Sand casting has existed for hundreds of years. The basic technique has changed little
over time: pour molten iron into a mold created in sand to create a shape. One creates a
mold by packing sand around a pattern, which when removed from the sand produces the void
space into which the iron will flow.
Sounds simple, doesn't it? From my own very limited experiences
in casting iron, any potato head can cast a small, simple, thick piece of iron (given
access to the proper materials, especially 2500 degree molten iron!).
Minor changes in the casting process allow for the approximate dating of pieces. The oldest pieces will have a circular "sprue" mark on the underside of the piece. This technique was used until the mid-to late 1700s. (Most pieces were cast upside down, to avoid having an unsightly sprue or gate mark on the top of the piece). The sprue is the point where the maker poured the molten iron into the mold. Any piece that is cast, even those today, will have at least one point on them where you can see where the iron entered the mold.
The next generation of cast iron have a "gate" mark, which will look like a long thin line on the bottom of the piece. This is where the iron entered the mold, and the technique lasted from the mid-1700s to the late 1800s or so. Pieces made around 1875 to the present time have the iron enter the mold from the sides (usually in two places, sometimes opposite one another), which creates a smooth bottom. Current techniques have the iron coming in from the side, so look for one or more places on the side of the iron with heavy grinding marks--that is where the iron entered the mold. The grinding was done to get rid of the excess metal from the pour.
Between 1875 and 1940, cast iron cookware manufacturers
periodically refined their casting techniques
to produce extremely well-made pieces.
The earliest pieces (1875 to 1900) were thinner than those produced later. This thinness
led to a high number of pieces that cracked or warped during use. Pieces produced during
this time also had fewer finishing steps on the production line, and many of the gem pans
have a slightly more primitive appearance.
Pieces made between 1900 and 1940 were both slightly thicker and more finished in their appearance, and the quality of the production process reached its pinnacle between 1920 and 1940. During this time, the manufacturing included a series of polishing steps that produced pieces with glass-like surfaces. One such step was as follows: Manufacturers would tumble the finished product in a large (several feet in diameter) rotating drum that contained small bits of metal (usually star shaped) that served to polish the pieces. Other pieces, such as skillets or dutch ovens, were turned on lathes to provide a very smooth cooking surface. You can often see the lathe marks on the inside of these pieces.
After 1940 or so, both Griswold and Wagner were no
longer controlled by family members, and the quality deteriorated significantly. Pieces
made after 1940 are downright clunky in appearance and do not have the same high quality
finish. You can tell these pieces from a distance due to their thicker skillet walls and
course looking castings.