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 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 which is made to be separated to remove it, 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 piece 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. Sand composition and grain size define the texture of the casting's surface, as do preparations applied to the mold which also facilitate the piece's release from it.
At the level of production of the major foundries, multiple patterns were required for high demand pieces. To insure consistency, a reference, known as a master pattern would be created. Initially carved from wood, it would then be cast in brass, aluminum or other materials. Often, a master pattern would be made up of various component pieces, perhaps to ease replication, or to accommodate variations.
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 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. 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, such handles are two curved prongs whose tips are tightly touching.
Later handle designs, while appearing to be aesthetic or ergonomic improvements, were actually labor saving innovations. A single handle pattern with a long, linear taper and a gentle curvature could be more easily removed 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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