Pot metal is
an alloy of metals with a fairly low melting point. This metal
was widely used in the cast parts of many radios of the
1920's & 30's such as tuning condensers, frames, dial drums and
other parts.
Pot metal
has become a negative word for many of use who restore old
radios as it is known for its tendency to self destruct. We
refer to this problem as; "Pot Metal Cancer".
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 start.
Another reason for the "cancer" is the
formula was corrupted before the piece was cast. This was 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 the recipe was most certain
one of (if not the main) caused for the self destruction of the
cast piece.
Because of
the instability of some of the metals within the alloy, repairs are not
very 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 it comes apart later. Pot metal
when formulated correctly is very stable and (in my opinion) will
likely last indefinably as I have seen some 80 year old parts
that are 100% perfect.
However when it's deteriorated, distorted, cracked or self
destructing, this process will continue.
Large, deteriorating pot metal housings can
be dangerous in rare occasions. I relate a story from a friend
of mine;
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, an 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.
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