Re: ATM: Dang hard speculum metal

[Anthony Stillman <atmer@flash.net>]

Charles,

>From my copy  "Amateur Telescope Making Advanced"  Copyright 1937, 1944,
1947 by Scientific American Inc. under the section entitled "Metal Mirrors
and Flats-a Composite Chapter" on page 62 paragraph three I read:

"Not everything regarding this work, noted in sulphurous private
communications, can be quoted, as this book must be maintained fit to
print.  One metallurgist writes that, after months of experimentation, he
finds that "every metal examined by him has surface flaws, in addition to
the well known physical defects.   Even at magnifications as low as 50, the
polished surface is seen to be more or less covered with tiny pits.  These
pits are distinct from the non-metallic inclusions and other impurities
inherent in the steel.  Some mirrors from ancient telescopes, which were
similarly examined, showed the same surface imperfections in the speculum
metal""

The section goes on to note that speculum, Cu3Sn, is not technically an
alloy but an inter-metallic compound or secondary solid solution and does
not have real "metallicity".  That is, it is hard and brittle and
non-plastic.

I recommend reading the section as the efforts of several workers are
detailed.  Most used a lathe to preform and or grind and polish the metal.

In my experience with aluminum I did not note any pitting.  However the
alloy I used, from Innovative Optical Materiel, was clearly plastic.  That
is, the edge deformed when accidentally strained (read, dropped on the
floor).

I'll add that I've done a fair amount of alloying and have yet to fabricate
an alloy that didn't include at least some trapped gas.  Though I don't
think that's what you are seeing.  Rather, I believe individual grains of
speculum are freeing themselves from the surface along the weak
crystal-crystal boundaries.  Some of the alloys I've created, particularly
those including aluminum and sliver or aluminum and gold show this effect
quite prominently.  Jewelers call these incompatible metals.


Anthony

PS  Aluminum silver is hard, brittle, grainy and gray, even with minuscule
amounts of aluminum.  Aluminum gold is hard, brittle, grainy and a
beautiful violet blue.  Sadly its impossible to forge and nearly as
difficult to grind.