Working on my Jocelyn Bell Burnell model, which is a representation of a bronze statue, made me curious as to how the real bronze casting process is done and what steps I would need to take if a statue such as mine was to be produced at full scale in real bronze. To research this process I identified several professional sources including Gabe Gabel; a sculptor and painter who specializes in sculpting for bronze cast outcomes and who wrote ‘A Survival Guide For Bronze Sculptors’. Also, the Southern Californian based American Fine Arts Foundry, who produced a video with a representative from Stan Winston, about the process a sculpture will go through in their foundry to create a bronze replica. Another interesting artist I discovered in my research is Rod Hughes; a sculptor and bladesmith who creates work using traditional saxon, viking or medieval techniques which includes bronze casting in a charcoal furnace. For some background reading I read the famous bronze sculptor Henry Moore’s book ‘Henry Moore: On Being a Sculptor’ which is not a process book about the use of bronze, but more his perspective on sculpture and people's capacity to truly see three dimensionally - calling people “form-blind” more often than they are colour-blind - plus his thoughts behind his own work among other things. Gabe Gabel’s book about the process of casting a sculpture in bronze gives good context from the artist's point of view working with the foundry to evaluate the sculpture and then prepping it. Gabel begins with going over her specific sculpting process, how she builds armatures, what materials she uses and how she manipulates them using heat or various tools. She then advises that the artist takes detailed photos of the model before it leaves the studio for later reference. When the model is ready the artist contacts the foundry for a price quote, they discuss the work so they have a full understanding of what has been used and discuss any changes needed in the design, if necessary. It is important that they know what armature techniques and materials have been used, also that it has been fully photographed, as many sculptures have to be cut into parts for the molding process. For this reason the foundry prefers artists to use aluminum wire for armatures. Once it is ready, the model is submitted to the foundry. This is the process of ‘lost wax’ bronze casting based on Gabel’s description of the process, the American Fine Arts Foundry summary and other information I discovered. Molding At the foundry the sculpture - made of any material of the artists choosing - has a silicone mold taken of it, this can be in few or many parts depending on the complexity of the sculpture. The mold is enclosed in a fiberglass or plaster shell for rigidity. Wax Cast The hot wax is poured into the silicone mold and poured out again, this is repeated until the wax is about 5mm thick. It’s important that the wax is hollow to save on bronze and to limit casting defects. Wax Chasing With the wax cast out of the mold the next step is cleaning it up to remove any seam lines from the silicone mold and fixing any imperfections such as air bubbles as these will all translate into the bronze cast if left. These are fixed using any heated tools on the solid wax (soldering irons for example). This stage is called ‘wax chasing’ or ‘perfecting the wax’. At this stage the artist can alter their model as the wax can be manipulated and altered; for example, Gabe Gabel uses the same model of a horse but alters it at this stage to make it unique for the client. Gating The gating system is created from the wax cast. Gating is for creating wax channels around the sculpture so the bronze can later flow in and let the air escape, preventing air bubble imperfections. Shelling Next is the shelling process where the gated wax model is encased in a sacrificial ceramic shell. To do this the wax sculpture is dipped into a slurry solution to make the surface wet and adhesive for the next step, which is appling silica sand. The first layer of sand is called ‘flower’ as it’s very fine and captures all of the detail, after this, increasingly larger crystal sizes are used. About 8 layers of the sand are applied to build up its strength. This process can take one to two weeks but varies between foundries, size of statue etc. Burning Out The burn out phase is when the bottom of the shell is cut off enabling a route for the wax to escape as the whole shell is turned over and heated. Next, all of the wax is melted out and collected. This can be then recycled for future casts. Vitrification This shell is now placed in a furnace and heated to 1500°F; this burns away any remaining wax (if it was left when the bronze is poured in the mold would explode), and vitrified the shell material. This means that the silicon based sand is turned into the ceramic shell, like glass. The outcome of this is very strong but it may develop or reveal cracks or gaps, so they must be filled in with a patching cement. This is crucial as it will prevent casting deformaities and stop any wastage of bronze. This is then left for a day before being brought back up to temperature. Bronze As the shell is brought up to temperature (1500°F) so is the bronze, approximately 2000°F. Once both are ready, the shell is carefully removed from the furnace and placed upright ready for the bronze, either on a secure rig or placed in sand to hold it steady. Many will be poured at the same time for efficiency. The bronze is taken out of the furnace too and the crew works fast to skim away any impurities (called ‘slag’) that have risen to the surface. (At this point of the process, bladesmith Rod Hughes’ charcoal furnace based heating process the bronze impurities would include whole chunks of coal as it is entirely covered. These would need to be scooped out too, then, same as the normal process, some bronze would be poured away to the side to ensure only pure molten bronze enters the mold.) The bronze must be poured in while the shells are still hot to ensure flow into every crevice, if it was cold it would set before reaching all of the sculpture. This is known as a ‘cold