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Overview
Sand casting bronze is one of many ways to make bronze door hardware. Quite literally, the definition of sand casting means making a sand mold based on a model and then filling that sand mold with melted bronze. When the bronze cools, the sand is removed and you are left with a bronze door lever, bronze door knob, or door entry set.
To those new to a bronze foundry, using sand may seem primitive. But it’s actually one of the best ways to achieve a quality bronze product with a precise finish, shape, and functionality. Unfortunately for us, we must create a new sand mold for every piece of bronze door hardware. And if the sand consistency isn’t perfect, the hardware won’t work. Fortunately for you, this means that every piece of your bronze door hardware is unique.
Door Hardware Models
The Teague family isn’t just filled with bronze foundry men and women, but product designers as well. For some door hardware models, we’ve hired outside designers to come up with the right design and mold. We typically make our molds from balsa wood. Or we use plasticine, a type of clay that never dries, to aesthetically shape the future lever, knob, or dead bolt. We use models to shape the roses and escutcheons as well. We know that when the bronze cools, it will be smaller than the actual mold, so the molds are oversized in the design phase. Door hardware models also have coreprints; these create registers within the molds into which are placed Sand cores. We use coreprints sparingly, so that our hardware is as solid as possible. We typically only use them to under cut profiles and holes which cannot be molded.
Sand Molds
The sand mixture we use is very particular. Too much moisture in the sand will cause steam to ruin the mold. We also want a mixture that adds a rustic finish to the bronze finish. Paths for the melted bronze during the pouring (casting) process create what’s called a runner system. Gas and steam generated during casting exit through the permeable sand or the runner system. The machinery holding the sand mold lets the gas escape through a device called a riser.
A multi-part molding box (known as a casting flask) is prepared to hold the sand mold. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object— such as a bronze door lever—the lower portion of the box, closed at the bottom, we fill with our own blend of casting sand—a slightly moist mixture of sand and clay. We pack the sand in tight with a vibratory process called ramming. The surface of the sand is then stabilized with a sizing compound. The model is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the model. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur. This would be bad. Teague has never suffered a moisture explosion, but they have caused injuries in other foundries.
Bronze Chills
Parts of the bronze door hardware is brought to a tough, ductile, state by inserting metal plates, which are called “chills,” into the mold, where the metal is to be hardened. We use chills for the fast moving mechanical sections of the door hardware. It forms a finer-grained and harder bronze at these locations. (The effect is similar to quenching metals in forge work. The inner diameter of an engine cylinder is made hard by a chilling core.)
Bronze Pouring
After the sand mold has reached the right moisture content, the box containing the sand mold is then positioned for filling with molten bronze. After filling with liquid bronze, the mold is set aside until the bronze is sufficiently cool to be strong. The sand is then removed revealing a rough casting that is usually still glowing red. After the new bronze door hardware has cooled, the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is cut from the rough casting. We apply various heat treatments to relieve stresses from the initial cooling and to add hardness. Finally, we add a compression treatment that adds resistance to tensile cracking, but we don’t want to smooth the outside surface too much. That’s what gives it that great rustic Teague look.
Finishing the Door Hardware
After the bronze door hardware has completely cooled, we use our hammers to further shape the hardware. We then grind and file the hardware edges that are used for mechanical purposes so that everything works. Finally, we polish the surfaces according to according to the desired bronze finish.
Conclusion
That’s how we make our bronze door hardware. We don’t explain what goes into our bronze ingots. That would be giving you too much information and you might try making your own Teague hardware.
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