Swelling pot-metal

[P3D bob wier <wier@xxxxxxxxxxxxxxx>]

I don't normally cross post from other lists, but from time to time
there are interesting discussions which are germane here. One currently
going on in one of the "antique radio" lists concerns the use of
"pot metal" and how it degrades over time. Since this might also be
found on older cameras, it's worthwhile knowing about...

THANKS

         mailto:wier@xxxxxxxxxxxxxxx
    05:53 PM Wednesday, February 19, 1997
     Texas A & M University - Commerce
 keeper of the Photo-3d,  Overland-Trails
ICOM radio and LDS State Research Outline Guides
 



>Subject: Swelling pot-metal
>X-Comment: Amateur Radio Equipment Using Vacuum Tubes
>Status:   
>
>>no 'probably' about the metal quality. i wish i could show you the coil 
>>turret from a Torn Eb, with the metal swelling and cracking so that the
>>turret cannot turn past the switch contacts. or the Torn Eb whose front 
>>panel has swollen so much that the set is jammed tight in its case,
>>cannot be withdrawn.
>
>Sure thing but this would not have been known at time of manufacture.
>Mazak, zamak or whatever you call it  can be cast to high levels of 
>dimensional accuracy. It can also take fine detail (it was used to make 
>Dinky Toys (Britain), Tootsie Toys (USA) and many parts in
>model trains in the 1930s, 40s and 50s) but its long-term dimensional
>stability is not so good, turning dry and brittle with age or high levels 
>of humidity. Worse still, it can distort badly and when forced back to its
>proper shape, break into many pieces. 
>
>Point is, it was OK when made and only 'crystallised' or deformed later.
>The folklore is that workmen used to throw the silver paper from their 
>cigarette packets into the ingots 'to add grist to the mill' and this of 
>course added impurities or destroyed the careful balance of ingredients. 
>The nett result is the mazak disease collectors like you and me have come 
>to recognise (known technically as intergranular corrosion). At the time 
>the castings were made, I don't think anyone really quite understtod the 
>need for scrupulous cleanliness and purity. It is of course tragic that 
>otherwise good radios cannot be restored; you have my sympathy.
>
>Twenty years ago, when I collected model trains, I saw a superb old 
>(German) model in a shop window. I said I'd buy it, the shopkeeper 
>took my money (quite a lot, proper collector's price) and handed it 
>over... and it crumbled to dust in my hands. "Nice model you've just 
>bought", he said.
>
>Andy.



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