The Lost Wax Bronze Casting Process: From Sculpture Concept to Finished Bronze
A Complete Guide to every stage of fine art bronze casting - a fine art bronze foundry look
At Carolina Bronze Sculpture we often work with artists, businesses, government committees, and the general public. Many people come to us with varying degrees of knowledge about how a bronze sculpture is made. While we do offer tours of the foundry to the public, we also want to help people understand what's involved in the process before they jump into having a bronze sculpture cast. Since our start in 1990 we've been helping to create fine art bronze sculptures and would like to explain the lost wax bronze casting process and provide some helpful tips for people who want a sculpture made.
What is Lost Wax Casting?
Lost wax casting (sometimes referred to as "Cire Perdue" which is French for "lost wax") is the primary way to create bronze sculptures. The short explanation is a wax version of a sculpture is encased in a heat resistant material and then the wax is melted out or "lost" and bronze is poured into the empty shell to make the sculpture. However, a lot of steps are involved in making that happen. The entire process actually began in ancient history.
Lost wax castings first date back to around the fifth or fourth millennium BCE (5000-4000 BCE.) The first bronze sculpture cast with the lost wax process dates back to around 2300 BCE. This sculpture is known as Dancing Girl and was found in the city of Mohenjo-daro in modern day Pakistan. The sculpture casting process was refined over time with the Egyptians and the Ancient Greeks making major contributions to the process. By around 500-480 BCE, the Greeks used the lost wax bronze casting process as the major technique for producing bronze sculptures. Because of the weight, expense and the possibility for bronze to warp as it cools the Greeks began to cast sculptures in hollow pieces so that any warping could be fixed. (You would need the strength of Hercules to straighten the leg of a life size solid bronze sculpture!) But where does the process start?
The Lost Wax Bronze Casting Process
Step One: The Design
Let's look at how a life size or larger sculpture is made. Because let's face it, they certainly are impressive. As most new things start, it begins with an idea.
Often, a business or organization will contact an artist wanting a sculpture made. They want it to represent a particular person or it may be based on a theme or concept.
So then the artist creates a design. Sometimes this is done as a sketch. Some artists design their work on the computer either on drawing programs or 3D design software. The artist then shows the initial concept to the organization for approval.
Sometimes the design process will involve engineering plans or architectural visualization designs. This can lead to multiple approvals and changes.
Step Two: The Maquette
Once the design is roughed out the artist sculpts a scale model which is known as a maquette. Because the clay is often not strong enough to support itself, the artist gives it support in the form of an armature which often looks like a stick figure made of wire. The armature gives the artist something to spread the clay over and still be able to tweak and fine tune the positioning and overall pose.
The artist may show this to their client to ensure they approve of the overall pose and look of a piece. Sometimes for very large sculptures the artist makes several maquettes in different sizes. These models are not as detailed as the full sized sculpture will end up being when finished. However, the closer the model is to what the artist and client want the finished sculpture to look like the better.
After the artist and their client are satisfied with the maquette, then a foundry can help the artist prepare for the full sized sculpture.
Step Three: The Full Sized Sculpture
A full sized sculpture will also need an armature to support the clay. However, wire isn't strong enough for a life size sculpture. Artists used to have to make armatures using metal pipes, rebar, wire and other materials. Often a life sized or larger sculpture would be so heavy they would need to hire an assistant.
However, modern technology can make this process easier. 3D scanners make it possible for artists to have their maquette scanned which creates a digital copy. The digital copy is resized to life sized or larger. A CNC machine carves a high density urethane foam to create a lightweight armature that the artist can use to sculpt the full size piece. At Carolina Bronze we offer 3D scanning and Foam Armature enlargement.
Artists can then take the foam armature (which is light enough to still allow an artist to move a 10 foot tall sculpture.) A thin layer of clay is spread over the armature. Usually this is an oil based clay. Water based clay can crack if it dries out, oil based clay keeps its shape without cracking. The artist uses a variety of tools to shape and sculpt details in the full sized piece that would not be possible at the maquette size. The finished sculpture is then approved by the artist's client.
Step Four: Mold Making
Making an accurate mold is a crucial step in turning an artist's full sized clay sculpture into a lasting finished bronze. The process begins with making a master mold. A master mold is called that because using a master mold is how copies or editions are made of a sculpture. A master mold is also where the expression "breaking the mold" comes from. When an artist wants to make a one of a kind sculpture or a limited run of sculptures they tell the foundry how many to make and once every copy is cast and broken out the foundry will break the master mold. Destroying a master mold ensures that no other original casting can be made!
