The Continuing Art of 
                     Bronze Casting  
                           
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WRITER
TIM SUNDERMAN




The Continuing Art of Bronze Casting, December 2011 Vhcle Magazine Issue 8, Art
 
2011: The Continuing Art of Bronze Casting by Tim Sunderman
 
 
 
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The next step in the process is to make yet another mold called the "investment". This one will be for the actual bronze pouring. The wax sculpture including all the gates and vents are immersed in vats of silica slurry then coated in sand and allowed to dry. This step is repeated nine times until the ceramic sand forms a layer about half an inch thick (12cm). The first layers of sand are hyperfine to most accurately record the shape of the wax model. The subsequent layers of sand are more coarse to provide greater structural support. This investment mold then will be fired in a kiln at a temperature of 2,650˚F (1200˚C) resulting in a medical quality porcelain, the wax is melted out of the mold in a high pressure steam autoclave providing the empty space for the bronze to fill.
At this stage, it is time to melt the bronze ingots in a crucible. Bronze is typically an alloy of 88% copper and 12% tin, but some foundries use a slightly different formula to assist the flow and annealing, or setting of the cooling metal, by using 94% copper, 4% silicon, 1% manganese and about 1% ace elements. Bronze melts at 1700˚F (930˚C) in a graphite crucible while the porcelain investment mold is heated to 1100˚F (600˚C). In this way, the bronze can flow into all the spaces of the mold without prematurely cooling.
Once the bronze and mold have reached the right temperature, the mold is moved to the pouring area and the "lead pour", or crane operator, lifts the crucible into place. The second team member known as the "deadman" maintains the balance of the crucible, and a third member clears the slag off the surface of the cooling molten metal. The bronze is poured into the mold through a funnel formed in the wax gating process and the air escapes through the vents.
As quickly as one hour after the pour, the bronze is now cool enough to have the ceramic mold broken off and the gates and spurs, now bronze, cut off with an electric arc cutter. One last pass with a sandblaster removes any remaining traces of the mold.
Now the sculpture moves onto the metal chasing and finishing stage where any evident flaws from the casting process can be corrected and perfected. Large pneumatic grinders and smaller pencil grinders are used depending on the size of the imperfections and the necessity to recreate the artist's original surface quality. On larger pieces that require the welding together of separately cast panels, the weld line in excellent foundries is rendered absolutely invisible by the skill of their metal chasers. If the piece is of monumental size, a framework steel substructure is created to provide support. And when necessary, a structural engineer is required to analyze and recommend a structure that can withstand the forces of high winds and earthquakes.
The final step is the surfacing patina or coloring of the piece. These are usually accomplished by applying chemical compounds with a spray bottle and fixed with a blowtorch to speed certain processes of oxidation. When the desired tone and transparency is achieved, a thin coat of wax is applied to protect the surface. The finished sculpture is now ready for installation in its permanent home.
There are many sculpture foundries in the United States, but one of the largest and best is the Artworks Foundry in Berkeley, California in the San Francisco Bay Area. Founded by Piero Mussi, who was trained by master craftsmen in his native Italy, they are producing some of the best work around. Tours are available to watch each stage of the production which also includes a gallery of incredible work produced there on site for some of the nation's preeminent sculptors. Go to artworksfoundry.com for more information.
 
