Re: Swelling pot-metal

[P3D George Gioumousis <georggms@xxxxxxxxxx>]

P3D bob wier wrote
> 
> 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
Bob Wier quoted
> >
> >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.
> >
Back in the late sixties I worked as a chemist for Shell Oil, and in the
process was exposed to some very learned car nuts. Pot metal is (was?) 
used for for brightwork on cars. It is cast zinc, and was valuable 
because it expands on cooling, so makes a precise impression of 
the mold. Its relatively low melting point allows steel molds, 
which cuts cost.

It was well known that with age the crystals it was composed of
grew bigger, and thus the piece got brittle and crumbly. It is
not that the manufacturers were ignorant of this property, it's
just that they felt that a car _should_ be junked after a few years,
so it didn't matter if parts just fell apart.

Rough on collectors and car owners like me that feel that a car
should last nearly forever. (Our oldest car is a 63, our youngest
an 81.)

Don't be too gentle on manufacturers; they know what they're doing.
-- 
                           Stereo                Stereo
George Gioumousis           /---\        |       /---\ 
                             o o         |       o o  
georggms@xxxxxxxxxx           |          |         |   
(415) 494-6276              \===/        |       \===/ 
                        Photographer          Photographer
                        & curmudgeon          & curmudgeon


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