[Milsurplus] Question for Metallurgically-Smart People

[David Stinson]

Thank you all for the lively discussion.
The reason I ask:
Off-and-on, I've been working with crystals, etching them to freq
or "pencilling" them down as needed.
Some crystals have a silver plating on the blank.  This isn't for
connectivity; it's to add mass and inertia to the blank in order
to bring the crystals oscillation frequency down.

I discovered this when trying to etch these crystals.
The blanks were in "regular" holders; i.e. between two
metal plate with raised corners- no direct contact
with the metal coating.
In the etchant, the metal coating bubbled vigorously.
To my surprise, etching that metal coating moved the
crystal's frequency up rapidly and by a large amount.
I didn't make notes at the time, but I estimate I etched it
for a couple of minutes, then became concerned at how
quickly the metal was dissolving.  I removed the blank,
cleaned it and tested, finding the crystal had moved a
huge amount- I didn't write it down, but it was tens
of KCs!  Way past my intention  IIRC, the blank started
at something below the AM window on 80 meters and,
after the short etching, ended above 4 MC.
That was hard to imagine, so I tried another on a
useless freq and the result was the same.  I did manage
the use some of these crystals, but the etching time
was in seconds!

This is the same principle as "pencilling-down" a crystal,
but with the uniformity and fine control needed to get
the crystal spot-on and still active.  One reason "pencilling-
down" often fails is because the random distribution of
material on the face of the blank can cause it to stop
ringing properly.  The metal coating is uniform, so it adds
to the inertial moment of the blank without the random
irregularities of "pencilling" that can kill oscillation.
If there were a practical way to deposit this material
on crystal blanks, there's just a world of now-useless crystals,
too high in frequency, which could be "re-tuned" to usefulness.

If I live long enough for another project,
I'll play around with this some more.
Be a while- still have parcels to ship and
radios begging for attention ;-).

  GL OM ES 73 DE Dave AB5S

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