
Rob Koehl's multi-metal sculpture of a cutthroat trout combines copper and stainless steel on a forged common steel body.
By Frank Ross
Sculpting in a single metal medium such as copper or stainless steel can produce some striking works of art, but combining the two into a single piece to achieve variations in color and texture presents the challenge of joining two dissimilar metals with different bonding and heat dissipating characteristics.
Rob Koehl, a metal sculptor in Cottonwood, Arizona wanted to emphasize the shiny silver back of a trout with the warm red tones of a copper under belly. If you have ever considered working in a multiple-metal medium but avoided it because of the difficulty factor, perhaps you can benefit from Rob’s techniques and tips.
This piece is built on a base of 14 gauge common steel that is forged to achieve the curved body, and then covered with 24 gauge stainless steel on the top and 20 gauge copper sheeting for the fishes belly. Rob uses a plasma cutter, MIG welder and oxy-acetylene torch, and begins the process by drawing out an outline of a fish on the steel with a piece of soapstone. Although the tail gets cut off due to size limitations of his forge, and recreated later, Rob finds that it helps to establish the correct proportions in the beginning. When laying out the trout, he leaves a little extra material to compensate for the curve of the body.
Once the basic shape is cut out, the slag from the plasma cutter is removed and the steel goes into a forge to heat it up for shaping the body using a wooden stump and hammer. Although Rob has done it both ways, his preferred method is to braze the copper onto the steel body first and then clean up the seam by grinding away any excess brass from the brazing process before joining the stainless steel. He begins by annealing the copper, so it is soft and ready to hammer into shape around the curved steel body. The annealing process is achieved by heating the copper in a forge until it is black hot and then dropping it into water. Once the copper has cooled a bit you need to clean up the surface with a finishing pad or brush and treat the surface with silver flux.
“When brazing the copper, you have to bring the area up to heat equally and copper is such a heat sponge so you have to move the torch around quite a bit. It helps to elevate the piece so you can apply the heat from the bottom as well. With brazing you’re not stacking metal like welding. You want to use the capillary action the flux creates to suck the bronze into the gap between the copper and steel. I use Wolverine silver flux with a small brass brazing rod and a number two tip on my torch to join the copper to the common steel. Then I run a bead down that and join the stainless steel next. I’ve found that by attaching the copper first it allows me to braze the copper on better. Ideally you want to get a good half inch of soldered area along that line to make room for the next process,” he explained.
“Once I’ve joined the copper, I hammer it down around the body and go back and clean up the line with a grinder to remove all of the brass so I’m not trying to weld into the brass when I lay down the stainless steel. When forging and joining the other metals together, it’s important to lay the project down on a flat surface to make sure you’ve got a clean line. It will save you a lot of work in the end if you keep the fish flat,” he said.
To complete the look, Rob adds a ferric nitrate patina to bring out the black speckles in the steel in addition to turning the copper a deeper red. “Another technique I’ve found to add color is to heat the metal up and then brush it with a fine brass brush. The brass comes off and attaches itself to the hot surface and adds both color and dimension,” he added.
When the final buffing is done, the result is a trout so beautiful its colors are only challenged by the original cutthroat inspiration.
If you learn better by watching, check out Rob’s YouTube channel for this and other metal working techniques, or learn more about his artwork at robkoehl.com .
For all of your copper, stainless steel or other metal working supplies, shop Rob’s favorite supplier; QuickShipMetals.com.























