclusions, they can be filled with a MIG welder and a bronze rod, and again chased to the desired texture.
Once the chasing is finished, the sculpture is essentially done, but it can be taken a few steps further. When it’s be freshly cleaned, bronze resembles rose gold. It rapidly oxidizes however, losing that luster and darkening. You can choose to let it age naturally, but that can lead to corrosion. Instead, by patinating the metal you can take advantage of the oxidation process and produce beautiful new colors in the metal. There’s a wide range of chemicals that can be used to patinate: ferric nitrate, cupric nitrate, and liver of sulfur are all popular, but you can get more exotic. I’ve even heard of people creating a patina from urine, although I don’t recommend that one. Once the desired color is achieved, a coat of wax locks in the patina and protects the surface from erosion.
From there, the final step is presentation. Not every sculpture needs a base, but in my experience, most benefit from one. At a minimum, it’s important to consider how a piece will be presented, and to factor that into the design process. I always like to add a hardwood base to my sculptures, unless they truly demand to be freestanding. It’s usually a good idea to drill and tap a small hole into the sculpture in order to fix it in place.
And then, suddenly, you’re done! Step back, crack a beer, and admire your work. Unless someone intentionally melts it down, it will look pretty unchanged 1000 years from now, when future archaeologists rediscover it. That’s about as permanent as your work can get.