Pot metal (AKA pig metal) is an alloy of metals with a fairly low melting point. This metal was widely used in the casting of some parts for many radios of the 1920's & 30's such as tuning condensers, frames, dial drums and other parts. Pot metal was also widely used in the automobile industry during that period.
Pot metal has become a negative word for many of us who restore old radios as much of it is known for its tendency to self destruct. We refer to this problem as; "Pot Metal Cancer". However, not all castings were prone to the cancer. If the recipe was followed and not corrupted, pot metal was a good product and fine examples made in the 1920s can be found to this day.
Zinc, lead, aluminum, tin & copper are the ingredients often used in the recipe. Some of the formulas used would vary and if a known stable formula was not strictly adhered to, deterioration was soon to start.
Another reason for the "cancer" is the formula was corrupted before the piece was cast. This was often caused by the workers who would often shovel up the dross and toss it back in the molten metal. Sometimes they would toss in other materials as well. This unbalanced recipe was the most certain (if not the main) cause for the self destruction of the cast piece.
Because of the instability of some of the metals within the faulty alloy, repairs are not always satisfactory due to the continuing instability. I have repaired pot metal using the steel filled (gray) epoxy (such as JB Weld) with some degree of success. It does a good job but don't be surprised if the piece continues to deteriorate or your repaired piece no longer fits where it should. Pot metal when formulated correctly is very stable and (in my opinion) will likely last indefinitely. I have (in my radio collection) some parts that are over 80 years old that are 100% perfect. However when Pot Metal has become, distorted, swollen, cracked or is in the self destructing process, this will likely continue. There is no way I know of to prevent further deterioration.
In rare situations large, deteriorating pot metal housings can be dangerous. I relate below a story as told to me by a friend of mine a few years ago:
One night while working in his garage
workshop he heard a loud pop and then the sound of something
zinging past his ear. At first he thought someone had fired a
gun outside and the bullet had penetrated his garage. He
immediately looked outside only to find all was quiet with no one
around. Surely if it were a gunshot, the neighbors would have
also been aroused. The next morning with that event of the past
evening still fresh in his mind, he went out to his workshop. As
he entered his now well lighted garage he noticed something
wedged in the wall about 6 feet from where he was standing the
night before when he heard the pop and zinging sound. It
was chip of metal about the size of a small arrowhead. As he
analyzed it's position and trajectory, he found a Radiola 100A
speaker sitting on a shelf on the far side of his shop with a
chunk missing. You guessed it, the piece that was wedged in the
wall matched perfectly the size and shape of the missing piece
of pot metal. A testament to the serious stress of this