Master Molds
Our master molds are made from a silicone rubber. The mold is applied as a liquid that solidifies into a flexible rubber. Mold makers apply the rubber using techniques we've perfected over the years to faithfully capture all of the fine details artists sculpt into their clay. It is important to faithfully capture the sculptor's marks in the clay so that it carries through to the bronze. In fact, we have molded life casts (sculptures molded from living people) and have captured the texture of their skin even down to their fingerprints!
When applying the mold it's important to make a way to remove the original clay. This is done by inserting shims into the rubber, creating a seam that can be used to open the mold and remove the clay.
Mother Molds
A mother mold is a protective mold that holds the master mold in the proper shape and prevents the rubber from being damaged or warping the waxes that will be made in it. Some foundries use a combination of fiberglass and resin for their mother molds. However, if the fiberglass is not reinforced it can be too flexible leading to waxes that are warped and don't fit together properly. Also using fiberglass and resin is a potential health hazard to workers and to the environment. We make our mother molds out of plaster which we reinforce with natural plant fibers which work like rebar in concrete to strengthen the plaster.
After the master and mother molds are finished, we grind the plaster edges smooth and then open the molds and remove the clay. This leaves an empty rubber negative mold to go on to the next step.
Step Five: Wax Casting
There are several ways to make a wax mold. some can involve simply pouring liquid wax into a mold and moving it around. This is called slush casting. Some use slabs of soft wax that are pressed into the mold.
We prefer brushing liquid castable wax into the master mold to ensure every detail is captured and prevent bubbles from being trapped in the wax. In the process we want to also ensure that the wax is a very even thickness. Typically the wax is less than 1/4 of an inch (6.35 mm) in thickness. Uneven thicknesses in wax can lead to casting defects and can impact both the assembling of the sculpture and the patina later on.
Chasing the Wax
When the wax cools it's removed from the mold and an artisan will begin to chase the wax. Chasing a wax means that they remove any seams where a two part mold meets. They'll also restore any texture that was where that seam met. Often they refer to photos taken of the original sculpture to ensure every detail is precise. We ask the artists if they want to come and approve their waxes to make sure every detail meets their approval. This presents the artist with an opportunity to touch up areas.
They can see their sculpture and if they notice a change they want to make they have an opportunity to do so. While a sculpture is in wax it can still be changed with simple sculpting tools. However, if an artist wants to make changes after it's in metal it becomes much more difficult. What would take an hour or so in wax may take days of welding and grinding in metal to change. At this point the wax will also be sprued up.
Spruing a Mold
Spruing or "treeing" a mold is when wax bars or tubes are attached to the wax mold. These sprues look like branches attaching to a main bar or trunk. (That's why it's sometimes called treeing.) A sprue becomes a pathway for the wax to melt out of the final heat resistant mold and is also the pathway for the bronze to fill the mold. After the mold is sprued and the artist has approved their waxes, metal pins are inserted in the wax. These pins are called core pins and they keep the outer part of the final mold from collapsing after the wax is melted out. This keeps the outer shell the same even distance from the inner core shell.
Step Six: Investment
The word investment in metal casting is a very old meaning for a very old process. It refers to an outer layer or covering. In this case, the final heat resistant mold or shell. Historically this might involve using clay mounds built around the wax. We use ceramic.
We take the sprued up wax mold and dip it into a liquid ceramic slurry, a watery mud-like mixture. We thoroughly coat the wax and sprue system making sure no bubbles are trapped in the slurry. Then a fine grain of sand is sifted over the mold making sure it is thoroughly coated. Each coat is allowed to dry before the process is repeated. This takes several days (the total varies based on the shape and size of the wax) of repeatedly dipping and sanding to make the ceramic thick enough to support the eventual weight of the bronze.
Step Seven: Burnout
Now we reach the "lost wax" part of the lost wax bronze casting process! We heat up a kiln to temperatures over 1000 degrees Fahrenheit. Then we place the ceramic mold onto a grate in the kiln to allow the wax to melt out of the mold into a catch pan below the kiln. At those temperatures the individual droplets of wax are often on fire as they drop into the pan bellow. So it looks like it's raining fire.
Once all of the wax has melted out of the mold, the bottom of the kiln is closed and the temperature raised so the ceramic can be fired much like clay pottery. This process is known as vitrification. It turns the sand and dried slurry into a hardened heat resistant glass-like ceramic. After the ceramic has been fired it's allowed to cool. Professionals examine the molds to make sure no cracks formed during the heating and cooling. We'll then vacuum any residue or carbon out of the mold. Then it's time to pour some bronze.
