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 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 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 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.