stop’. Shell Removal As the shell and the bronze within cools down causing pressure changes from shrinking metal and the ceramic splinters and shatters off; this is called sloughing off. If this doesn't come away on its own a hammer can be used to give it its last push. The ceramic shell part is the only element of the molding and casting that isn’t reusable. Bead Blasting Next the bronze statue is bead or sand blasted: glass beads or sand are used to remove any lasting bits of shell. Metal Chase The bronze statue is next taken to the metal workshop to have the sprues (which is the name for the stems of wax created as channels for the bronze in the gating process) removed and recycled by melting down for future sculptures. If the sculpture was cut into parts the re-assembly is done now too. The welding rods for the metal chasing on a statue can be cast alongside the model to ensure the alloy of bronze is identical resulting in an invisible seam line. To camouflage the welding joints, sprue residuals and any casting imperfections an expert metal worker, called chaser, will reproduce the textures of the original sculpture using the reference images taken by the artist and the foundry at the start. Approval Now the sculpture is in bronze and whole, the artist must approve the outcome before it can progress. If it is to their expectation and they are satisfied then the statue has no more metal work to be done it receives its final sand/bead blast. Patination Stage Finally it’s time for the bronze statue to receive its patina. To understand this, here is a very brief background of bronze and what is involved in patination. ‘Patination’ is from the italian word ‘patina’, and it refers to the green hue or film copper takes on. The word originally came from Latin to mean “a shallow dish”; the italians would have seen the same green hue on their copper dishes. Patination has now evolved to refer to the process of colouring a sculpture's surface in any form, not necessarily bronze or even a metal at all. Bronze Alloy consists of copper and tin usually - this can vary between foundries. It is the copper content that makes a statue oxodise when in contact with certain chemicals, developing a green hue or film, an example of this occurring would be the Statue of Liberty’s famous green tone. This process can be utilised by a patina artisan, called patineur, to create different finishes on the bronze statue. The vast amount of looks that can be achieved are too many to list but things that patineurs use to create these chemical changes in the bronze (catalysts to cause chemical reactions they control) can be:
Once these techniques have been done and the statue is at the desired point, the work will be completed with a “passivation layer”: this prevents further corrosion. Despite this, the environment will still affect a statue's chemical composition if exposed enough; a car will rust eventually if left in the rain for long enough, and the same goes for unmaintained statues. However, the Statue of Liberty, as I mentioned previously, sports a green oxidized coating but it is left as a form of protection against further more damaging types of corrosion. A chemical balance has to be struck which depends on what the sculpture is likely to experience, this is all taken into account in the patination stage. Another example of inevitable changes to bronze statues patina is the human desire to assess through touch. Those bronze statues which reside in public places, such as parks and memorials, sport shiny brighter bronze areas, the duller patina turned gold. I am not referring to the gentle changes in the patina caused by natural elements exposure such as rain water but the brightest spots are where peoples hands have transferred natural acids onto the bronze. These cause chemical reactions and over long exposure the bronze statues’ tend to brighten on places such as hands, breasts and noses. I wondered what alternatives there is to lost wax bronze casting but I only found two related techniques: Cold casting: Mixing metal powder into resin to achieve a metallic looking finish. This could be bronze to mirror a bronze cast which is sometimes called “bonded bronze”. Digital techniques: A digitally scanned sculpture can be 3D printed and/or milled from foam. The benefit of this process is being able to replicate a sculpture at any scale and reproduction. Bibliography:
Alec Steele. (2018) CASTING A BRONZE SKULL!!!. Youtube. [online] Available from: https://www.youtube.com/watch?v=-JAZ_W-oH3Q&feature=youtu.be&ab_channel=AlecSteele [Accessed 16 February] All Classics, Ltd. (n.d.) Bronze FAQ. [online] Available from: https://allclassics.com/bronze-faq/ [Accessed 17 February 2021] American Bronze Foundry. (n.d.) Molds. [online] Available from: http://americanbronze.com/2016/how-the-art-casting-process-works/expendable-mold-casting-methods-sand-lost-wax-shell/ [Accessed 16 February 2021] American Fine Arts Foundry. (n.d.) About Us. [online] Available from: https://afafoundry.com/about-afa/ [Accessed 16 February 2021] Gabel, G. (2008) A Survival Guide for Bronze Sculptors. [Kobo eBook]. Available from: https://www.kobo.com/gb/en/ebook/a-survival-guide-for-bronze-sculptors-1 [Accessed 15 February 2021] Merriam-Webster. (n.d.) Patina. [online] Available from: https://www.merriam-webster.com/dictionary/patina [Accessed 19 February 2021] Moore, H. (2013) Henry Moore: On Being a Sculptor. [Kobo eBook]. Available from: https://www.kobo.com/gb/en/ebook/henry-moore-on-being-a-sculptor [Accessed on 16 February 2021] Smooth On. (n.d.) How To Make a Metal Cold Casting With Smooth‑Cast® 325 Resin. [Online] Available from: http://smooth-on.com/tutorials/metal-cold-casting-smooth-cast-325-resin/#:~:text=%22Cold-Casting%22%20is%20a,metal%20(lost%20wax%20process.) [Accessed 19 February 2021] Stan Winston School. (2012) The Lost Wax Bronzing Process - Stan Winston from Sculpture to Bronze at American Fine Arts Foundry. Youtube [online] Available from: https://www.youtube.com/watch?v=XJsAFLM0RWk&feature=youtu.be&ab_channel=StanWinstonSchool [Accessed 16 February 2021]
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