you are so right al
what we collect today were consumable goods and not supposed to last
forever
zono
In a message dated 8/20/2013 2:13:12 A.M. Eastern Daylight Time,
clockworkh...@aol.com writes:
While the term 'potmetal' was used for many low melting point alloys the
modern name for the potmetal we are familiar with is 'die cast zinc.' Zinc
is actually a wonderful metal to cast with a melting point of 419.53 °C or
787.15 °F. This is low enough so that a mold can easily be made of many
materials. Today the purity of zinc is very high, around 99.999% before it
is mixed with other metals to make alloys. Aluminum is common but any
alloy commonly raises the melting point so pure zinc is preferred for most
applications. Some metals cannot be added to the zinc in even tiny amounts
without creating a dimensionally unstable alloy.
The potmetal problems we see on phonographs is mostly due to 'inter
granular crystal growth' and is not temperature related. The old timers
called
this 'dezincification' which is a misnomer since the zinc really does not
go anywhere unless a piece falls off. If even a tiny amount of certain
elements like tin, lead, cadmium, sulfur, or other contaminate is introduced
into the liquid casting metal the result with time will be slow deformation
of the casting as atomic bonds dissociate along crystal lines while other
crystals actually grow. Thus, some portions of the solid casting may
contract while others expand. A very pure potmetal will be dimensionally
stable
for very long periods of time.
The contaminated batches may have been inadvertent. Old pipe organ pipes
made of zinc could have been used for scrap in the melting pot without the
people thinking about the lead/tin solder seams. Just that small amount
of contamination is enough to ruin a full batch of zinc. Some batches were
very pure and that is why you can find a perfect potmetal reproducer body
with some regularity.
Another problem is the cooling rate when the casting was removed. A rapid
cooling of a die cast zinc will leave stress in the metal. A slow cooling
allows for the metal to naturally anneal at lower stress. Die cast zinc
today that needs to be accurate to a high degree is removed from the mold
and allowed to go through a series of cooler furnaces before reaching room
temperature.
There are environmental elements that can add to a bad batch of zinc. If
the casting was exposed to salt air (a.k.a. sodium chloride) or acidic air
such as the sulfur dioxide coal plants produce, then any porosity may
become a source of oxidation and the casting will suffer. Zinc oxide will
form
along the microscopic lines between zinc crystals and they too will cause
expansion but much less than a bad batch would cause.
But HEY, these things were not supposed to last more than a couple of
centuries anyway! ! !
May all your finds be rare ones,
Al
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