ATM Speculum Blank No. 5

[Anthony Stillman <atmer@flash.net>]

I've been working on my speculum efforts in odd moments and have made some
little progress.  At one point I had one of those buttons I cast, sanded
flat and polished by buffing wheel.  Several folks at the mirror making
workshop noted how reflective this metal is.   Our resident professional
pointed out how not flat I had made it.  That chunk of metal was relegated
to the fire along with it's siblings to cast the 60mm blank.  My forth
attempt at this was no better than my first.  My second was, and I was sure
I could do better.

My fifth attempt at casting speculum bronze has yielded a blank worthy of
continued work.  It's 9 mm thick, 63 by 66 mm oval, and weighs about 1/4 kg
There are defects, its wedged and its going to end up thinner than I'd
like.    Probably 8:1.  Anyone want to plop this?

I've also alloyed two ten gram buttons of inter-metallic compound.  One was
speculum, Cu3Sn, three copper atoms linked up with one tin atom.  The other
no doubt also has a name, I just don't know what it is yet.  This compound
is Ag3Sn, three silver atoms linked up with one tin atom.  Both metals are
hard and brittle and clearly display crystallization.

On sand paper I sanded both buttons flat to 1500 grit and have polished
them on an unchanneled flat pitch lap.  The speculum took a good polish
with cerium, but the other metal developed scratches.  I switched to rouge.
This helped but the Ag3Sn was still finely scratched.  I then tried black
rouge with some noticeable improvement.  I'm still working on a way to
develop a scratch free polish on this stuff.

I finished polishing, as best I could, both pieces of metal in the same
manner and at the same time.  I have been comparing the reflectivity of one
to the other as the surfaces age.  Fresh, the silver alloy was more
reflective than the speculum, though it also scatters more light.  After a
few days this was no longer so.  The Ag3Sn does not have the warm hue of
speculum.  I think if I can put a scratch free surface on it, it will make
a better mirror.  The down side, ATMing always seems to have a down side,
is the cost.  A three inch disk would have about a hundred dollars worth of
silver in it.

Other inter-metallic compounds of which I have become aware are PdSn2
(palladium tin), Ni3Sn4 (nickel tin) and Mg3Al2 (aluminum magnesium).  The
first two I've yet to try.  I did try the magnesium mix.  A sticky slushy
metal resulted.  Almost worked, but it caught fire when I tried to remelt
it into a nice looking button.  Yea I know, should have swamped with an
inert gas.  Next time.

In the past,  while just fooling around, I made aluminum silver.  I found
no reference to this being an inter-metallic, but it is very hard and
crystalline in nature.  Looking at, and plotting the melt data of copper
tin alloys, (CRC Handbook of Chemistry and Physics) I see that there is a
cusp at the 2/3 - 1/3 constituent ratio, what speculum is.  Looking at a
plot of the melt data for aluminum silver I see that there are cusps at the
90-10 and 70-30  silver-aluminum ratios.  Messing more with this metal is
on my list.

More to follow.

Anthony


finis coronat opus
The end crowns the work.