This 4' tortoise sculpture was formed and fabricated from a single 4' x 8' sheet of 16 gauge copper. I will take you through the construction, step by step. There is a video slideshow of the build at the end of this article.
I purchased a 4' x 8' sheet of copper and cut it into more manageable pieces. I use a Bosch electric shear to rough cut sheet metal if it is thinner than 14 gauge.
I started the shell by cutting an over-sized oval pattern. I hammered this flat sheet into a high dome. Notice the tiny deformations left by each hammer blow. I am using delrin mallets, which are softer than metal, so they do not scar the surface.
Excessive hammering stiffens copper considerably. This structural change in the metal is called work hardening. Heating the copper to temperatures near it's melting point renders the material soft and pliable. This process of making the metal malleable is called annealing. I am using a large "weed burning" propane torch available from Harbor Freight. After heating, I quench the metal in water to quickly cool it down and blast off some of the oxides left by the annealing operation.
To stretch the sheet farther I hammered it into an opening that I cut into my workbench, in essence creating a giant silhouette die (or matrix die). In this photo you can see the edges of the hole digging into the shell form. Because the shell was larger than the hole it did not slip through during hammering.
You can see the opening in the table here. After I had reached the desired depth I trimmed the edges of the copper with the electric shear.
I covered a stack of styrofoam with wet clay and placed it inside the copper dome. The clay supported the metal as I hammered in the contours of the shell. It preserved the shape of the copper during forming while allowing it to continue moving.
I mapped out the sections of the shell with a sharpie and tapped the lines in with my chasing tools. From this point forward the shell was formed using a hand held pneumatic hammer. I use an air chisel designed to cut and chip metal. I modify the chisel tools by grinding and forging them into hammers.
I chose to work each section of the shell to completion before moving on to the next. I like to get an idea of how things are going to end up looking while I am in process. These stepped layers were pushed out from the inside of the form and then refined from the outside.
I formed a simple plastron (the lower shell) and tack welded the pieces together. A tack weld is a quick, superficial weld used to temporarily hold metal together. I would need to separate the shell later to attach the body parts. A turtle's upper shell is called the carapace.
To form the neck I made a copper cylinder and crushed it in my hydraulic press like a giant soda can. I helped direct where the metal would crinkle by hammering on isolated areas, creating work hardened zones and softer annealed zones.
The fit between the head and neck was not quite right so I added a patch to fill in the gap.
A tortoise's front legs are flat and wide. I broke the legs up into three sections - a front and back below the knee, and a cylinder above the knee. After the formed parts were welded together I began playing around by positioning them to find a dynamic pose. I decided to add the scales later and continued making body parts.
The toes were welded to the inside of the legs.
The paper templates for the bottoms of the feet. If you can make it out of paper, you can make it out of metal.
To fill in the spaces between the head and legs, I created sections of webbing based on paper patterns.
The final touch was giving the sculpture a nice, even color called a patina. I chose to use liver of sulfur (liver potash) which gives copper a weathered, aged look. Liver of sulfur is an acidic patina chemical that darkens the metal. Copper and copper alloys like bronze and brass will develop their own patinas naturally over time. The liver of sulfur is simply a kick start for the process.
Here is a video slideshow of the build from YouTube.