 
The long run of history is not written in books, and certainly not in the fleeting twinkle of electronic storage, but with stone and metal artifacts. And of all the metals, bronze has become the keeper of records. Lead is too soft, gold and silver too rare, and iron rusts. But bronze, essentially a copper alloy, is incredibly strong, made from common minerals, and is relatively resistant to corrosion, so it lasts a very long time. Until recently, the Bronze Age was believed to have started around 3300 BCE, but there are other examples from India that suggest that bronze casting could go back as far as six to ten thousand years. Tools, weapons, jewelry, and art have held intact over the millennia. And in regard to these, art reveals the greatest part of history.
Many of the great monumental sculptures around the world become defining icons of civic identity, typically in bronze like the colossal Tian Tan Buddha of Hong Kong, or the Statue of Liberty in New York. Even in modern times, bronze work continues to be the prevailing medium of metal sculpture. And there are some contemporary artists doing incredible work today. But sculptures are only as good as their casting, and this process of casting bronze is an art in itself.
So, this is a description of how bronze sculptures are made. Nearly all bronze casting still uses a method developed five thousand years ago called Lost Wax where a wax sculpture is replaced by bronze in a mold.
Artists can spend hundreds of hours on a sculpture. A good foundry wants to assure an artist that the quality of their casting is equal to the skill of the created work. So there are a number of refinements to the basic technique of Lost Wax that render excellent detail, surface quality and strength. Sculptors commonly work with oil clay because it holds sharp, clean edges and sags less than water-based clay. But, regardless of the original medium, the first step in the process of bronze casting requires a mold for the finished artwork. To do this there needs to be one or more seams in the mold to separate it to release the original sculpture. One half of the sculpture is covered in silicone rubber and reinforced with another coating of hard plaster. This step is repeated for the other side. After it has dried, the mold is separated. The original clay sculpture often sustains significant damage during the separation because small elements like curled fingers for example, will break off during the process.
The next big step is critical in the final outcome. The mold is reassembled and wax is poured into the silicone mold. Obviously, bronze statues are not solid. If they were, they would be too heavy, they would be prohibitively expensive, and they would be structurally unsound as major rifts would form through the center as it cooled. So, there is a real skill at maintaining a consistent thickness of wax throughout - roughly a quarter of an inch (5cm) depending on the size of the piece. This involves "slushing" the hot wax through the mold in three separate pours using cooler wax each time so as not to melt the previous coat.
Whatever the wax is is what the bronze will become. So there is a step called "chasing" where the solidified wax piece is tediously corrected. Seams are removed, any scratches, deformations, and imperfections are expertly resolved by production artists - sculptors in their own right - to finalize the wax work. These artists will even project light through each area of the wax to check its thickness by seeing its translucence. If it is too thin, they will melt additional wax onto the inside surface to firm up the area. Watching them work is a real demonstration of artistic skill and craftsmanship. Oftentimes at this stage, separate panels of wax need to be assembled seamlessly and with no evident change of surface quality across the seams. Clearly, it is far easier to correct wax than metal, so every effort is made at this stage to create a perfected version of the original.
Once the finished wax is approved (usually by the original sculptor), channels need to be constructed from wax bars that will become the pathways for the molten bronze to be poured into a new mold and pathways for the air in the mold to escape. If it were not vented, the super-heated air trapped by liquid bronze on top of it would expand and erupt back up through the pouring channel resulting in a blistered, bubbled finished surface of the final piece, and likely unfilled pockets in the mold. Spruing and gating are terms used to describe the sometimes intricate attachments of wax veins for the pour which are simply melted to the sculpture. But even this step requires considerable experience on the part of the craftsperson to predict the flow of the bronze through the shape and the likely path for the venting air. Trapped air means a hole in the finished sculpture.
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Tim Sunderman is a Graphic Designer in the San Francisco Bay Area whose first love is drawing and painting, tries to avoid computers until there is no other recourse, and because there is no other recourse, yearns for the open spaces. Tim is a graduate from the Academy of Art in San Francisco, and majored in Philosophy at the University of Pittsburgh. He is a college art and design instructor and freelance artist.
Read other articles by Tim Sunderman as well as see his photography work.
 
          

     

     

     

     

        
                                                                      
 
 
01 Finished sculpture in oil clay.
02 Silicone rubber mold painted over sculpture.
03 Silicone rubber mold painted over sculpture.
04 Silicone mold sections with plaster support backing.
05 Wax casting of sculpture from silicone mold.
06 Spruing and gating pouring channels and vents.
07 Silica slurry for building the ceramic investment mold over the wax model.
08 Porcelain sand for coating the wet slurry wax model to build the investment mold.
09 Porcelain ceramic investment mold.
10 Heated investment molds emptied of wax, ready for bronze.
11 Firing bronze ingots in a crucible.
12 Crucible being lifted by crane.
13 Pouring team guiding the crucible.
14 Molten bronze being poured into ceramic mold.
15 Full bronze slug after the mold has been broken away showing the gate and vents.
16 Cutters and grinders are used to remove the pouring channels.
17 Metal chasing is the process of assembling and perfecting the raw bronze cast panels and surfaces.
18 Separate cast panels are welded together with no trace of the original seam.
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*see captions at the end of article
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19 Chemical patinas are applied by brush and oxidized by blowtorch to attain the final surface color.
20 Finished sculptures are ready for shipping and installation.