Step Eight: Bronze Pour
The bronze pour begins with filling a crucible (which often looks like a large pottery vase) with bronze ingots. The crucible is set inside of a furnace and then the furnace is used to heat the bronze to around 2100 degrees Fahrenheit. As the metal melts and fills up the spaces in between the ingots more metal is added. This is called charging.
Preparing for the Pour
While the metal is melting, the kiln is also heated up to 1000 degrees. Then the molds that'll be filled are placed in the kiln. Preheating the molds prevents the bronze from cooling too fast before it fills all the details of the sculpture. It also keeps the hot bronze from making the ceramic mold explode due to temperature shock.
Once the crucible is full and the metal is at the proper temperature the action really starts. First the pouring crew will take the molds out of the kiln and set them in a sand bed and make sure they are secure. You never want a mold of hot bronze to tip over and spill.
Skimming the Metal
Then the crew will skim the metal. Impurities in the metal float to the surface, sort of like grease in a stew. Those impurities are called either dross or slag and if they end up in the mold create casting defects known as inclusions. The crew skims the impurities out leaving the metal as pure as they can make it.
The Actual Pour
The crucible is then secured in what is known as a pouring shank. This is a set of handles that clamps down onto the crucible with metal arms known as "dogs." The crucible is then lifted and positioned to pour. The bronze is poured into each mold filling it completely so that as the metal cools it doesn't shrink and cause holes in the metal known as shrinkouts.
Once all of the molds are full the crucible is placed back in the furnace. We typically have several pieces ready in batches so we end up pouring several times in a day. (As a note: we offer tours of the foundry and will often post on social media near or on a day when we will be pouring bronze.)
As the metal cools it does shrink slightly. However, the ceramic doesn't. As the metal solidifies it shrinks and causes the ceramic to crack. This actually lets the pouring crew know when it is cool enough to move the molds.
Step Nine: Breakout
After the bronze has been poured the ceramic shell is then broken away and removed. The sprue system is also cut away. Core pins are pulled out of the metal and the remaining ceramic residue is sandblasted away. Then it is ready to be assembled.
Step Ten: Assembly
Before a sculpture is assembled each piece of metal is inspected and prepped. Core pin holes are filled and ground smooth. Casting defects are corrected if found and then metal working artisans begin to put together a very heavy 3D puzzle. Modern technology has made assembly easier than in the past. Modern Tig welding has replaced the use of sockets and pins or the soldering of pieces with lead or other metals.
Armature
For life size public sculpture armature is a must. To make a sculpture safe in a public setting internal structures must be installed to allow it to be securely mounted on a base or installed on a foundation. Engineering designs are consulted and as the sculpture is assembled the support is built throughout the figure.
Chasing
As the sculpture is put together any warping that occurred as the metal cooled is corrected by chasing the metal. The metal is bent back into it's proper shape and then welded together. At first those welds are easily noticeable but just like the seams in the wax the metal welding seams are smoothed away. However, while simple sculpting tools work on the wax. In metal it takes grinders of various sizes and grits to clean those seams.
After the seams are ground down the metal artisan then needs to add texture back where those seams were. Using reference pictures of the clay and a variety of often handmade custom tools, artisans restore the texture that the sculptor made. After the sculpture is assembled a mounting template is made. The sculpture is also sandblasted to remove oils from tools or handling from the metal before a patina is applied.
Step Eleven: Patina
A patina simply put is the oxidation of the metal. While many will recognize oxidation on iron as rust, bronze and copper alloys like it will oxidize to a point but will not crumble into pieces. This is one of the reasons why people say bronze lasts forever.
The way a patina develops changes based on several factors. Humidity, temperature, environmental conditions, and various chemicals can create a wide variety of colors in the way the metal oxidizes. This is actually a very good thing. Since the patina is an actual change of the metal itself it has an advantage over paint. Paint will eventually chip and peel off of a sculpture. A patina is part of the sculpture. Most of the time artists have a particular patina in mind for the finished sculpture. (I say most of the time because sometimes an artist may want the patina to develop naturally) In order to create the patina artistically it takes a skilled artisan known as a patineur.
These artists are part alchemist. As they change the metal to create the desired colors and effects that meets the sculptor's designs they use torches and an array of chemicals which are applied with various techniques. After they finish they seal in the patina with a clear coat to protect it from moisture and the effects of people handling it. Usually a clear coat of wax is applied to the sculpture providing it with extra protection as well as a finish that is buffed to an even shine.
Then the sculpture is ready to be shipped out or installed.