How It's Made: Sand Mold Casting

Whether made by hand by a late-19th century foundryman or by a modern automated molding production line, the basic methodology for the manufacture of cast iron cookware remains fundamentally the same.

The process begins with the creation of a pattern. The pattern can be considered an original from which many duplicates are made. It is in most respects identical to the pieces it was created to produce, with the exception of its size. The pattern is always made slightly larger than the intended size of the duplicates, to account for the fact that the molten iron shrinks as it cools and solidifies.

The pattern is used repeatedly to create many molds, each of which are ultimately destroyed in the course of making, typically, a single pan. A mold consists of a damp sand-based mixture packed around the pattern in a two-piece box made to be separated to remove it after the sand has hardened, and then put back together leaving a cavity in the pattern's shape. Molten iron is then poured into the mold cavity via a passageway created through the packed sand. Once the iron solidifies, the hardened sand is broken away to release the casting, and any excess iron not a part of the finished casting is trimmed or ground off.

That's the simplified explanation. The actual process is a little more involved, requiring the observance of close manufacturing tolerances, and the accommodation of variances in how the molten iron might tend to flow into the mold cavity for a particular shape. Patterns may consist of multiple components used to create the cope and drag separately, and with casting channels attached. Sand composition and grain size define the texture of the casting's surface, as do preparations applied to the mold which also facilitate the casting's release from it.

At the level of production of the major foundries, multiple working patterns were required for high demand pieces. To insure consistency, a reference, known as a master pattern would be created. Initially carved from wood, a master pattern would often be made up of various component parts, perhaps to ease replication, or to accommodate variations. It would then be cast in a brass version to which runners and gates were added. From the completed master, aluminum versions were cast to actually use to make sand molds.

Tricks Of The Trade

While the basics of sand mold casting may extend even to present day automation, there are instances where a good degree of creativity was required to accomplish certain design features. A prime example can be found in the casting of lids with loop handles.

As sand must be tightly packed around a pattern to create a mold, loop handles present a unique challenge. As a fixed protruding part of the pattern, a loop handle would not be feasible, as the pattern could not be removed from the sand mold without disturbing the sand packed around it. Through handle designs and casting artifacts one can observe the usage of removable or hinged sections in the manufacture of loop-handled lids.

Early loop handles were usually wider at their "attachment" points, and tapered narrower in the middle. In order to facilitate this design, two removable segments were required, one for each half of the handle, as a single piece in this case would still not allow for its removal without disturbing the sand already carefully packed around it. This was achieved by having each half of the handle on the pattern hinged so that they could extricate themselves from the sand mold as the pattern was removed, but without ruining the mold. That two part handle patterns were used is evidenced by the casting line which can often be seen and felt at the apex of such handles. Essentially, most early loop handles are actually two curved prongs whose tips are tightly touching.

Later handle designs, while appearing to be aesthetic or ergonomic improvements, were more likely labor saving innovations. Rather than two hinged segments forming the handle, a single handle pattern with a long, linear taper and a gentle curvature could likely have been more easily retracted from the sand mold than the earlier design requiring two pieces.

The above is true of lids whose undersides show no evidence of handle attachment. Those that do indicate the maker chose instead to cast the handle as a separate piece in advance, and used a technique to embed it in the sand mold before pouring the iron for the main casting. The molten metal surrounded the tips of the embedded handle, firmly anchoring it once cooled.

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