Sand Casting

Once I decided to do my own casting the next step was to build the furnace and tools. The furnace was built using half of a 55 gallon drum. I cut a hole in the side at the bottom of the drum and placed the jet in such a way that the flames would spiral around the crucible. The inside of the drum was lined with a 1" layer of 2100 degree fiber board and next with a layer of 2500 degree zircon fire bricks. The naturally aspirated jet was attached to a manifold connecting 2 50 lb propane tanks, each with a regulator capable of 30 psi. The regulators control the pressure and in order to control flow I installed a ball valve at the jet. To preheat the crucible slowly, I partially closed the ball valve and ran the pressure at between 5 and 7 psi. Once the inside of the furnace reached 1400 degrees the valve was opened completely and the pressure was raised to 20 psi. At high flow, it was necessary to keep the tanks in a hot water bath to keep the regulators from freezing.
















The lid of the furnace was made of fire bricks drilled and sandwiched between two angle irons on threaded rods. A small hand pivot crane was attached to the base to make moving the lid easier and safer.
















The process produces a small amount of slag that floats to the top of the bronze. I used a bent piece of sheet metal to scrape this off.



Before I could do any of this I had to make my own tools. The tool to lift the crucible out of the furnace is similar to a post hole digger made out of steel but instead of shovel blades, the tongs were shaped to fit a number 10 crucible. (The number "10" designates that this crucible holds 10 lbs of aluminum which is relative to 33 lb of bronze.)






An artist friend gave me a pouring ring but it needed to be modified. I added length to the arm, made the rings slightly smaller and welded a movable latch onto the side which keeps the crucible in the ring when it is tipped on it's side. The pouring ring is set up to pivot over the mold. After the crucible is pulled from the furnace, it is set into the pouring ring, locked into place and is easily moved over mold for pouring into the sprue.
















I consulted with several metal craftsmen and read as much as possible about the process. The first few pours were trial and error with temperature. I'd read that 1950 degrees would be sufficient to pour a large casting and that fine detail work required 2100 degrees. At 1950, the metal cooled before reaching the end of the mold. After that, I brought the metal up to 2100 degrees and as an added precaution (see the first photo) I preheated the sand with a heat gun. This worked beautifully.




This photo shows the cope and drag - the two parts of the mold were built from 2 x 6s and 2 x 4s. Internal guide pins and external, tapered guides kept the two parts aligned properly. I chose Petro-bond, a casting sand that uses oil rather than water to bind the sand together. It eliminates the possibility of uneven water distribution which could cause a steam explosion. I made other tools for this process including a hand ram for ramming into the mold and around the pattern. I used various small shaping and cutting tools to cut sprues and vents. Talcum powder was used to keep the two parts of the mold separate and to keep the patterns from sticking to the sand.